Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. oh Pe ea lee UNITED STATES DEPARTMENT OF AGRICULTURE. at. ns iO teg _ Miscelianeous Publication No. 540 aio a So | ISSUED June, 1944 - VEGETABLE AND FRUIT _ DEHYDRATION aS = k Manual for Plant Operators i Prepared By so 4 i Bureau af Agricultural and Industrial Chemistry Sire Agricultural Research Administration 4 For sale or the Superintendent of Documents, U, S. Government Printing Office, Washington 25, D. C. Z mes . Price 30 cents — eS = nies 2 i ~ - . ra ADMINISTRATIVE PERSONNEL __ Emergency Dehydration Research Project _ Western Regional Research Laboratory, Albany, Calif. Bureau of Agricultural and Industrial Chemistry — W. W. Skinner, Chief of Bureau = T. L. Swenson, Director of Laboratory — E. M. Chace, Public Relations W. B. Van Arsdel, Engineering Investinationg L. B. Howard, Technological Investigations _ H. C. Diehl, in Charge, Dehydration aes Schools. R. T. rea’ Editor : Se CONTRIBUTORS _ AGRICULTURAL RESEARCH - ADMINISTRATION Bureau of Agricultural and Industrial Chemistry -E.A. Beavens - J. R. Matchett — Horace Campbell _ a? AL Pima —Martha E. Davis ae William Rabak — H. J. Dutton a 'W. D. Ramage — G.T.Hemmeter - #£=C.L. Rasmussen L. B. Howard W. B. Van Arsdel Hans Lineweaver” K. T. Williams B. Makower - a: Bureau of Plant Industry, Soils, and Agricultural Engineering Y-R. Boswell: — DF. Fisher _ J.-S. Caldwell = Ww. T. Pentzer WAR FOOD ADMINISTRATION ~ Office of Distribution R. W. Kueneman UNIVERSITY OF CALIFORNIA _ E. O. Essig __ J. E. Knott G. C. Hanna ~ E. M. Mrak OREGON STATE COLLEGE E. H. Wiegand MASSACHUSETTS INSTITUTE OF TECHNOLOGY B. E. Proctor Washington, D. C. UNITED STATES DEPARTMENT OF AGRICULTURE MISCELLANEOUS PUBLICATION NO. 540 Issued July VEGETABLE AND FRUIT DEHYDRATION A MANUAL FOR PLANT OPERATORS Prepared by Bureau of Agricultural and Industrial Chemistry, Agricul- tural Research Administration CONTENTS : Page Page Introduction esse = 20s sea ee eae 2} Effect of drying conditions on quality of prod- Locating new plants--_-------------.---------- WO rac Seace nas a Saceige = Soe seen EES Plant lay-out, equipment, and capital invest- Heat damage—scorching.___.2____-__--____ 74 a eee eer Beh ee ee ee 3 gamers to pulriiive quality. See 75 ulldings and lay-out_-_------------------ amage to color and flavor._-____________ 75 Preparation Line eee ae 11 | Conveyor-beit dehydrators_____.--.----.----- 76 Packaging rool... ww. 1| THHBeland-truck dehydrators._...-.---_-.--. 77 Plant and equipment costs_-------.-.----- 16 Buvallel:tlow Senet Rit eer Sek oe es Handling capacities and utility require- 9 Combination arrangements Pencil Se Recneee 80 IMETICS Hes Ses Soe ee ee es Wo & (Ghaaaames sa avarc(® GEA EE ne Sroring and handling fresh fruits and vege- of Ce pane aes cata Waimea ear ae a tables_------.------------------------------ Combined blanching and predrying...--. 84 ce the product... - DESMA Re ee 2D Starting and stopping the tunnel drier____--__- 84 Transp ATEN HT yibl ba tac ea ed Re Oe RED 21 Simple counterflow tunnel__.____________- 84 Preparation of raw Tn Lorin ae 29 Parallel-flow tunnels in multistage driers _ 87 ashing _- i eah eae Deke | Reais CRO 29 Counterflow tunnel as second-stage drier - 88 Gradinotaats eee ee Gee er) 04 | Cabinetidehydratorsss2 0. = (oes. 22 ee 8 88 Root peeling Peet ats ee ote ee oy Types of cabinet dehydrators__.._......-. 0 GMatibinthites oo eared wa nae Sei a eRe 31 Preparation equipment for use with @ucting te ole ent a ede 34 CAbinetidnlersss-seos= oe Seas eee 92 Blan chinese owen tk awe anaes Doh oat 36 Operation of the cabinet drier_-___-------- 92 Adequacy of blanching as measured by Continuous dehydration in cabinet driers-. 93 tests for enzyme activity________________ 36 Establishing a time and temperature Methods of blanching.._________________- 37 schedule for cabinet dehydration------_- 94 Vegetable blanchers__.--___- Se Bay BEN ae 39 sOUnCES of heat $e dehy drators sss eip ee Sa aa - Sulfuring vegetables: <2: 222 ee 42 irect-combustion heaters_._-_-_-._------ Principles involved in the drying process-____- 46 Indirect heating system_.___.--__--.--_-. 98 The vaporization of water___-_-_--------- 46} Temperature controllers=_ 222-222-2223 22= 27 99 Properties of air and water vapor. -------- 47 | Mechanical movement ofairin dehydrators_.. 103 Volume of mixtures of air and water vapor- 49 Centrifugal or rotary impeller fans_______- 103 - Specific heat of air-water vapor mixtures_- 51 Propeller, axial-flow, or disk fans______-_- 104 ee ee w-2---------~---=--- oh Application of fan laws in choice offan__. 104 Characteristics of evaporation from a = | MPA SO bin driers e108 moist solid - -_.----------.-------------- 53 Operationlofbini driers] sane 107 Papo ion from the surface of a wet Determining needed capacity 108 IMATE Al ee eee ee Se eee ees 54 : ATES AG Es. De fallin CARNITINE I Removal of water from deeper layers-_---- 54 eee aes Soe eTRSSS G mene Gy ee et Effect of thickness of piece___-____________- 56 TERS ae eo See Se Ogee “Case hardening’....._____.............. 57 | Other types of dehydrators and their uses--__-_- 111 Equilibrium moisture content_---.-_-__-- 58 Spray dr ‘ani Sey Te OS DT its Typical drying curves, constant drying Rotareduinidniase itis wage a 113 COTY LET OT Se ee Bt ees Se 59 Loe arte ee yah MRR LMT OA BT See Effect of wet-bulb depression_____________ 60 Rotary ora Fae BRE BS ag Effect of temperature level__..___________ 61 Apple and prune kiln driers__------------- Wiflect of air velocity...) 2 2.0) 1224) 2 3 61 | Multistage dehydration ---._-- --------------- 114 Manner of exposure to airstream_________ 62 Changes in material as drying progresses. 115 Effect of thickness of layer_..._-_________- 63 Examples of multistage systems---------- 115 Character of the support___._---_-- in aaa 64 | Finishing and packaging___.__---------------- 116 Menuet of Dreparation Bu Ata n eae endo a eel insecuen of ihe ay, prod eta nee a 119 ape and size of pieces__.-.___-_________- 5 rades and specifications for fruits an Nature of the fruit or vegetable_____.____- 65 vegetables ESE i ee Se Oe ee 120 Estimation of drying time____..______._-- 66 | Temperature reduction to maintain quality in Drying conditions within a dehydrator--. 66] dehydrated products_-__-------------------- 120 Cooling of air due to evaporation -____-___- 67 | Standard types of packages__-__--__------_-_- 122 Maximum evaporation capacity of a de- Packaging equipment and methods_--_-_----_- 124 hydrator___------__- ie eae 68 Cylinder-and-meter method ---_-_-.------ 124 Bepermmental determination of drying Vacuum-chamber method-_--------------- 126 RING he ot = ns eS EG as eee Sa 69 Gassing by means of “‘dry ice’”’____------- 126 Effects of recirculation of air___.---__-___- 70 Seaman spase ee Lee Soa : ewe URES 128 Heat usage in a dehydrator______________- 72 ipackagin ey ine cartons se ssa eee eee 128 Balance between efficiency and effective- Labeling packages and cases-_------------- 128 MESS Oa ae ae ee Roe a Se Nm ie ee 74 BSCS Sea REE: Rien SE ee NE Re ele 128 2 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE : ; ‘ Page Page Substitutes for tin-plate containers___________ 129 | The control Jaboratory—Continued. Functions of the package________________- 130 Analysis of atmosphere in cans of de- Single sheets, double laminations, and hydrated vecetables==— a eee 157 compound laminations_-._--__---______- AST | VEY GTA GIOGCS ES 159 Glaze packaging for compressed dehy- IMethods222. =. 32 ee a eae ee 160 drated foodSi22 22822 Sk eee 132 Sources oferror_.. 2-—-_ ee 161 Compression to high density for packaging___ 132 Composition and amount of water_______- 162 Presses ANG prOlesses) =) 45 sees ene 134 Serial tests=—2 2 ee eee ee 164 Holding ipresses= 42 & B28 e Boe eR eens 137 Nature and condition of material______--- 166 Sanitation on oe eee te Pete a ae 137 Definition of tenms== =o eee 167 Watersuply iss sale e te Seen i Wen) Summary of rehydration tests_____.-.___- 169 Wastexdisposale: 22 eat Pisa rit Bes 139 | Quality testing ---—-___ joo -2- = 2-2-2 - = === == === 169 Operating iersone le ae aa 140") Taree Golection aid teaining: ena 71 COAG NO MESSI G6) NOS 2222222 saa M41) Summary of quality testing... 172 UM ALIO Mas eee wee eee entrees nee ees eee esha 141 P ; t Sg ee a 1A | renee ecnirerient ia Seca a 172 Temperature and moisture -----..------- 142 Segregation and analysis of “processing M@he*controllaboratorys=2 = eee 142 COSTS. Bee Ee an ae ee 173 Examination of raw product___---------_- 143 Packaging and warehousing costs___-___- 181 MMolstute CE Seen ee aes 144 Indirect and overhead costs___...--__---_- 181 etermination of adequacy of blanching - 147 Estimated processing costs in vegetable de- Vitamin determinations_____--_-------_-- 150 hydration-=- 22-0225 JS ee 182 Determination of sugar (vegetables) -__-_- 153 | Handling various vegetable and fruit crops___ 183 Determination of starch (vegetables) _____ 155 Vegsetables 225 2 hs ee Sees 183 Determination of sulfur (dried fruits and TUitS 3 oe Bid ees 2 ee eee 210 Veretables) 2 =a eee a ae 156 |sbiterature cited a= = sea ee ee ee 216 INTRODUCTION More than a year before the attack on Pearl Harbor military leaders and food technologists in the United States realized that technical information on the dehydration of foods might become vitally important. As a result of the war, the production of de- hydrated foods has increased rapidly. Prior to the year 1941 only relatively small quantities of a few vegetables were dehydrated com- mercially, but at present vegetable dehydration has become a large industry. The dehydration of fruits has been a well-established and important industry for many years and considerable published infor- mation on the subject, including results of investigations by this Bureau, is available. The part of this manual dealing with fruits is not intended to be a complete guide for operators in the dehydra- tion of fruits but presents only some of the most salient points of common information which may be of incidental interest to vegetable dehydrators. The developments in the dehydration field have been accompanied by large advances in the production of dehydrated milk, eggs, and meat.* In 1940 the Bureau of Agricultural and Industrial Chemistry started new investigations on the dehydration of foods. The objec- tives were to discover (1) methods of decreasing the weight of foods, (2) ways of saving shipping space, (3) types of containers requiring less metal, (4) how to lengthen the storage life of dehy- drated products, (5) techniques for retention of the greatest possible nutritive value of the foods. Two dehydration training schools were conducted by the Bureau to assist the many food manufacturers who were engaged in, or about to become engaged in, the dehydration of vegetables and fruits. ‘The first school, lasting 2 weeks, was held at the Western Regional Research Laboratory, Albany, Calif., in September 1942; and the second, lasting 2 weeks, was held in Rochester, N. Y., in October 1The term “dehydration” is commonly used to denote specifically a controlled process of drying with forced circulation of heated air. VEGETABLE AND FRUIT DEHYDRATION 3 of the same year. The results of the past year’s and previous research on dehydration were conveyed directly to commercial de- hydrators. Lectures were given at the schools by people from the Department of Agriculture and other governmental agencies, and also by people from the Massachusetts Institute of Technology, the University of California, Oregon State College, and private indus- tries. From these lectures and from the results of current research, a large body of information has become available. A large number of problems have faced the new dehydration industry. The technological and engineering problems, with which this publication is concerned, are widely inclusive, ranging from the suitability of raw material to those involved in final packaging, storage, shipment, and reconstitution for use. The present publica- tion is based on the results of both earlier and more recent work and is designed to serve as a manual for commercial operators. The detailed results of research are thus not included here; instead each division of the general subject is treated in a manner designed to facilitate application in commercial production. LOCATING NEW PLANTS An adequate supply, or potentially adequate supply, of suitable raw material is the primary consideration in the location of a dehydration plant, but there are really many factors involved, especially in wartime. The more important are as follows: (1) Availability of an adequate supply of suitable raw materials, (2) sufficient suitable labor for pro- duction and processing, (3) suitable fuel, (4) electric power, (5) an ample supply of pure water, (6) adequate facilities for sewage disposal and prevention of nuisance odors, (7) sanitary condition of surround- ings, (8) adequate transportation facilities, (9) experienced and finan- cially responsible management, (10) suitability of existing facilities for expansion or conversion, and (11) suitability of location with respect to war strategy. Some of these factors are self-explanatory as listed; others are discussed in the paragraphs that follow. The conversion of tunnel-type fruit dehydrators to vegetable dehy- dration has been successfully accomplished in a number of cases. Other converted fruit dehydrators have proved unsatisfactory. Most fruit-dehydration facilities are not suitable for conversion to the pro- duction of “quality” dehydrated vegetables. Diversion of existing dehydrators to vegetable dehydration should be made only after a critical engineering analysis of the problems involved in conversion and the probable efficacy of the converted plant. The problem of raw-material supply must be considered from two points of view: (1) Large production of suitable material and (2) dislocation of established economy. Large production is not sufficient justification for the establishment of a plant, since the market for fresh produce may be large enough to absorb a large part of the production. From the latter point of view, the best areas for new dehydration plants are those in which the market prices are lowest. These low prices usually result from large production and high yields. combined with relative remoteness from large consuming markets. Suitability of material is also important. Before the operator locates a plant in an area he should obtain information on the suitability of the crops for dehydration, through tests and pilot-plant operations if possible. 4 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE He may discover that a crop grown in a given area requires different treatment from that required by the same crop, and even by the same variety, grown under different conditions. He should assure himself that the crop or crops to be dehydrated are high in original nutritive and other quality factors. Large quantities of heat, power, and pure water are required for the operation of a dehydration plant and these should be available at a reasonable price. If steam is used as a source of heat, the type of fuel is relatively unimportant. Heat consumption is lower with direct heat and this requires the use of gas or oil fuel, preferably the former. Difficulty in disposing of wastes from the preparation line, including the wash water, has resulted in the closing of some plants. The starch from potato plant wash waters tends to settle rapidly in sewer lines and may result in clogging unless a heavy flow of water is main- tained. The large tonnages of solid wastes must also be disposed of in such manner as to avoid the creation of a nuisance. Attempts have been made to recover the starch from potato plants in a salable form. The low value of the recovered crude starch makes the advisability of this process somewhat doubtful. Plants using abrasive peeling offer the best opportunity for this type of saving, because of the large peel- ing losses in these plants. Plants drying onions or garlic must be located so that prevailing winds carry the nuisance odors away from populated areas. On the other hand, all dehydration plants should be located where odors from other sources will not trouble them. The flavor of dried vegetables may be damaged by absorption of such odors and the acceptability of the product seriously impaired. PLANT LAY-OUT, EQUIPMENT, AND CAPITAL INVESTMENT A properly planned dehydration plant is not built around a particu- lar piece of equipment or around a certain step in the process. The different operations must be balanced, with no “bottlenecks.” The capacity of each piece of equipment should therefore be somewhat flexible so that an operating balance can be obtained without seriously impairing the efficiency of any part of the plant. Plants in the ca- pacity range of 5 to 100 tons per day, unprepared basis, are discussed here, and most of the statements are equally true for larger plants. Figures 1 to 5 illustrate floor plans that are considered later in more detail. Vegetable rather than fruit dehydration has been considered chiefly in the preparation of these plans and in the discussion of plan- ning. Many of the general statements also apply to the dehydration of fruits. , Each prospective dehydrator will have individual preferences con- cerning many features of lay-out, construction, and operation. ‘The equipment, the lay-outs, and the operating steps outlined herein are offered as constructive suggestions. Many important matters have not been illustrated because custom tailoring is necessary in almost every case. Those omitted include such operations as air desiccation, grinding, storage of raw material or finished product under special conditions, and numerous techniques still unproved on a commercial scale. Plants much smaller than 25 tons per day are not usually in a com- mercially competitive position unless they have some special advan- tages, such as low-cost raw material or low-cost labor. Small plants, VEGETABLE AND FRUIT DEHYDRATION ‘009 ‘esvaaMS $00), ‘aredor Lv17 puv Coys ouryorut ‘O09 ‘soygo { ON8‘T ‘SUIOOI YSVA PUB IOIOT * OOF ‘A107 B.10QVT * OO8 ‘uI00L Jopiog {Qo0'T ‘Surseyovd ooo‘eT ‘Surdap ‘ 000‘9 ‘Uolvavdaid 'Q0G‘'g ‘ese10}s Sor[ddns-Suiseyovd puv jonpoid-poystuy * 0098 ‘edv.104s8 [VII9]VUL-MLY 3; SMOTLOF SV oIB (Joos VIvNbDS) soovds 100, q “queyjd uoreapAyop odAJ-JouUN} ‘9Se4ST}[NU ‘U0}-OOT JO JnO-AVT—T wWAn9LA MOVYL YNdS 4 WHOSLV 4d ®yOWIKXOsddoO Ajuo 8: B\D9S 02 si Oo! § O waopioia Ey =A y004 | posoneya |_| uo oqo) uo}; 9edeu) 39vyOLsS SJ9VYOLS ae Se SND PD Sep a5 —_— _—~— WWIN3SLVW S3IIddNS-ONIDVMOWd Brom] — $159 -go_usod_=7— WyOsLVid Diy pup yona1. euur Ty \ 8 LONGOUNd-G3HSINIS k A sokonuog ©1q0;20g Jouo90g q sokeauog Apdsy SANZ NZN Bs Z SO WAS DYN Y ZBI 40hoauog uo Buyys0g ye Wom ss0youolg IM WZ 30)0A0)3 40pjoos [ Buo0g i Gy Ae Za ra Taw Owns e1SOMm 04019009 yOLVYVdIS SWOOY HSVM ONV | ¥3N9007T J9VM3S MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 6 ‘OOP ‘edBIBMOS !OOG ‘aIedor Av1) puv dogs suryqovul £O0G ‘adqJo ‘ OOT'T ‘Su001 YSVA PUB AJDAIOT ‘NOE ‘A10}B10GR] + OOG ‘W001 190g ! OO, ‘surseyovd {o08'2 ‘surdéap ‘00%‘E ‘uoT}Bredeid ' O09'F ‘93v10}8 sorddns-surseyoed puve yonpoid-poystuy + O09P “93v.104S [VIO] CVUI-MBY :SAMO[[OJ SB 91B (joaJ oIBNDS) sadeds Joo,yA “yueld uoweapAqop od4j-[ouun} ‘MopaojyuNod ‘04-OG JO jno-AvJ—zZ wunowy WOWHL YNdS M4 WY¥OI1V 1d eyow!xouddo Ajuo.s1 e@j09S uo} ;9edsu} oz gi Ol G O 4994 J9VYOLS S39VYOLS SVRERRT, Sal1ddNS-ONIDVHOWd Male a6o4045S| | Aoiaj pud wonsy WYOSLV 1d 8 LONGOYd- d3HSINIS ATSLVWIXO¥ddV eboioys O01 dOHS 2 | | i k INIHOVW Hea nee eee SWOOY H9VM GNV 4¥3xD07 | | | es =! yoor, 40y8U04L Jepjo9S { buijeeg 4914450 yO LWuVd3s 39vmMas WOOYU Y3I1I0"8 94nSS04g-yBIH Burwwis | VEGETABLE AND FRUIT DEHYDRATION 7 operating as community projects or on individual farms, often justify themselves by making possible the saving of crops that have no ready market. Their value in wartime is limited by the fact that the output per unit of operating labor and construction materials is low. APPROXIMATELY 14 BOILER LABOR ROOM ATORY| SEPARATOR 3 Transfer Track _ 3 | RAW-MATERIAL STORAGE Wosher High-Pres&ure Tray Blancher PLATFORM Trimming Table NT FINISHED-PRODUCT & aoa Truck and Troy Storage PACKAGING-SUPPLIES APPROXIMATELY ( \ ==" Path of Cars Through Tunnel <+- _— STORAGE Ne Feet ili O35 10-152. 20 Transfer Track = Drying Bins Inspection Scale is only approximate FIGURE 3.—Lay-out of 25-ton, counterflow, tunnel-type dehydration plant. Floor spaces (Square feet) are as:follows: Raw-material storage, 2,600; finished- product and packaging-supplies storage, 2,900; preparation, 1,800; drying, 4,000; packaging, 500; boiler room, 300; laboratory, 150; locker and wash rooms, 700; office, 450; machine shop and tray repair, 300; sewerage, 300. APPROXIMATELY 165' Rotory Peeling wosher §5'evotor LOCKER Fe Scalder, sees [ee Cutter, wosher High-Pressure WASH RGOOMS MACHINE SHOP Sorting Conveyor 1 Ie Conveyor — - off . Chutes BOILER RAW-MATERIAL ROOM STORAGE FINISHED-PRODUCT & PAGKAGING-SUPPLIES STORAGE PLATFORM OFFICE : Feet Oo 5 10 15 20 pe Ee EE ae Scale is only approximate Figure 4.—Lay-out of 25-ton, conveyor-type dehydration plant. Floor spaces (square feet) are as follows: Raw-material storage, 2,500; finished-product and packaging-supplies storage, 2,500; preparation, 2,100; drying, 2,600; packag- ing, 500; boiler room, 500; laboratory, 150; locker and wash rooms, 700; office, 400; machine shop and tray repair, 200; sewerage, 250. 8 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE BOILER wg ee a a eee __~—— Path of Cars Through Tunnel ——— Transfer Trock 1 > v ” o e = oe Trimming = R ¢ Table Wouee | eae, ! Roised Platform = > | : N7 Incline =) = a Orying. Bin LOCKER = x Secling FINISHED -PRODUCT RAW-MATERIAL ° El AND AN a on Filing PACKAGING-SUPPLIES STORAGE WASHROOMS STORAGE PLATFORM Feet. ° 5 10 15 20 [ees ee ee ee Scale is only approximate FIcurRE 5.—Lay-out of 5-ton, counterflow, tunnel-type dehydration plant. Floor spaces (square feet) are as follows: Raw-material storage, 500; finished-prod- uct and packaging-supplies storage, 500; preparation, 450; drying, 1,400; pack- aging, 150; boiler room, 100; locker and wash rooms, 250. Buildings and Lay-out The building need not be expensive, but certain features are essen- tial. It must have good concrete floors throughout and proper drain- age, so that walls and floors can be washed down and kept clean. Built-in waste flumes in the floor of the preparation room are an aid in this respect. All outside openings should be screened so that flies and other insects cannot enter, and outside screen doors should have automatic closing devices. Rodentproof construction is highly desirable. The plant lay-outs presented here show practical floor plans and will serve as guides to floor-space requirements and arrangements for the different operations. Buildings of rectangular shape are discussed because they are commonly used. If the plant is to be located in existing buildings, the lay-out must be modified to take advantage of the available space in the best manner. In some cases it may not be feasible to locate all parts of the plant within the limits of a rectangular building. Boiler and sewage-sep- aration rooms can be conveniently located in small adjoining buildings, or in an extension of the main building. As a safety and fire-preven- tion measure and to eliminate excessive temperatures within the plant, it is better to locate the boiler in a separate building. Boilers should be designed and installed in accordance with the code of the American Society of Mechanical Engineers. It may be advisable to place the office in a position overlooking the receiving and shipping VEGETABLE AND FRUIT DEHYDRATION 9 platform for convenience in keeping receiving and shipping records and checking the movement of all materials in and out of the plant. Raw-material storage must be located conveniently to the receiving platform. It should be dry, well ventilated, and cool. Refrigera- tion will be necessary in many cases. The size of the storage room is governed mainly by regularity of raw-material delivery. Where the plant is located close to adequate supplies and harvest is continuous, a smaller storage room is necessary than where supplies are hauled long distances or harvest is irregular. Storage requirements vary with the product. Perhaps as much as 10 days’ supply of potatoes, sweetpotatoes, and onions can be kept on hand, since these products do not deteriorate in that time, but in many cases not more than a day’s supply will be necessary. Cabbage, carrots, rutabagas, and beets are usually harvested as they are needed, but since rains delay the harvest of these vegetables, many operators believe they should keep at least 10 days’ supply on hand. Others say it is not advisable to keep more than 2 or 3 days’ supply since these vegetables show signs of wilt after that period. One day’s sup- ply of leafy vegetables is the most that should be kept on hand unless cool storage is available. Bulk storage may be necessary if there is a shortage of sacks or boxes. This will decrease the amount of space needed but will either complicate the handling problem or necessitate special storage facilities. The storage space for finished product and packaging supplies must be dry, cool, and insectproof and rodentproof, and should be adjacent to the packaging room and the shipping platform. The regularity of outgoing shipments is an important factor in determin- ing the size of the finished storage room. ‘The availability of pack- aging supplies is another factor to consider. Small packages of con- sumer goods require considerably more storage space than large packages, such as 5-gallon cans. It is advisable to have additional storage space in adjoining build- ings or on mezzanine floors to take care of unforeseen storage needs, and it may be desirable to provide clear space around the building. This space can be used for movement of trucks and additional tempo- rary storage in emergencies. ‘The storage space should be constructed to withstand heavy loads. Tunnel driers require considerable floor space because of the need for transfer tracks, car tracks, car and tray storage, tray-washing equipment, and the tray conveyors used in loading. The conveyor- type drier requires relatively little floor space in addition to that oc- cupied by the drier itself. Through circulation of air permits heavy loading on the belt, thus reducing its required size and minimizing needed floor space. For tray-type driers, adequate storage for cars and trays must be provided. A covered platform alongside the tunnel on the outside of the building provides an inexpensive storage space. Facilities for washing or cleaning trays must be provided. Ample floor space and proper tools are necessary. The laboratory can be conveniently located near the preparation line and the packaging room, but this 1s not essential. The time spent in getting a sample is only a fraction of the time spent in analyzing it. A location near the machine shop is not desirable because vibra- tion may affect the analytical balances. 10 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE The main locker and wash rooms should be located near the prep- aration line since many workers are employed there. Care should be taken to avoid the movement of employees across the line of product flow. Labor requirements are closely related to plant lay-out and equip- ment. For example, the length of trimming and inspection belts depends upon the number of women needed for these operations. The spacing of equipment and the flow of product are influenced by the number of workers needed for the various operations. Proper plant lay-out saves space and reduces the requirement of labor and equipment. Flow of products through the plant in a manner that eliminates cross traffic decreases the amount of labor required. Ele- vators and conveyors also save labor. In many instances, the use of gravity flow will decrease the required number of employees, and will eliminate elevators and conveyors at some points. The work in the machine shop will consist mainly of repair of trays and adjustment and maintenance of preparation equipment. It should be located so as to require the least movement of the items to be repaired. Some machines are too large to take to the machine shop, and it is necessary to allow for working space alongside this equip- ment or provide for its removal to the outside. In many cases, equip- ment is shipped to the manufacturer for repairs. Storage space should be provided for spare parts and equipment and supplies. On the basis of actual floor space in operating plants and an objec- tive appraisal of the adequacy of these allowances, approximate floor-space requirements for various parts of the plant are given in table 1. TaBLE 1.—Approzimate floor space requirements in square feet for dehydration plants handling carrots, potatoes, and rutabagas ; 5-ton plant 2 25-ton plant 2 50-ton plant 2 | 100-ton plant ? Item = mai ral tee |= Low? | High | Low? | High | Low? | High | Low: | High Raw-material storage 4_____-___------ 400 800 |} 2,000 | 4,000 | 4,000} 8,000 | 8,000} 16,000 Finished-product and packaging- SuppliesistorageyS: =e ee 400 800 | 2,000 | 3,500} 3,000 {| 6,000 | 6,000} 12,000 Preparations) eee n Soe en ee ee 400 600 | 1,500 | 2,500} 2,500} 3,500} 4,000 6, 500 1D erat etsy Vang SRE So SE Oe Ne 1,000 |} 2,000} 3,500 | 5,000} 7,000} 9,000 | 10,000 | 14,000 Packaging 22 otek) eh Sie eae 100 200 400 600 500 800 800 1, 000 BOleriroo mye ae ee een ee ee 100 200 300 500 500 800 800 1, 200 1 BrpN oY Oy 720 60) Ca aes use Ak pace Shh SWE ae SE ta PE 2 A a | ee 100 200 200 400 300 500 Locker and wash rooms_-__-___--___-- 200 400 500 | 1,000 1,000 | 1,500! 1,500 2, 500 OPH CORSE ANT TEE 0 ES RINE oo Re REE Semen A eS peel ace ae 300 500 400 600 500 750 Machine shop and tray repair___---_|--------|-------- 200 400 400 800 500 1, 000 S@Werage sen tates ccs ste mene EN hee 2 Nl ee [oo eee 200 300 400 600 500 1, 000 Total Seite oa a ae 2,600 | 5,000 | 11,000 | 18,500 | 19,900 | 32,000 | 32,900 | 56, 450 1 Because of their drying characteristics, sweetpotatoes require more tray area than allowed here and therefore have not been included. Other space requirements for sweetpotatoes are substantially the same. as those listed. 2 Capacity given in tons per 24 hours, unprepared basis. 8 The low limits of floor space will be undesirable in many instances. 4 The space indicated for raw-material storage will provide from 2 to 3 days’ supply of root vegetables in sacks or boxes. Additional space must be provided if a larger supply of raw material is to be kept on hand. x it is aot feasible to have this storage space in one building, adjoining buildings or covered platforms may e used. 5 Additional storage space, 50 percent or more of that indicated here, should be provided on mezzanine floors or in separate buildings for storage of chemicals, spare equipment, and other items that accumulate. It is assumed here that these dehydration plants are on a war basis and finished goods are shipped as soon as shipping facilities are available. However, for normal operation in peacetime, plants of the same capacity will ordinarily need more space for storage of finished goods. 6 Floor-space allowances for the dehydrator are based upon truck and tray tunnel driers. 7 Floor-space allowances for the boiler room are based upon the use of steam for blanching and incidental uses only. If steam-heated driers are used, this item must be raised. 8’ Inmany instances no space will need to be allocated for sewerage. Space indicated here is for settling and separation of solids from liquid wastes and for trimmings from the preparation line. VEGETABLE AND FRUIT DEHYDRATION 11 Preparation Line Figure 6 presents the lay-out of the preparation line for the 100- ton vegetable plant. Both side elevation and floor plan are shown. The line need not be straight; it can be turned at any one of a number of convenient places as illustrated in figures 1 to 4. Plants processing potatoes have been selected for illustration. Other vegetables, for example carrots and rutabagas, can be handled on the preparation lines illustrated with little or no change. Sweet- potatoes can be handled if lye peeling is used. The use of the lye peeler as a scalder also enables the lines to operate on tomatoes al- most as outlined. Other vegetables may require considerable change in these lines. Beets are commonly cooked before peeling. Cabbage requires the use of, kraut cutters and the addition of coring machines over a suitable conveyor belt; considerable rearrangement and a different type of blancher are also necessary because of the desira- bility of blanching on trays. Only properly designed and carefully built machinery should be used. A poor cutter or slicer may cause damage to the product and increase washing losses. Incomplete peeling necessitates excessive trimming labor, and drastic peeling wastes the product. The cost of a good blancher and its operating costs are small compared to the loss that will be incurred by the use of one poorly designed. Im- properly designed elevators, conveyors, and washers may be too rough in their action, resulting in damage to products. Some vegetables cannot be handled easily on elevators or conveyors. This is espe- cially true of leafy vegetables. The plant should therefore be ar- ranged to give these vegetables a minimum amount of handling. Ruggedness of equipment and long operating life are important. High maintenance charges may soon offset any saving due to low initial investment. Repairs cause grief and expense due to inter- ruption of production and improper handling and processing. When there is a possibility that the stopping of any machine will interrupt the continuous flow of the product through the plant, some means of substitute operation should be available, or else there should be storage facilities for the product so that it will not de- teriorate. In larger plants it may be justifiable to provide two of almost all major items of equipment. Two or even three trimming belts are preferable to one from an operating standpoint and because of the possibility of break-down. It may be desirable to provide two smaller blanchers instead of a single large one. This arrange- ment has particular value when two products are run simultaneously or a product is being prepared in two forms. Oversized equipment may be a wise investment. Various parts of the preparation line are then able to handle increases in throughput which may occur as a result of improvement in quality of raw ma- terial or changes in labor and equipment. On the other hand, much can be done to reduce investment in processing equipment. The number of elevators and conveyors can be reduced by placing some machines on elevated platforms directly over other machines, thus utilizing gravity flow. This also reduces the floor space required. A properly constructed water spray over the front uncovered section of the blancher belt may satisfactorily wash the product in lieu of an expensive separate mechanical washer. In addition, a water spray MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 12 ‘jueld uoeipAyep u01-QOLT 10J JusudInbs uojviedead Jo yno-Aey—9 FANdI,y buipoo7 BENGIASIG puippeads 8,OwWIxosddo Ajuo Si aj09S 0} polrebbo4s ‘ a JONUDW 4O O1yDWOyNY (ie eae ss9youo|g aN ford sl ioaH fe} ie) poet suoiy9esS AnsidS—4040M Ssayyn Xi sano seen SUN [rigs suneos YR tous eeu (TET) JOFOABI soKkenu0 9 4BYSOM As0}OY OYyy 40 4109 B90 UI paynsusgnS aq AOW Joysom sax0US Vv Nvqdd 4O0O14 i 6u07 09 % ,Ob ~J sejqoy, Bulwwisy ony ats OY] wiasola_ ere : Pea Cy ) Jokenuod SS See | Nan 1298 ose v1010- ee emmy yds ENG: 40j0Ae13 CO Sees RW | Japjo9s Buljeed iy He aA \ Jeu SDM TAD IO’ a yong ulosig seusom Asojyoy dost puos Bulpoo 7-A0d) S4QyoUDIG OAL . einceoranuonn (ase TUDO S40JOAC|3 JOYSOM OML SS ee =f Buljeeg yOeH pinbi7 s t K pesojoug \ soyyo 30 Seuisg ‘eq sejqoyobean NN JO $®ZIS SNOI4OA 40 UOILIOdOIg ie Se Buneeoimuctenign 8uy uo Builpuedeg ‘saynyo Aq 40 3821S Wa : 1ONUDW oq AOW s4ojaed Of posy / NOILVA313 3al¢ $49|00g anisnuqy k \ \ \ ol Ee se 4 die | Gwar Seaver VEGETABLE AND FRUIT DEHYDRATION 1g at this point tends to prevent excessive humidity in the preparation room by condensing steam escaping from the front end of the blancher. Elimination of all unnecessary handling of the material reduces the amount of labor and equipment needed and results in a, better finished product. To be effective, the preparation line must be carefully laid out, with sprays at intervals to keep the product moist, and with no pockets that pou accumulation of vegetables which later contaminate the entire ot with discolored, oxidized products. Washers, either spray or rotary types, placed at intervals in the line tend to keep the product wet and clean. This is good sanitary practice. Arrangement of floor drains beneath the preparation lines to carry off the water is necessary. ‘This prevents accumulation of waste water and residues from the line which would soon become sour and create odors in the plant. - The best preparation line is one that continuously gets the product to the drier in the shortest time allowed by proper operating pro- cedures. These procedures vary slightly from plant to plant and considerably with one vegetable or another. Dehydrator Choice of drier—Three types of vegetable dehydrators are shown in figures 1 to 5. Figure 1 shows a plant capable of handling 100 tons of raw product per day, in continuous operation. The dehy- drator is of the multistage tunnel type. Figures 2 and 3 show 50- and 25-ton plants with dehydrators of the counterflow tunnel type; and figure 4, a 25-ton plant with a conveyor-type drier. All are planned to include finishing bins. This presentation is not meant to imply that a multistage unit is better for a 100-ton plant, and a conveyor type for a 25-ton plant. The examples are presented for illustrative purposes only and it is possi- ble that each of these types will prove to be suitable in a wide range of plant capacities. The capacities indicated are only nominal; the true capacity of each is dependent upon the product, the drier design, heat input and air circulation, and the use of finishing bins. In the multistage drier, the material passes first through a parallel- flow tunnel, then through a counterflow tunnel, and finally into fin- ishing bins. If properly designed, this is a very flexible type of unit, permitting the adjustment of drying conditions to the optimum for product quality. The second-stage tunnels, used alone, are suitable for fruit drying. The counterflow tunnels illustrated in the 50- and 25-ton plants are a conventional type. Drying times are not as short as in multi- stage units because the maximum temperature of the air is limited by the highest temperature that the product at the dry end can stand. The use of finishing bins, permitting removal of the product from the tunnels at a higher moisture content, partially offsets this draw- back. The conveyor type of drier shown in the 25-ton plant has shown promise in commercial operation and will doubtless be used increas- ingly as its operating problems are overcome. Figure 5 shows the lay-out for a plant handling 400 pounds of potatoes per hour. If the operation is continuous, the plant will 14. MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE process 5 tons of raw product per 24 hours. The dehydrator is a nine-truck tunnel of small cross section, and it is assumed that one truck will be loaded every 40 minutes. The preparation line will probably be operated only one or two shifts per day; the drying will ee continue only until all the product in the tunnel has been ried. The operation of plants much smaller than those handling 25 tons per day is likely to be intermittent, and batch-type driers or tunnels smaller than the usual commercial type may, therefore, be preferable. The use of tunnel-type driers in a discontinuous operation is feasible only if close control of temperature and humidity is maintained dur- ing the starting-up and shutting-down periods. The choice of drier may be influenced by the amount of labor required. If tray-type driers are used, all practical labor-saving methods and devices should be installed. Conveyor driers require less labor. Tray handling and washing may entail a considerable amount of hand labor, whereas belt cleaning may be almost entirely automatic. Where labor rates are high, the rehandling costs involved in multistage drying may be sufficient to cause a reconsideration of the system to be installed. Automatic movement of the cars in and between the tunnels may overcome this disadvantage. The lower labor cost in operating a conveyor drier may offset the higher initial capital cost. The output per dollar of investment for a conveyor dehydrator is generally less than for a tunnel drier. It may not be possible, however, to determine which type of dehydrator is preferable on the basis of cost alone. It is probable that the choice will be determined mainly by technological factors. It may depend also upon the availability of construction materials. Availability of operating labor and materials must also be considered. The upkeep of the drier is important. The cost of maintaining the trays in proper condition can be balanced against the upkeep of a large and costly belt or conveyor. Ample capacity in the dehydrator is usually a good investment. Since the fuel and power costs are relatively low, an increase or decrease of even a substantial percentage does not seriously affect the total processing cost. Increased labor costs due to inefficient use of labor in the preparation line, when the dehydrator is unable to handle the output of the line, usually amounts to far more than any additional drying cost resulting from the use of a slightly oversized dehydrator. | Finishing bins used in conjunction with the dehydrator increase the capacity of the dehydrator eee by shortening the time of the main drying operation. ‘This shortening of drying time may result in an improvement in product quality. The over-all cost per unit of drying capacity will usually be less when finishing bins are used. Loading and stacking trays—One tray line should ordinarily be adequate for plants handling up to 100 tons per day. Proper timing of tray loading, stacking, drying, and tray scraping is essential for efficient operation. This is especially true for large plants. A mini- mum of about 10 to 12 seconds should be allowed for handling each tray at the loading pee although the actual operation of taking the tray from the loading table and placing it on the truck requires somewhat less time. On this basis, the 100-ton plant is near the upper limit for one tray line. It should be borne in mind that if the rate is VEGETABLE AND FRUIT DEHYDRATION 15 increased so that the handling time is less than 10 to 12 seconds per tray or if the flow of product is not uniform, two tray lines will be necessary. Spreading the product on trays is slightly more difficult than spreading on a flat belt, because the sides of the trays are higher than the material. Leafy vegetables, such as nonblanched shredded cab- bage, are an exception since this material is stacked higher than the sides of the trays. Several suggested means of spreading on trays are sketched in figure 7. The blanchers shown in figure 7, C and DP, are especially well suited for uniform spreading of products. VIBRATOR OR SHAKER TOP VIEW Conveyor ZL SIDE VIEW DIAGONAL FEED FROM BLANCHER €\vibrator Tray Conveyor Blancher A SYNCHRONIZED SPREADING BELT SPREADING DEVICE - This moy be a stationary bar, straight, curved, or ongulor; or it may be a revolving Blanchers drum with brush bristies,fingers, or Product is fed from end “other suitable protuberances of blancher either porallel or at right ongles \ (GY \erotection Shield SEEDER y —_—_—_——_ The movement of the trays is D B synchronized with the movement of the spreading belt DIAGONAL FEED FROM TWO BLANCHERS Tray Conveyor FIGURE 7.—Various methods of automatically spreading product on trays: A, Vi- brator or shaker type of spreader; B, spreader utilizing spreading belt lo- cated above trays; C, a 4-foot blancher will spread evenly across 3-foot trays, at approximately a 45° angle. This lay-out is designed to fit into floor plan shown in figure 2. D, two blanchers, each 4 feet wide, will spread evenly across 6-foot trays at approximately a 45° angle. It is important that tray handling be avoided wherever feasible. One possibility is illustrated in figure 8. After the trays are scraped and dumped, they are placed immediately on the tray conveyor which takes them back to be loaded again. Tray cleaning can be accom- plished on this conveyor by means of high-pressure, hot-water sprays, revolving brushes, etc. A car standing alongside the conveyor can be used to furnish extra trays when necessary. Two conveyors in 16 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE series, the first running at a faster speed, help to maintain a continu- ous line of trays for loading. If this system is used, tray scraping and tray loading must be coordinated for efficient operation. Some operators have found it necessary, when using wooden trays, to soak them before they are washed in order to obtain satisfactory cleaning. ‘Trays so washed must be dried under proper conditions SIDES High-Pressure Tray Cleaner Gor Being Water, Spray Unloaded WASZANN. : ASE Conveyor Tray Conveyor Hopper FIGURE 8.—Lay-out of tray line to avoid excessive handling of trays between unloading and loading. to avoid damage from checking. It is desirable to dry the trays before reloading so that the large amount of water carried in the wet wood will not cause an increase in the drying time and a correspond- ing decrease in quality of the vegetables being processed. Packaging Room The packaging room should be enclosed, thus excluding damp air from the preparation room and dehydrator. Air-desiccating equip- ment is advisable in many cases. If a refrigeration system already is available, desiccation based upon refrigeration can be used. Where no such equipment exists, nonrefrigerative types are generally in- stalled. When a product is dried to an extremely low moisture con- tent, desiccation of air is essential and will more than pay for itself in improving the quality of the packaged material. Where a shaker-sieve is used to remove the fines from the dried product, the economical use of these fines is a problem. If the quan- tity is large, installation of grinding equipment may be advisable. The necessity for grinding equipment also depends largely upon the demand for soup stocks, purees, and seasonings. Onions, celery, and garlic have been quite generally prepared in powder form, and pow- dering equipment will probably continue to find its greatest use for these vegetables. An extremely dry product and dry air are essen- tial in any powdering operation. Plant and Equipment Costs The costs of the building and equipment vary considerably. The construction of new buildings or the use or rental of old buildings, the purchase of new or second-hand machinery, and the wide varia- tion possible in the type and size of each item of equipment are fac- tors that make an over-all estimate unreliable in many cases. As a guide, however, tables 2 and 3 have been prepared. Table 2 pre- sents rough cost estimates for the preparation, final inspection, and packaging equipment indicated in figures 1 to 5. Table 3 gives the approximate costs of constructing and equipping these same plants, including all usual items. Serr VEGETABLE AND FRUIT DEHYDRATION 17 TABLE 2.—Preparation, final inspection, and packaging equipment costs for dehy- dration plants handling carrots, potatoes, rutabagas, and sweetpotatoes 5-ton plant ! 25-ton plant 1 50-ton plant! | 100-ton plant ! Item Se oe ee SSS SS = Low | High | Low | High | Low | High | Low | High ———— Preparation equipment: and itrulckcte Soe ae $15 $25 $50 $100 $100 $150 $100 $200 CONVEY OTS esse eee eee oo eee Seen ee eg OE 500 800 800 | 1,000] 1,000 1, 500 MTG viatOnseie sce eek ee teed ale oe mree a ay aee Ue 500 700 600 800 800 1, 000 Wiashenreties me eave Gato a 300 600 600 800 | 1,000} 1,200] 1,100 1, 400 IReelerss: sss see Ae ae Sa eee 300 ESS OY 0) be ea a | ete nee Vc [as Lae 26 Wop eee iReelineiscaldersi ben Ree een ee 1,000 | 1,500] 1,500 | 2,000} 2,500 3, 000 High pressure washers and J OLD UaDL) OVS\ ses erate ecg Behe ta Si aN IC en (al es 1,000 | 2,000] 1,500 | 2,500} 2,500 3, 000 Trimming belts or tables_________ 75 100 1, 500 3, 000 2, 000 4, 000 4,000 8, 000 Conveyors and elevators_________|_--_----|-------_- 600 | 1,000 800 | 1,200] 1, 500 2, 000 Cutterssis tia ene 500 700 800 | 1,300 800 | 1,300] 1,500 2, 500 Blan Chers sae eceiges te ano 1,000 | 1,200} 2,000} 3,000] 3,000] 5,000} 6,000] 10,000 MO GA ere ARS Rei ea 2, 190 3, 125 8,550 | 14,200 | 12,100 | 19,150 | 21, 000 32, 600 Final inspection and packaging equipment: Hoppers and shaker sieves- -_-__-- 50 100 300 500 400 600 400 600 IGASORA MOA ORS ee Se ea 400 600 600 1,000 | 1,000 1, 400 Hoppers, scales, and packaging EQUipmenties sa ee 100 150 300 600 400 800 500 1, 000 VOUMET CONVEY O Tet eee ese | ae eG NS 200 400 250 500 400 600 Hand trucks and tools__________- 50 100 100 200 150 300 250 400 TINO Genes oi Pe an A 200 350 1, 300 2, 300 1,800 | 3, 200 2, 550 4, 000 Total cost of equipment ________ 2,390 | 3,475 | 9,850 | 16, 500 | 13,900 | 22,350 | 23,550 | 36, 600 Approxim ate installation costs (25 percent of equipment) ---.._______- 600 850 | 2,450] 4,100 | 3,500 | 5,600 | 5,900 9, 150 Total cost installed._.____-____- 2,990 | 4,325 | 12,300 | 20,600 | 17,400 | 27,950 | 29,450 | 45, 750 1 Capacity given in tons per 24 hours, unprepared basis. TABLE 3.—Building and equipment costs exclusive of boiler equipment* for dehy- dration plants handling carrots, potatoes, and rutabagas * 5-ton, 25-ton plant, | 25-ton plant, | 50-ton plant, | 100-ton plant, counterflow conveyor counterflow | counterflow multistage Item of plant tunnel type 3 type 3 tunnel type 3 | tunnel type 3 | tunnel type 3 Low | High | Low | High | Low | High | Low | High | Low | High Preparation, final inspection, and packaging equipment-__| $3, 000] $4, 300/$12, 000/$21, 000}$12, 000/$21, 000/$17, 000)$28, 000/$29, 000|$46, 000 Drying equipment-__-_________ 4,000} 6,000] 30, 000} 35,000} 12, 000) 15, 000} 25, 000) 30, 000} 50, 000} 60, 000 Building space at $1 per Squarevoots2=--32 = eas 2,600} 5,000) 10, 000) 17,000} 11, 000] 18, 000} 20, 000} 32, 000) 33, 000} 57, 000 Seweraces cme mame fa oo 1,000} 2,000} 1,000} 2,000] 2,000] 3,000} 3,000] 4, 000 Office and laboratory equip- OCS aN oes A ee ae eee an ee eg 100 500 500} 1, 000 500} 1, 900 500} 2,000} 1,000} 3,000 Machine shop tools and equipment__._...._-__.-_-_ 100 200 250 500 250 500 500} 1, 000 500} 1, 500 Total cost exclusive of boiler equipment 1___| 9,800] 16, 000| 53, 750} 76,500] 36, 750) 57,500] 65, 000} 96, 000)116, 500}171, 500 Cost per ton of daily capacity (unprepared basis) _.____-_- 2,000} 3,200) 2,200) 3,100) 1,500} 2,300) 1,300} 1,900) 1,200) 1,700 1 No cost allowances are included for boilers because many dehydration plants install second-hand boilers at a fraction of cost of new ones. For example, 1 plant purchased a second-hand 125-horsepower boiler at an installed cost, including accessory equipment, of approximately $8,000. Estimates of costs of new boilers including piping and auxiliaries, but not foundations or buildings, are as follows (from 1 to 2 boiler horsepower are required for each ton of daily capacity, unprepared basis, for blanch- ing and incidental uses only): Heveloped horsepower: Price per horsepower Developed horsepower: Price per horsepower Bem Speer 5 We tee sees Sr NS Lo Se QV Sr est a HO ATES 8 CAA STE UO es aL OO ee eee Ser SNe Ns 200 SOOE ieee eR SE tsp ar ate a 100 0 BANE Be aera ee awe alee Be Ae 170 BOON Se saa eee BARNA Eh Sete SE 70 * Because of their drying characteristics, sweetpotatoes are not included. The drier and the floor-space requirements are different because of the need for more tray area. * Capacity given in tons per 24 hours, unprepared basis. 569074—44 2 18 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE The over-all costs, as given, without boiler equipment, vary from $1,200 to $3,200 per ton of daily capacity, unprepared basis, depend- ing upon the size and type of plant and the low-high limits upon which the estimates are based. The low cost of $1,200 for the 100-ton plant and the low limits for the other sizes cannot be considered practical since they are a summation of the low estimates for the items of equipment. There is only a remote likelihood that any plant will or should be constructed at a minimum of cost for all items. Unless constructed under unusual circumstances, such a plant would probably have op- TABLE 4.—Capacities per unit of time in a vegetable-dehydration plant capable of handling 50 tons per day, unprepared basis Sweet- Table | Cab- - Pota- Ruta- Item beets bage Carrots Onions bagas pota- : s | (slices) , (shreds) ey (slices) (strips), (slices) (slices) Unprepared basis: ; Weight per hour (operating 21 hours | DenGay) eee en Bis Eee pounds__| 4,750 4, 750 4, 759 | 4, 750 4, 750 4, 750 4,750 - Weight per minute____________- doz =| 80 80 80 | £0 80 80 80 Women coring (estimated)__number__|________! Oxie 222s 2= eae es pe ee eee eae ee Weight per woman per minute ; POUNGS==|—ee ees 13 | OG ey aes | eset Ce eis ee | Prepared basis: } Assumed culling, peeling, trimming | lOSS =a et ee re ent eae percent __ 25 25 20 10 25 15 25 Weight per hour (operating 21 hours DEMO AY) pete eee oe pounds__| 3,570 3, 570 3, 810 4,290 | 3,570 4, 050 3, 570 Weight per minute ____________ dor2ne 60 60 64 72 68 Women trimming (estimated) number__) 2373 see 20 30 40 20 30 Weight per woman per minute pounds__ 246| Sees BE 2.4 1.5 3.4 Ze Assumed blancher loading per square : [TOOL Meee brie ee ee pounds__| (2) 2 a (3) 4 a ay Assumed blanching time___-_minutes__} (2) 3 6 (3) 6 6 6 Weight in blancher at any one time‘ | pounds__ (2) 180 380 (3) 360 405 360 Active blancher surface___square feet__ (?) 90 95 (3) 90 100 90 Assumed tray loading per square foot pounds__ 1.4 1.0 15 125 1525 1.4 Hee) Weight per 3 by 6 tray_________- do== 25 18 } 7 2255 2205 25 22.5 Weight per 22-tray car__________ doze 550 395 595 495 495 550 495 Cars Pennoni nese eee number__ 6.5 9 6. 4 8.7 fae 7.4 i. Time percar =.= eae a minutes__ 9.2 6.7 9.5 vs 8 8 8 hraysi per nouns. = eee number _-_ 145 200 140 190 160 160 160 aLTAYS DCEMU Le seas eee doze) 2.4 3.3 2.4 3. 2 2.7 2.7 2.7 Pime Per way= 2 ses eee seconds__| 25 18 25 19 22 22 22 Approximate active length of blancher | 6 feet wide, if product is blanched on | SPD: Gib Ay S oe eee cee a feetes esau 30 ANA |e een en 50 50 50 Dried basis: Assumed over-all shrinkage____ratio__| 13 tol | 20to1} 1lltol |511lto1/| 7to1l | 10.5tol 5 tol Weight per day222 2 aie pounds__| 7,700 | 5,000 | 9,100 9,000 | 14, 300 9,500 | 20,000 Weight per hour (operating 21 hours Peniday) see ee ee pounds__ 365 240 430 430 680 450 950 Weight per minute________-___- dose: 6.1 | 4 769 1e2 ll “5 16 Weight per 5-gallon package (esti- MALE) ee a ee pounds__ 8 7 17 12 10 12 12 Packages per day__________-- number__ 960 715 535 760 1, 430 790 1, 670 Packages per hour (operating 21 hours | Der dav) st ee number__ 46 | 34 25 36 68 | 38 80 Time between packages____minutes_-- 163%") 1.8 2.4 Le, 0.9 1.6 0.8 Be ee ee ee eS Eee eee eee 1 Recent experiments indicate that under best blanching conditions the blancher can be loaded more heavily than indicated. Some uncertainty also exists in regard to the blanching time required to secure satisfactory results. At high blancher loading, the retention time must be longer; conversely, at light load- ing (e. g. on trays) time may be shorter. 2 Precooked whole. 3 Not blanched. ‘ 2 4 The blancher size and the number of trays and cars handled are besed on the total weight of trimmed material. The actual weight handled will decrease during washirg, cutting, and blanching, because of leaching and loss of fines. On the basis of the loadings irdicated, the size of the blancher and number of trays handled will, therefore, be somewhat less than shown. 5 The over-all shrinkage ratio of onions may vary from & to 1 to 14 to 1, dependirg on the variety and con- dition of the raw material. The ratio of 11 to 1 has been eccpted for estimating purposes only. VEGETABLE AND FRUIT DEHYDRATION 19 erating difficulties due to lack of equipment and limited floor space. Dehydration plants should be balanced units, and the costs of various parts will be low or high in accordance with the circumstances affect- ing each particular machine, operation, or floor-space requirement. These costs must be considered as only very rough estimates since they cannot possibly include all items. Even a plant that has been completely engineered before construction may present the owner with additional cost items before it is finished. Conditions vary throughout the country, and these variations materially affect any attempt to arrive at generalizations regarding costs. Handling Capacities and Utility Requirements The capacities per unit of time at various points along the proc- essing: line of a 50-ton plant are shown in table 4. Data are given for seven vegetables important in the present program. Such tables are of assistance in estimating labor requirements and equipment sizes for each operation. Facilities must be available to provide approximately the quantities of heat, power, and water indicated in table 5. The figures in this table allow for the differences in consumption of utilities under various operating conditions. The indicated demand load for electric power is really total connected load. The average operating load will usually be smaller. TABLE 5.—Approximate utility requirements for dehydration plants of various sizes Requirements per hour for plant of— Utility and application 100-ton 50-ton 25-ton Water: Potatoes and sweetpotatoes_______________________= gallons__|10, 000-20, 000 | 5, 000-10, 000 | 2, 500-5, 000 Carrots, beets, rutabagas, and onions________________- do____| 8,000-16,000 | 4,000-8,000 | 2, 000-4, 000 Can Dates mem ster hes eee tes Ob re a do_-__| 2, 400-4, 000 1, 200-2, 000 1, 000 Electricity: : = pemeud NO ER Cl eae en Pet eNO alg ne ay ok SS Oe kilowatts__ 150-250 80-125 50-70 uel: Dehydrator: DiTecteheater atan see ee millions B. t. u.1__ 15-20 74-10 346-5 MnTdiTech Neat meee es eT SU Sk ie Ee dose: 30-50 15-25 8-13 SLeAME RCA Ge i oa wet en Ps doe 20-30 10-15 5-8 pBlancher4cand:incidental_ 5.2 22-2 22 22 e dota 4-8 24 1-2 Boiler capacity: Blanching and incidental_____________- boiler horsepower 1!_- 100-200 50-100 25-50 DCH y OTA LOr ae eee eee Ne) eS doves 500-700 250-350 125-175 1 The lower limits of heat requirement and boiler capacity for the dehydrator are considerably larger than needed for some vegetables under good operating conditions. On riced potatoes, for example, the mini- mum heat requirement may be less than two-thirds of that indicated. 2 Low limit is based on continuous-type blancher. If batch-type blancher is used, the ‘blanching steam demand will be higher. The quantity of heat required varies widely with different methods of air heating and from one vegetable to another. The necessary size of steam plant, even on the same operating procedure, varies with boiler efficiency and care exercised in avoiding steam waste. The lower limits of heat and steam requirement for the dehydrator are consid- erably higher than the actual operating usage for some vegetables under good operating conditions. They allow a margin of safety - adequate to permit full-capacity operation at all times, even under bad. atmospheric conditions. Many operators will not find it necessary to have such a large amount of surplus heating capacity. 20 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE STORING AND HANDLING FRESH FRUITS AND VEGETABLES Proper handling of fresh produce is an important step in the prepa- ration of a high-quality dehydrated food. Serious losses in edible quality and nutritive value may take place as a result of failure to recognize the perishable nature of vegetables and fruits. Rots may attack even the least perishable commodities and make them unfit for use. Spinach, sweet corn, and peas, for example, are so perishable that they should be processed within a few hours after harvest. If unavoidable delays are experienced, such products require refrigera- tion to slow down deterioration. Others, such as potatoes, can be held for some time without refrigeration but they should be protected from the sun and from heating. Receiving the Product The minimum requirement at the dehydration plant is a receiving shed where the fresh produce can be stacked out of the sun, wind, and rain as it is delivered from the field. In cold climates a tight, insulated building is needed. When produce is received during warm weather and is to be held overnight or all day before processing, it should not be stacked tightly ; instead, an air space should be left on two sides of the containers to permit ventilation and prevent heating. All produce generates heat; leafy products generate enough to cause deterioration in a short time if stacked in unventilated piles. Produce is delivered to the dehydrating plant in many types of con- tainers, the selection being dictated by experience. Ventilated crates or hampers, or small lug boxes are often used for such perishable commodities as peas, corn, and spinach, and crushed ice is sometimes packed in the middle of the container to prevent heating during long- distance shipment. Shallow field boxes are widely used for stone fruits, apples, pears, and some vegetables, and bushel boxes, baskets of various sizes, and barrels are used in other districts for fruits and vegetables. Such vegetables as potatoes, onions, and other root crops are commonly sacked. It is often more convenient and better practice to handle the com- modity in the field container rather than to empty it into bins. Emp- tying it into bins entails extra handling, increasing chances of bruising, and necessitates provision for ventilation if the produce is to be held for some time. Some products, for example cabbage, car- rots, or other root crops, may be delivered in bulk, and receiving bins are then used to good advantage. Storage Since a long season of operation is advantageous, the problem of laying up a supply of raw material is important. Storage facilities will also aid in smoothing out production peaks and preventing shut- downs. If commercial storage plants or suitable farm storages are conveniently located, they can be used. It may be necessary, however, for the operator to provide his own storage, and how elaborate this will have to be will depend upon the storage requirements of the com- modity, on the temperature and humidity it needs, the normal storage life, and whether or not outside temperatures can be used to approxi- VEGETABLE AND FRUIT DEHYDRATION Dal mate these requirements. For the storage of root crops, such as potatoes and carrots, in the cool fall months nothing more than a covered shed may be needed. The special requirements of each crop are discussed in a later section of this publicaticn and these, together with commercial experience in the district, should be consulted to determine whether or not unrefrigerated storage will suffice. Plans for farm storage plants that use outside air to maintain temperatures can be obtained from the United States Department of Agriculture, Washington, D. C. If refrigeration is needed, the kind of plant that is installed will be determined by the availability of equipment and the investment warranted. Storages refrigerated by ice bunkers and fans instead of mechanical refrigeration have been constructed in recent years for crops that have a short season, such as stone fruits and grapes. The initial investment is lower than for mechanically refrigerated plants and desirable humidities of 85 to 90 percent are easy to maintain. Combination ice-bunker and mechanically refrigerated plants have been constructed that utilize the high refrigeration capacity of ice bunkers to cool the commodity and a small compressor to maintain temperatures during storage. The more common type is the storage plant refrigerated entirely by mechanical refrigeration, and it may be one of several designs. Whatever the type of plant installed, it should have refrigeration capacity sufficient to cool the produce to within a few degrees of the recommended storage temperature in 18 to 24 hours, and it should maintain temperatures throughout the rooms within a degree or two of the specified temperature. The correct humidity should also be maintained within 2 or 3 percent. These specifications if insisted upon will insure ample refrigeration capacity, good air volume and distribution, and suitable air-temperature control. The lay-out of the plant for convenience in loading and unloading will likewise be an important matter to discuss with the engineer designing the plant. Transportation If the dehydration plant is located close to the area of production, as it should be, there will be few transportation problems. Generally the methods found adequate for the trucking or carlot shipment of produce for the fresh market should suffice for the dehydrator. When perishable crops, such as spinach, asparagus, peas, and sweet corn, are to be in transit for 10 or 12 hours or longer, icing the container or the load with crushed ice will help to preserve quality and nutri- tive value. This may be advantageous for even shorter hauls. String beans, which are often harmed by wetting, may be benefited by precooling in cold air before shipment. For products that are shipped some distance, refrigerated trucks or refrigerator cars will be needed. The refrigerator car lines serving the district can be consulted as to the refrigeration services in general use for specific commodities. A new type of car, equipped with fans to circulate air in transit, is available in limited quantities in some districts. These provide better refrigeration in transit than standard cars, and will be found especially adaptable to perishable commodities that are shipped without top icing the load. 22 ~=MISC. PUBLICATION 540, U. 8. DEPT. OF AGRICULTURE PREPARATION OF RAW MATERIALS Washing Root vegetables.—Root crops received from the field are sometimes laden with mud and debris, and must be thoroughly cleaned before they are peeled, since dirt or sand on the vegetable will interfere with any kind of peeling operation. The difficulty of cleaning depends on such factors as type, variety, age, and condition of the product, as well as type and condition of the soil. In localities where produce has previously been prepared for the fresh market, adequate cleaning procedures have probably already been established. In such cases, it 1s suggested that a study be made of the adaptability of these methods to a new processing line. Root crops are usually cleaned in two or more steps. A dry shaker screen will remove much loose dirt and trash. Loose dirt is then washed away in a water spray or bath. The equipment required for this preliminary wash is generally incorporated into a unit which performs a dual function. For spray washing the sprays are located on the elevator, which also serves as a conveyor to the main washer; for washing in a bath the product is allowed to soak in the boot of the same elevator. The latter method is preferred, as it tends to even out the surges caused by intermittent loading (fig. 9). Figure 9.—Hlevators: A, Equipped with water sprays; B, equipped with boot washer. The final washing operation can be carried out in any of several commonly used types of washers, such as the rotary drum, the brush, and the shaker washer. Most of these units are equipped with water sprays, the intensity of which may range from normal city water pressure to several hundred pounds per square inch. High-pressure water sprays ordinarily require the use of booster pumps. Because of its simplicity, the rotary-drum washer is the one most widely used. The severity of its action can be varied by changing the roughness of the drum surface. If longitudinal slots are used, open spaces between the slots will facilitate the ejection of sticks, rubble, and peewee-sized products. The retention time will determine the degree of washing for any given washer. Control of this factor in a con- tinuous washer can be accomplished by varying the degree of tilt of the drum, varying the drum speed, or more positively by providing an internal helical guide fence to regulate the advance of the product. | VEGETABLE AND FRUIT DEHYDRATION 23 Batch-type drum washers are generally suitable only for use in the very smallest dehydration plants, or in connection with institutional or community dehydrators. The brush washer is quite commonly used, but it is limited because of mechanical complications due to brush failure. The product in the washer is caused to pass over or along between rotary brushes. These brushes are usually made from water-resistant, tough fiber bristles or, in some cases, of rubber fingers. The adjacent brushes usually run in alternate directions or at different speeds in such a way that the product is rotated and brushed simultaneously as it pro- gresses. The retention time is governed by the flight path or by the rate of charging, which has the effect of crowding the washed product out of the washer. The units are best suited to removing sandy loam but may be used to remove heavier soils. Shuffle or shaker washers, although very efficient, are mechanically complicated. Their vigorous reciprocating action produces violent scrubbing and tumbling of the product, and hence even the most stub- born mud can be removed. When equipment of this type is used the supporting structure must be firm in order to prevent objectionable vibration. In any type of mechanical washer the rubbing and abrasive action can be powerfully reinforced and supplemented by proper distribu- tion and control of the supply of wash water. The water may have two distinct functions. When it is supplied in the form of vigorous sprays it has a very effective scouring action which will cut away tight- ly adhering patches of clay and dislodge dirt from the bottom of wrinkles and eyes that are out of reach of surface rubbing or brush- ing. In addition, the flow of water down over the material flushes away the loosened dirt and removes it from the washer. Both of these actions are desired. ‘The first calls for sufficient water pressure and careful placing of the sprays. The second calls for a sufficient volume of water. In a rotary-drum washer the stream of vegetables is carried by the rotation of the drum well up on the ascending side of the drum, whence they are constantly rolling downhill toward the bottom. The water sprays should be carefully directed to cover thor- oughly this sector on the ascending side of the drum. Drain lines beneath vegetable washers should be provided with easily accessible dirt traps, since much of the dirt and sand carried by the wash water will be so heavy as to settle immediately in an or- dinary sewer line and plug it. The amount of washing required will vary throughout the season, and therefore ample facilities should be provided to care for the prod- uct under the most adverse conditions. The installation should also be sufficiently flexible to permit rearrangement of the equipment whenever necessary. Loose leafy vegetables—Cut vegetables, such as spinach, chard, parsley, etc., are usually washed in drum washers fitted with water sprays, orin tanks. The former means offers a continuous process and is, therefore, most commonly used. Shuffle and brush washers are obviously unsuitable. Retention time in a drum washer will vary with the condition of the product, and the character and quantity of soil to be removed. Spray nozzles that provide a good scouring action without tearing, ripping, or otherwise injuring the product should be chosen. 24 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Cabbage—Head cabbage requires special handling during washing to prevent breaking up of the head. Usually, to prevent double han- dling, the loosened and damaged leaves are removed and the product is cored and quartered before washing. In this condition the leaves are loose and the head may be broken if it is roughly handled. Since most of the dirt and other foreign material is removed along with the loose and damaged leaves, washing becomes relatively simple. In most cases the cored and quartered product is placed on a wire-mesh belt conveyer and sprayed from above and below so as to dislodge en- trained soil. Consideration should be given to the use of a water flume for conveying and cleansing the cabbage simultaneously; it is probable that eddy currents caused by the water in motion would bring about effective washing. Grading Following the washing operation it may be desirable to pass the product over an inspection belt for the removal of cull and foreign material, especially if the lot is of poor quality. This will be useful in saving time in the subsequent trimming and inspection operations. Size grading of raw vegetables is a desirable operation to reduce peeling losses, especially when abrasive peeling is practiced. This use is discussed more thoroughly under the subject of abrasive peelers. If root vegetables are to be blanched or cooked whole, grading to size is almost essential to assure uniformity of cooking. Size grading may be accomplished either manually or mechanically. Manual grading can be confined to removal of culls and other un- desirable products, or special grading tables can be used which permit each inspector to grade the product into several sizes. By a system of belts and dividing fences, inspectors segregate the product into lots according to size. The equipment is in general similar to that usually employed for grading potatoes. As would be expected, the process requires semiskilled inspectors or operators. Mechanical graders are available in several types. The most com- mon are the rubber-spool grader, the rotary-drum grader, and the shuffle grader. All operate by rolling the vegetable over a support fitted with openings that are small at the entrance end and larger further along, so that pieces of different sizes drop through at different places. In general, selection of a grader should be governed by its ruggedness and reliability. The unit likely to require the least main- tenance should be selected, all other conditions being equal. To summarize, it can be said that grading has application if field- run products are handled, if selective grading for marketing purposes is desired, or if abrasive peeling or cooking of the whole vegetable is practiced. Root Peeling A satisfactory peeling method is of vital importance to all dehydra- tion plants handling root crops. Unsatisfactory or inefficient peeling may spell the financial doom of an otherwise successful plant. The peeling method should be selected only after careful weighing of labor cost against cost of raw product. Other considerations that affect the selection are available means of waste disposal, uniformity of the product size and quality, and ability of the product to resist dis- coloration or damage during or after peeling. The most commonly VEGETABLE AND FRUIT DEHYDRATION 25 used. peelers for root products are the abrasive, brine, flame, lye, and retort peelers. Brief discussions of the units are presented below. Abrasive peelers Abrasive peelers are suitable for processing all root vegetables and are generally divided into two groups, the batch or bucket type and the continuous. The first consists essentially of a cylindrical abrasive-lined drum having a revolving abrasive bottom or floor plate, and is provided with water sprays. A side door facili- tates removal of peeled product. When in operation, the product is dumped into the drum and the revolving floor plate jostles the vege- tables, causing them to be thrown against the sides and bottom until all surfaces are rubbed smooth and the skin removed. The flow of water flushes the finely divided skin through a waste opening in the bottom. Naturally the operation must be continued until all pro- tuberances are ground away if the valleys, eyes, and imperfections are to be removed (fig. 10). ees | FicurE 10.—Batch-type abrasive peeler. The continuous abrasive peeler works on a slightly different prin- ciple (fig. 11). The product passes in a circuitous course among and over the rotating abrasive cylinders. Water sprays flush waste ma- terial away. The rate of feed determines the retention period in a peeler of this type. Skill must be exercised in determining the proper degree of abrasive peeling. If the product is retained for a relatively short time, only partial peeling will be accomplished. Excessive retention will result mM excessive waste. Since the weight of the product governs the pres- sure at the region of contact, the rate of peeling will vary according to the size and shape of the product. Flattened pieces which refuse. to roll may be entirely ground away. If hand peeling is depended upon to finish the semipeeled product, a balance must be struck between the cost of hand peeling and the cost of raw material. As machine peeling is increased, the waste of flesh underneath the skin increases, but the amount of hand peeling required becomes less. As previously pointed out, products of different sizes peel at different rates, and if peeling is to be accomplished at maximum efficiency, grading is necessary. Plants operating on potatoes have experienced as low a recovery as 60 percent (40 percent peeling and trimming loss). At best the peeling and trimming loss with abrasive peelers and hand finishing will be in excess of 20 percent. The abrasive peeler is being replaced in a large number of plants by more efficient peeling methods. In general the field in which abrasive peelers are best suited comprises relatively small and inter- 26 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE mittent operations, which include not only community and institu- tional dehydration, but also restaurants and Army kitchens. Brine peeling—Brine peeling, recently developed, may be found suitable for peeling potatoes, rutabagas, sweetpotatoes, and beets, but not carrots or parsnips. Essentially, the process consists of holding the washed vegetable immersed in boiling saturated salt-brine for a period ranging from 6 to 15 minutes, depending on the kind, variety, maturity, and condition of the product. After the skin has been ade- quately loosened, the product is removed from the boiling solution and placed in a rotary-drum slot washer having rough inner surfaces Sh Fo NS) ey arg a J FEED HOPPER & ? 5 Cs . SF Se tO%6 o gc ots | mo SO e Oe : YOY ies CO: {S ) A ' B FieurE 14.—Methods of coring cabbage by hand: A, cutting to minimize hand labor. Only three knife cuts are necessary. B, six knife cuts are necessary. to reject such heads will lead to increased inspection costs for the dried product and possible rejection of the product on the basis of defects above the tolerance permitted. The system also has a dis- advantage in that if loose heads are being processed, the complete cabbage may disintegrate during the coring operation, thus causing considerable waste. In some foreign countries, the core is sliced finely and dried along with the cabbage. ‘This process is not permissible un- der United States Government specifications. After coring has been accomplished, the product is washed as described in the section on washing. Many types of trimming tables are in commercial use. Probably Fixed Knife edge FIGURE 15.—Stationary knife for coring cabbage. the most common for root vegetables is the straight-flow type in which a belt carries the untrimmed product along each side of the table, and the trimmed material is placed on a center belt which may be at the same level as the side belt or elevated to allow room for a return belt (figure 16). The “merry-go-round” type of trimming table has proved very popular. The product is brought around again to the trimmers if it is missed the first time. VEGETABLE AND FRUIT DEHYDRATION 33 Some operators prefer a type of trimming belt which permits a defi- nite check on the work of each trimmer. ‘This may be accomplished by having the feed belt move forward intermittently, each trimmer handling all the material on a definite section of the belt. In another type the operator opens a gate from a central feed line and puts the trimmed vegetables through a counter chute. The trimmings in all FicureE 16.—Trimming carrots in preparation for dehydration. these cases may either be carried away on a belt or put into a recep- tacle provided for each trimmer so that individual trimming losses can be determined. In planning the trimming belt it is preferable to allow about 3 feet of space for each worker. It is possible to operate with only 30 inches with some crowding, but it is not desirable to plan on this basis. The larger allowance makes possible the addition of extra workers if needed. Preparation labor varies considerably for different vegetables. 569074—44——3 34 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Five to ten trimmers may be required in a 50-ton cabbage plant (un- prepared basis) ; from 15 to 25 for the same tonnage of carrots, ruta- bagas, or beets; from 25 to 35 for onions or sweetpotatoes; and from 30 to 50 for potatoes at the same tonnage input on an unprepared basis. Cutting Following the trimming operations the product passes to the cutting equipment where it is cut into the desired form and size. For the root crops the form may be strips (julienne), slices, or cubes, depend- ing on the market demand. The size of the cuts is governed by the market requirements, the effect of size on drying rate (the larger the size the longer the time required for drying), and the characteristics desired in the finished product. The different cuts are as follows: Slices one-eighth to one-fourth inch thick. Strips three-sixteenths to three-eighths inch wide or thick and not less than three-fourths inch in length. Cubes three-sixteenths to three-eighths inch on a side or half cubes. From the standpoint of the conservation of shipping space the cubes are preferred, since they make a more solid pack than the slices or strips; that is, more weight can be packed into a given volume. Several types of machines are manufactured for the cutting of root crops into the desired forms and sizes. A very flexible type is one that can be used for the cutting of all three forms into different sizes by simply changing the cutting parts. Leafy vegetables, with the exception of cabbage, usually do not re- quire division into smaller pieces after trimming. Cabbage requires cutting or shredding into pieces ranging from one-eighth to one-fourth inch in width. The larger width is preferred, since there is likely to be less loss of ascorbic acid from the product. The machinery used for the cutting or shredding of cabbage for dehydration is the same as that used by sauer-kraut manufacturers and canners. For the cutting of root crops, machinery capable of handling from a few bushels to 12,000 pounds per hour, depending on the form and size of the piece, can be obtained. For cabbage, cutters handling from 10 to 50 tons per days are available. The knives on the cutting machinery should be kept sharp because the use of dull knives will result in pieces of irregular shape and lack- ing in well-defined cut surfaces. Furthermore, considerable bruising of the tissues will occur, which has the effect of accelerating the meta- bolic processes, leading to rapid deterioration of vitamin quality as well as some other quality factors. As a safeguard against serious damage to the knives, precautions should be taken to remove small rocks, bolts, nuts, and nails. This may be accomplished by passing the material over a shaker screen with strong sprays of water. A magnet can be used just ahead of the cutter to take out tramp iron missed in the screening operation. Much damage and lost time can be avoided by such simple precautions. Washing of the cut product is desirable for sanitary reasons, for the removal of foreign material and fines, and, in the case of starchy vegetables such as potatoes, for the removal of loose starch from the cut surfaces. The presence of the water film on the cut surfaces also tends to protect the product from discoloration by oxidation in travel- VEGETABLE AND FRUIT DEHYDRATION 35 ing from the cutter to the blancher. This operation is usually ac- complished by means of cold-water sprays over the conveyor leading from the cutter. None of the cutting equipment. mentioned above is suitable for pro- ducing a riced product. Ricing equipment is usually one of three types: namely, rotary, screw, or plunger. The essential action of each consists of mashing the product and extruding it through perforations of suitable size, not exceeding three-sixteenths inch in diameter. For ricing, the product is first thoroughly cooked, and best results are usually obtained if the operation is carried out while the product is still hot. If the product becomes cool, it may become pasty or gummy, which will make it difficult to obtain a uniform spread on the drying trays. 6 In the preparation of material for ricing, the precooking can be carried out on the sliced, whole, halved, or quartered product. The procedure using slices is probably best from the standpoint of prac- tical operation and labor saving, since slicing can be accomplished mechanically while halving or quartering is accomplished by hand labor. Furthermore, slices can be cooked throughout, without over- cooking the surface. Cooking the whole, halved, or quartered ma- terial has the disadvantage of longer cooking time and additional labor of size grading. It has an advantage over the use of slices in that better retention of water-soluble material is obtained, since less surface area is exposed to leaching action and oxidation. When potatoes were sliced three-sixteenths of an inch and blanched in steam for 4 minutes, the loss of ascorbic acid was 68.4 percent; when they were quartered and blanched in steam for 20 minutes the loss of as- corbic acid was 23.5 percent. The cut product intended for ricing can be precooked by means of flowing steam, boiling water, or steam under pressure in a retort. Cooking in flowing steam or by steam under pressure is preferred over the water cooking method since water cooking may lead to excessive loss of water-soluble nutritive materials from the product. | Cooking in flowing steam can be accomplished by one of two meth- ods. The first method consists of spreading the material on trays and placing them in a cabinet or a retort into which live steam is allowed to escape for the time required to thoroughly cook the mate- rial. The second method consists of passing the prepared material on a belt through a continuous steam blancher, as described for slices or cubes. In this case, however, the retention time may be somewhat longer than required in ordinary blanching in order to effect thorough cooking. For the cooking of whole, halved, or quartered pieces the cabinet or retort method is the most satisfactory, since the retention time is so great as to make it impossible to handle a large volume of material by the continuous method. Cooking with steam under pressure has the advantage of shortening the retention time for thorough cooking, and is especially useful for cooking whole, halved, or quartered material. Regardless of the method, the cooking time will vary with a num- ber of factors, such as type of vegetable, variety, maturity, and qual- ity, size of pieces, rate of loading, temperature of the cooker, and. uniformity of heat distribution in the cooker. Because of these fac- tors each operator must determine the cooking time by trial, the criterion of adequate cooking being the production of a product 36 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE readily passing through the ricer free of lumpy material. For the cooking of quartered material in flowing steam the retention time may range from 20 to 40 minutes. BLANCHING The terms “blanching” and “scalding,” as used in the canning, freez- ing, and dehydration industries, refer to the practice of heating the prepared raw food product, in live steam or boiling water, for a short period prior to the principal preservation treatment. In dehydration, blanching is practiced for two primary reasons: (1) To prevent or check the development of undesirable colors, flavors, odors, and the loss of vitamins during dehydration and storage, and (2) to obtain a finished product that will rehydrate and cook rapidly and yield a cooked product of desirable texture. In addition, blanching destroys many of the microorganisms in the raw products. Fruits are not blanched, although blanching has been suggested for certain cut fruits. Recommended blanching treatment varies with the vegetable; specific recommendations are presented in a later division of this publication. The importance of blanching as a means of preserving quality is — exemplified by potatoes; if not blanched, they will come from the dehydrator in a discolored condition and after a few days of storage will develop rancid odor and flavor. Similarly unblanched dehy- drated carrots lose their characteristic color during storage and soon develop a stale odor and flavor. With green snap beans blanching influences the time required for reconstitution and cooking and also the quality of the cooked product. In one experiment conducted at the Western Regional Research Laboratory it was observed that un- blanched dehydrated snap beans absorbed about 1.1 grams of water per gram of dry product on soaking in water for 4 hours, as compared with 1.9 grams for material that had been blanched in steam for 2 minutes, and 2.3 grams for material blanched in steam 10 minutes. On rehydration, the difference between certain blanched and unblanched materials is apparent. The unblanched material tends to remain shriveled and tough in texture, whereas blanched material becomes plump and tender. Adequacy of Blanching as Measured by Tests for Enzyme Activity It is generally believed that at least a part of the deterioration in quality that occurs in unblanched dehydrated vegetables is the result of enzyme action. Enzymes are substances present in all living cells, and their function is to accelerate chemical reactions in the cell. In the absence of these accelerators, life would not be possible, since the chemical reactions necessary to life would proceed too slowly or pos- sibly not take place at all. It may be assumed, then, that if the enzymes are inactivated or destroyed, some of the chemical reactions leading to the development of undesirable qualities in dehydrated vegetables will be retarded or prevented. Since enzymes can be destroyed by heat, it has been assumed that the tendency of blanched dehydrated vegetables toward longer storage life than unblanched is due, at least in part, to the inactivation or destruction of the enzymes as the result of blanching. On the basis of this assumption, tests for the presence or absence of certain enzymes have been adopted as criteria of the adequacy of VEGETABLE AND FRUIT DEHYDRATION 37 blanching in dehydrated vegetables. Unfortunately, existing infor- mation does not enable one to tell which enzymes may be in- volved in the quality changes that occur in unblanched dehydrated vegetables during storage. Accordingly, one of the most heat-stable enzyme systems (peroxidase) with some exceptions has been chosen as the test enzyme, on the assumption that if this system is destroyed it is highly probable that the enzymes that may actually be involved will likewise be destroyed by the blanching treatment. However, it is known that the long blanch required to destroy the peroxidase system in some products, as indicated by the interpretation of the tests em- ployed at present, is in excess of that actually required for the preservation of quality in the dried products. This is an unfortunate situation, since it may lead to the excessive loss of nutritive values from the products as the result of overblanch- ing. Nevertheless, since adequate information concerning the relation between peroxidase activity and the keeping quality of dehydrated vegetables is not available, it is necessary to adhere to the present test as an indication of adequate blanching in order to be on the safe side. There are cases in which judgment must be exercised, since in some products, even under ideal blanching conditions, it seems impos- sible to inactivate the peroxidase system, as now interpreted, within a blanching time compatible with practical operating conditions. Rutabagas are an example, for it has been demonstrated that inactiva- tion of the peroxidase system, as indicated by the benzidine test, re- quires over 30 minutes of blanching. (See section on “Determination of Adequacy of Blanching,” p. 147.) Methods of Blanching There are three common methods of blanching: Steam blanching, water blanching, and series blanching. Steam blanching consists in subjecting the prepared product in suitable equipment to the tempera- ture of live steam (212° F. at sea level) or to higher tempera- tures obtained with steam under pressure. Water blanching in its essential features consists of dipping or passing the prepared prod- uct through boiling water. Series blanching is a modification of water blanching in that the soluble solids leached from the products during blanching in water are allowed to accumulate to a certain concentra- tion in the blanching water. This concentration (4 percent has been recommended) is then maintained by the gradual introduction of fresh water and removal of spent water. Each method offers advantages and disadvantages. With the steam blanch it may be more difficult to obtain a uniformly blanched prod- uct than with the water blanch. It is generally conceded that water blanching results in a greater loss of water-soluble materials, such as sugar, minerals, and certain vitamins, than does steam blanching. This appears to be true from comparisons made on the same blanching- time basis (table 6). However, the time required to blanch a given quantity of material adequately in water may prove to be less, under certain operating conditions, than that required to obtain the same degree of blanching in steam, in which case the difference between steam blanching and water blanching with respect to loss of water- soluble materials might not be so great. Series blanching has been recommended by British investigators on the grounds that there is 38 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE less loss of soluble materials from the blanched material. Investiga- tions of the Western Regional Research Laboratory tend to indicate, however, that losses due to series blanching may be less than those encountered in ordinary water blanching, but not necessarily less than with steam blanching. TABLE 6.—Effects of water blanching and steam blanching on losses of vitamin C, sugar, and protein from dehydrated vegetables Vegetable 3-minute blanch in— via aoa poe Percent Percent Percent Pens ee 2a ae a et eRe 49.0 - 682256 PRAM PRE TE RTE RS eR aa, Seat a SR Sa 28.0 7/5 22 AUG Tae ee aa eR GS 38.0 WP 7 18.5 Carrots----.------------.------ {Steam PERE: ewes foo 20.0 18.5 4.5 SGC S eae ee tit nee ge 46.0 ao 0 Beans._------------------------ {Steam hyre eal gto es 36.0 3.3 0 Brussels sprouts______________-- 1a Bue Re SiN NR CET Tes aS a2 3 ae PSE See, ee Se 1 Data from Adam, Horner, and Stanworth (1). The carrots are reported as diced and the beans as sliced and green. 2 Data from Horner (2/). Very largely because of differences with regard to loss of soluble materials, steam blanching is generally recommended. In fact, Gov- ernment specifications for most dehydrated vegetables require that the product be steam-blanched. It is not possible to state accurately the blanching times required for different products, since the time is dependent upon a large number © of factors that must be controlled. Among the factors that may in- fluence the blanching time are the following: (1) Size of pieces. Since the product should reach a temperature of at least 190° F. in the center of each piece, it is obvious that the larger the piece, the longer will be the time required to reach this temperature. (2) Depth to which the material is loaded on the blanching trays or belt or amount of material loaded into blancher. It is obvious that the greater the depth of the load, or amount of load, the longer will be the time re- quired for penetration of heat to the center. (38) Uniformity of heat distribution in the blancher. If there are pockets or areas in the blancher in which the temperature is low, a longer blanch will be re- quired to compensate for the low temperatures. (4) Ability of the blancher to maintain a high temperature. If the temperature should drop for any reason, it is obvious that a longer blanch will be required to obtain the same degree of blanch had the temperature not dropped. (5) Characteristics of the raw material, such as variety and maturity of the product. An additional factor that may influence the blanching time is altitude. This factor applies to both steam and water blanchers operated at atmospheric pressure in those cases where the blanching temperature is specified as that of live steam and boiling water. In general, the greater the altitude the lower will be the temperature that can be maintained, and consequently the longer will be the blanch- ing time. Variations of as much as a degree or two may occur as a result of day-to-day changes in barometric pressure. The following tabulation shows the effect of altitude on the average boiling point of water. VEGETABLE AND FRUIT DEHYDRATION a9 Boiling point Boiling point Altitude (feet) of water (° F.) Altitude (feet) of water (° F.) (fc Ne as 18 Be ee AE cae nee br tik ee BaD) StS eOA Yt Sates tL Sali Tacks bh Lead eed amine ee 202 Ti, | EERO Pe ot ea ee Ree Salen ee DOE YEO OO se me be. ir raid pee eye tS 200 Fes (0 1 age are Suet AI eh ccs SS ae et DS Ae OO sas SS SR ad 197 3 ICD UR Vine 98 ey a ae MS a eee ee DOG eS OOO ated SL ree se AB 196 Asin UO): 2 Sistas ere ee te ae Ee Ai Oh OO tee Sears are eee ene ee 195 Since blanching is an essential part of the dehydration process, and since the purchase of finished product by the Government is partially dependent upon adequacy of blanching, the processor of dehydrated vegetables must maintain careful control over the blanching operation. This means that he must be certain that the blancher is designed to insure unifcrm heat distribution throughout. He should be certain that his steam supply is adequate to maintain a constant, uniform temperature, and must provide for uniform loading and spreading of the material. The blancher should be provided with thermometers at various points so that frequent checks can be made on the mainte- nance of temperature and uniformity of heat distribution. As it comes from the blancher the product should be tested at frequent inter- vals for adequate blanching by the use of enzyme tests. If a positive test 1s obtained, steps should be taken immediately to discover the cause and remedy it. Under ordinary blanching conditions, where the op- eration is carried out under atmospheric pressure, the operator should strive to maintain the temperature in the blancher as close to 212° F. as possible. Vegetable Blanchers _ The dip method or tank blanching requires a steam-heated water vessel. If the unit is used for batch blanching, the produce is placed in perforated metal baskets and held submerged in hot water until all of the product has been raised in temperature above 190° F. If dip blanching is to be continuous, the blancher is equipped with a double draper belt similar to that shown in figure 17. The upper belt le D las Ray Tee aa. as. FIGURE 17.—Dip blancher equipped with double draper belt. is used to hold the product submerged in the tank during the blanch- ing process. Another method of adapting the tank blancher for continuous processing is to provide an endless chain fitted with hooks to carry metal mesh baskets through the bath (fig. 18). Naturally. FIGURE 18.—Dip blancher equipped with baskets. the retention time will depend upon the speed of the belts and the length of the tank. Atmospheric steam blanching can be either a batch or continuous process. The method consists essentially of exposing cut vegetables to steam in a booth or chamber. 40 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE The batch-type steam blancher is usually a chamber in which trays of cut vegetables are placed. After the door is closed, steam is ad- mitted at various points along the box so as to fill the chamber com- pletely, thus exposing the product to steam in motion. The resulting washing action by the steam increases the rate of heat transfer from the steam to the product. Entrapped air, being heavier than steam, 'is discharged through ports located at or near the bottom of the cabinet. Without these ports, air locks may form, which will prevent effective and uniform blanching. The continuous atmospheric blancher usually consists of a tunnel or large cabinet containing a wire or perforated-plate belt on which the product is loaded. The tunnel is fitted with steam sprays both above and below the belt throughout its entire length. Reinforcing the steam supply at the initial phases of the blanching cycle is some- times practiced, since the additional steam is useful in rapidly heat- ing up the product to the blanching temperature. A modification of this form of blancher consists of a similar tunnel fitted with conveyor chains upon which product-laden trays are placed and conveyed through the tunnel (fig. 3). Usually the same trays are used for both drying and blanching. This type of blancher is particularly suitable for blanching cabbage and other leafy vegetables which cannot be easily handled, once blanched. Inasmuch as tray loading in this case is determined by drying rather than blanching limitations, blanchers of this type must be of considerably greater proportions than simple belt blanchers of equivalent capacity. The repeated exposure of wood trays to blanching will result in rapid deterioration. Trays that have absorbed moisture during the blanch- ing operation naturally increase the drying load. It is desirable that the center or active section of all tunnel blanchers be located at a higher elevation than either the entrance or discharge ends so as to form a heat lock. Heat lock is caused by the difference in density between steam and air at ordinary temperatures; thus the steam is trapped in the upper portion of the blancher (fig. 19). It |A B C Curtain Figure 19.—Hump-back blancher showing heat lock. is desirable to minimize the clearance between the tunnel walls and the belt so that all of the steam admitted to the chamber comes into inti- mate contact with the product being blanched. It is important that all air be kept out of a blancher. To prevent suction of air into the blancher, steam jets should be positioned so as to neutralize the kinetic energy of the jets. This is accomplished by dividing the branch lines into pairs and drilling the steam orifices so that the jets oppose each other. If steam is allowed to impinge directly on the product, furrows will be cut in the bed, thereby increasing the depth of the bed between furrows. This will retard the rate of blanching, inasmuch as blanch- ing can be considered complete only when the innermost product in the center of the bed is heated up to or above 190° F. VEGETABLE AND FRUIT DEHYDRATION Al Stacks connected directly to the blancher either at the ends or at the midsection are undesirable. They serve no useful purpose and are in fact detrimental to the blanching process, because live steam is induced to escape. Unattached hoods located at the two ends are desirable to take away unavoidable steam leakage. Some leakage of steam is necessary, however, to insure a full chest of air-free steam. If water-wash or sulfite-spray sections are provided either before or after the blancher, these should not be constructed so as to cause a draft in the blancher. A satisfactory practice is to eliminate the tunnel cover over the water-wash or sulfite-spray sections. A heavy canvas curtain should be provided at both ends of the blanching section so as to minimize leakage. Spreading the product on the blanching belt or trays can be ac- complished manually or mechanically. For mechanical spreading, use is made of revolving brushes or drums, stationary bars (straight, angular, or curved), or vibrators and shakers. At least two additional procedures are in common use. The material discharged from the cutter may pass into a water flume which empties on the front end of the blancher belt. The water drains through the belt into a sump, leaving the cut material in a uniform layer on the blancher. Another method uses the momentum of the vegetable pieces as they come from the cutter. The discharge spout is removed and replaced by a suitably shaped deflector plate. Proper use of this method on diced vege- tables gives a satisfactory spread over a width of three feet or more. The multiple-belt blancher, as its name implies, comprises a number of belts. The product is dropped from one belt to the other as it progresses through the blancher (fig. 20). It has the advantage of ‘Ficure 20.—Multiple-belt blancher. being considerably more compact than the ordinary continuous blancher. Designing such a blancher, however, is a difficult task and requires considerable mechanical skill. Because of multiplicity of parts and the alternate stresses produced in the side walls of the unit, it cannot be readily constructed of wood. Retort blanching requires a means of subjecting cut vegetables to steam above atmospheric pressure. This method is quick and positive; however, the equipment is costly and, being essentially a batch process, must be supplied in duplicate so as to produce a steady flow of product. The labor required also may limit its use in dehydration. Improve- ment in product quality resulting from this process is probably neg- ligible and the method cannot be justified unless retorts are available and other equipment cannot be procured. Summary of blanchers.—Of the various types of blanchers now em- ployed in the dehydration industry, the simplest and most commonly used is the belt blancher. If this unit is modified for use as a con- 42 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE tinuous-tray blancher, the effective blanching area must be greatly enlarged to achieve the same blanching capacity (fig. 21). This is necessitated by the lighter tray loading as compared to belt loading. The multibelt- eonUaiote blancher is not suitable for use with cabbage or other products that are difficult to handle after blanching. It is compact, but more difficult to build than the continuous-belt blancher. FIGURE 21.—Steam-blanched diced carrots being loaded directly from the blancher on drying trays. The tank-dip blancher is the simplest form of unit, is generally ac- ceptable aie for moderate-scale operation, and is not conducive to highest quality of product. SULFURING VEGETABLES The treatment of certain vegetables with sulfite solutions prior to dehydration has been used and recommended considerably in England, Australia, and Canada during the past 2 years, but has not been applied commercially in the United States until recently. It has been adopted with some reluctance in the United States because of a general antipathy toward sulfur dioxide in any form as a food pre- servative. However, its accepted use in dried fruits and the definite improvement in retention of palatability, color, and ascorbic acid through processing and storage of dehydrated cabbage treated with a sulfite solution, ultimately forced its acceptance. Recent Govern- VEGETABLE AND FRUIT DEHYDRATION 43 ment purchase specifications for dehydrated cabbage provide that the product shall be so sulfited that it contains sulfur dioxide within the limits of 750 to 1,500 parts per million. Sulfiting of white potatoes and carrots has been under consideration, but up to the present specifications have not been issued in which toler- ances for sulfur dioxide have been included. The sulfiting of white potatoes does not present unqualified advantages. From the stand- point of acceptable palatability they tolerate substantially less sulfite (perhaps not over 500 p. p. m. as SO,) without exhibiting an un- desirable sulfur flavor in the reconstituted product. Moreover, the B vitamins are unstable in the presence of sulfites, and potatoes lose these vitamins when so treated. Application of sulfite to some potatoes may result in better color as the product comes from the dehydrator and also better retention of quality through storage under adverse con- ditions. Carrots will probably respond more favorably to sulfite treatment than potatoes, from the standpoint of improved storage characteristics, and will probably tolerate a higher content of sulfur dioxide without undesirable sulfite flavor. Apart from the beneficial effects that sulfiting offers in the retention of quality, it has made possible the use of higher finishing tempera- tures in dehydration and consequently shorter drying times. Particu- larly with cabbage, finishing temperatures 10° to 15° F. higher can be employed safely without scorching, as compared with unsulfited cab- bage. This has reduced the drying time as much as one-third in some cases. The effect is also observed with onions, potatoes, and carrots. It is possible to accomplish the sulfuring by any one of several methods. In England where blanching is done by immersion in hot water, it is relatively easy to incorporate the sulfiting treatment by merely adding sulfite salts to the blanching bath and maintaining the desired concentration by addition of salts at fairly frequent inter- vals. The Canadians also have recommended the immersion proced- ure and the Australians and New Zealanders have experimented both with dipping and with application by spraying the trayed product. In the United States blanching is done almost exclusively in con- tinuous-belt steam blanchers or in cabinet or pressure retort units, also with steam. Application of sulfite by any dipping technique thus requires the addition of another operation to a processing se- quence already firmly established. Since by far the greatest number of blanchers used in this country are of the continuous-belt type, efforts were first directed to the development of a spraying procedure which could be made an integral part of the blanching operation and equipment. This has been successfully accomplished with cabbage and the method has been used in the production of more than a half million pounds of dehydrated sulfited cabbage up to the present time (1944). The method involves application of a sulfite solution by means of a spray on the cabbage as it is conveyed through the steam blancher. Application to the raw shredded cabbage before it enters the blancher has not proved desirable because of relatively poor absorption of sulfite. Undesirably high concentration of sulfite salts must be used in the solution if enough uptake to give 750-1,500 p. p. m. in the dried product is to be achieved. On the other hand application to the blanched product as it emerges from the blancher will result in very 44 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE good absorption and will permit the use of relatively dilute solutions. This method has a disadvantage in that excess solution will continue to drain from the trays as they are stacked and may lead to variations in sulfite content between the upper and lower trays and, with wood slat trays, to staining unless the trays are new or very carefully cleaned. It appears that objections to spraying before or after blanching can be overcome by application during the blanching process. In this way the partially blanched cabbage absorbs the sulfite efficiently and the product is discharged from the blancher with an essentially dry surface, so that no draining from one tray to the next lower results. Blanchers and raw stocks of cabbage vary so much that it is impos- sible to give a detailed procedure that can be applied under all condi- tions with equal effectiveness. In general the spray should be located at a point one-third to one-half the length of the blancher from the entrance end. In long blanchers (50 feet or more) best results have been obtained by installing the spray at one-third of the way. Ina 20-foot blancher best results have been obtained by placing the spray at the midpoint. The best location is apparently critical and should be determined by trial. In order to obtain good coverage and uniform results in spite of unavoidable variations in tray loadings, it 1s necessary to apply the spray at a rate that will result in some run-off. A rate of 5 to 10 gal- | lons of solution per 100 pounds of trayed cabbage is recommended. Various types of sprays can be used, such as single or multiple (3 or 4) pipes extending across the blancher at right angles to the direction of belt travel and a few inches above the cabbage, drilled with 345- to 55-inch holes about 1 inch apart. Still another type consists of a | single transverse pipe with the holes so drilled that the jets play against the roof of the blancher and the solution is splattered out rather uniformly over the product. At present it is not known which type is most effective. i The sulfite solution is conveniently supplied to the sprays from two wood or concrete tanks of such size that each will carry enough for an 8-hour shift. On this basis 3,500-gallon tanks should easily supply a plant with 50 tons of daily capacity. The second tank is used in making up fresh solution to desired strength while the first is in use. Each tank should be equipped with a mixer that will give vigorous stirring without beating air into the solution. The tanks should be set at a level that will supply a minimum head of 12 feet, or if this is not practicable a pressure pump must be provided to deliver the solution to the sprays. Control of the sulfite content of the dehydrated product is effected by adjustment of the concentration of the sulfite-in solution. For cabbage the required concentration will usually le within the range of 0.15 to 0.30 percent, calculated in terms of SO,, depending upon cabbage variety, blancher, and dehydrator conditions and perhaps other variables. Either the normal sodium sulfite or mixtures with sodium metabisulfite can be used. At present the influence of pH of the applied sulfite solution on product quality is inadequately understood. From the corrosion standpoint it is advisable to use a predominant amount of the normal salt to keep the pH at 7 or above. Moreover, with green varieties of cabbage better color results when the more alkaline solutions are used. However, alkaline sulfite solu- VEGETABLE AND FRUIT DEHYDRATION 45 tions are more readily oxidized by air than are those at lower pH, so that important amounts may be converted to inactive sulfate. In the absence of complete information it seems best to use a mixture of the two salts which will give a solution in the range of pH 7 to 8. A convenient proportion consists of 3 parts by weight of sodium sulfite to 1 part of the metabisulfite, which yields a solution of ap- proximately pH 7.2. If desired the cheaper metabisulfite can be used and the correct pH achieved by addition of soda ash. Hard waters cause precipitation of the highly insoluble calcium sul- fite, which results in a milky appearance of the solution and upon accu- mulation may cause clogging of spray jets. This precipitation may be effectively inhibited by dissolving sodium hexametaphosphate (Cal- gon) in the water to give a concentration of 5 p. p. m. prior to addi- tion of the sulfite salts. The sulfite salts dissolve somewhat more slowly in the presence of the phosphate and a large excess is to be avoided because of its potentially unfavorable tenderizing action on the cabbage. Reasonable caution should be exercised in handling sulfite solu- tions to avoid contamination with metals such as iron and copper, since these catalyze the oxidative conversion to inactive sulfate. Con- tact with air should be minimized, especially with solutions of high pH. If a solution is held more than a few hours it should be mixed, sampled, and rechecked for sulfite content and refortified, if necessary, before use. Corrosion of metal trays will not be serious, provided solutions of pH 7 or higher are used. Tinned trays must be kept well coated; otherwise cabbage coming in contact with exposed iron will be seri- ously stained as a result of the relative positions of tin and iron in the electrochemical series. Galvanized-iron trays are less likely to cause staining of the product, since the zinc is more active than the iron and hence soluble iron does not reach the cabbage. Cloths over the tray screens have been tried to avoid staining and to facilitate removal of the dried product without excessive fragmentation. Un- fortunately, although the cloths are very helpful in detraying opera- tions, they do not work satisfactorily when sulfite is used. Metal trays can be treated with a light coat of mineral oil to avoid sticking of the dried cabbage. In plants equipped with cabinet blanchers sulfiting can be carried out by dipping baskets of the raw shredded material in a tank of sul- fite solution. If dipped cold it is necessary to use a solution of 0.5 percent as SO, or greater to obtain the desired uptake. The cabbage should remain immersed a minimum of 15 seconds, should be removed and drained another 15 seconds, and then dipped in or sprayed with fresh water for 5 seconds. ‘Then it is spread on trays and blanched in the usual way. Varieties of cabbage may be expected to show considerable variation in absorption of sulfite from cold solutions; therefore trials must be made with different sulfite concentrations on each new lot of cabbage. Perhaps a more satisfactory procedure is to dip the raw cabbage in a hot (180°-190° F.) sulfite solution for 5 seconds, drain briefly, dip momentarily in fresh water, and then spread on the trays and blanch. Lower concentrations of sulfite can be used when the solution is applied hot—0.2 to 0.4 percent as SO, ordinarily being adequate. Dipping of cabbage after blanching is not satisfoctory because of 46 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE the difficulty of traying the softened product. It is possible to apply the sulfite as a spray, followed by a fresh-water spray on trayed blanched material. This procedure, however, is not wholy satisfac- tory and cannot be recommended for cabbage because of draining complications. Laboratory and pilot-plant experiments have demonstrated that both potatoes and carrots can be successfully treated by application of sulfite solutions as a spray during blanching in much the same manner as for cabbage. If future specifications require sulfiting of these commodities, the required solution concentrations will of course be influenced by the specified content of sulfite in the dry product. PRINCIPLES INVOLVED IN THE DRYING PROCESS The drying operation is a major step in the manufacture of dehy- drated foods and may be defined as controlled evaporation of nearly all of the water present in the fresh product. The evaporation of water under these conditions is, however, a complex process, and its princi- ples are sufficiently important to justify discussion here. The Vaporization of Water If a container is partly filled with water at a temperature of 100° F. and the air is removed from the space above the water by means of a vacuum pump, and then the container is tightly closed, a sensitive manometer connected to the vapor space will indicate the gradual de- velopment of a pressure within the “empty” space. If the temperature of the water is maintained at 100° this pressure will increase up to 1.93 inches of mercury. The higher the temperature of the water, the higher the pressure of its vapor will be; at 212° the vapor pressure of water is 29.92 inches of mercury—the same as average barometric pressure at sea level. Table 7 presents values for vapor pressure of water over a range of temperatures (39, p. 960). Suppose that the container of water at 100° F. is left open to the air, so that the space above the water must remain at atmospheric pressure—say 30 inches of mercury. Water evaporates from the sur- face of the liquid, and the water vapor pushes out some of the air, since the pressure cannot rise above 30 inches. When the system comes to equilibrium, the space above the water will be filled with a mixture of air and water vapor. Each of these components is contributing a part of the total pressure of 30 inches; at a temperature of 100° F. the water vapor is contributing 1.93 inches, the air 28.07 inches. These two quantities are termed “partial pressures.” Before the system reached equilibrium, the space was not yet saturated with water vapor. It became saturated when the partial pressure of water vapor in the space rose to equality with the vapor pressure of water at that temperature. The fluid content of the cells of vegetables and fruits does not consist of pure water, but is a complex solution of salts, sugars, and other substances. The vapor pressure of such a solution is always somewhat lower at any given temperature than the vapor pressure of pure water ; the more concentrated the solution, the greater the difference. The liquid in a fresh vegetable is dilute, but as dehydration progresses the liquid left behind becomes more and more concentrated and its vapor pressure becomes lower and lower. ‘This is one of the major reasons 47 VEGETABLE AND FRUIT DEHYDRATION why the final stage of drying a fruit or vegetable proceeds so much more slowly than the initial stages. Returning to the example of evaporation at 100° F., we find that as evaporation proceeds the liquid cools down below 100° unless addi- tional heat is supplied. Actually, 1,037 B. t. u.* would have to be supplied to the liquid at 100° F. for every pound of water evaporated. TABLE 7.—Properties of water vapor and air Vapor ren iM bsolute Specific volume ! Temperature (° F.) pressure vaporiza- humidity at of water tion saturation ! Dry air | Saturated air Inches of Lb. vapor/lb. Cu. ft./lb. mercury B.t. u./lb. dry air Cu. ft./lb. dry air (ON OIA SU EEN DO i EO 0. 0375 1, 094 0. 00079 11, 58 11. 59 OER e SS Se ae etn eee a eae ew . 063 1, 088 . 00132 11. 83 11. 86 ZO AERC OR EN eer CUE eee See eR ESS . 103 1, 083 . 00216 12.09 12.13 aD SA Re lt et Sei ae ses a . 165 1, 077 . 00347 12. 34 12. 41 CU eS COs ate Dee er ee we eee ee . 248 1, 071 . 0052 12. 59 12.70 Oe eee earth eras MOTTE CIE LTA Ye tact ty bo . 362 1, 066 . 0077 12. 84 13. 00 C3 OES eet EN TE aA Wa Seat a Say? 1, 060 -O111 13.10 13. 33 CAO gp ele Se a age LE eee Pe Seeker g . 74 1, 054 . 0158 18. 35 13. 69 SO) ea a RE AS eA PIAS 1B aletas Dy EAE 1. 03 1, 049 . 0223 13. 60 14. 09 (RR a SOS a Sa) eae Sa aes Cee 1. 42 1, 048 . 0312 13. 86 14, 55 VU 0) a Se a aah ke ce ecm ee eae ee 0G 1. 93 1, 037 . 0432 14,11 15. 08 ASE Raa ee eS eee a ae Gage 2 Pa ST 2.59 1, 032 . 0595 14, 36 bs E20 Bare ark: eaeW eeeteetan etter kes sea 3. 44 1, 026 . 0815 14. 62 16. 52 VSO ee ER Si a I eh Sas, 4. 52 1, 020 . 1114 14. 88 17. 53 TICE) ep are oe Nee Ste Sin Si ip ora fe ee BO Seee es teeth §.88 1, 014 . 1532 15. 13 18. 84 ENS (eae eres Dene Se ae Nee eee VAY A 1. 008 . 2122 15. 39 20. 60 EGE earn USS eI Fee ges Se Es 9.65 1, 002 . 2987 15. 64 23. 09 IZ eters Fete Re Gir ak eI 12. 20 996 . 432 15. 90 26. 84 IN (Ve Mite LN SS Bees ates ae oO i TE; a 06 URE 1 06 > | ~ Lay om .05 0 + = 05 E : fe) Tig = 04 a Oh rar (fa z | fe) ot) f : 1 3S .03 s Ee «x 2a wW WwW KE z= .20 oO © @ Température m wee Sw rs A om = = — 100 120 140 160. i80 DRY-BULB TEMPERATURE (°F. ) Ficurp 25.—Approximate equilibrium moisture content of common vegetables. general average of these equilibriums for the vegetables which are commonly dehydrated. Such an average is sufficiently precise for many dehydrator calculations. , Knowledge of these equilibrium values is important both in de- hydrator design and in analyzing the pick-up of moisture into de- hydrated vegetables through sheet packaging materials. The finish- ing end of a dehydrator cannot dry the material to any lower mois- ture content than the value corresponding to equilibrium with the air at that point. If the finishing air is high in humidity, a very dry product cannot be made. Furthermore, the closer the approach to equilibrium, the slower becomes any further removal of moisture. VEGETABLE AND FRUIT DEHYDRATION 59 If the finishing stage is to proceed at a reasonably high rate, there- fore, the humidity of the air must be decidedly lower than that corresponding with equilibrium at the desired final moisture content. Typical Drying Curves, Constant Drying Conditions General physical laws and the known properties of air and water vapor make it possible to calculate accurately some important char- acteristics of a dehydrator. The methods used and some of the results are discussed in a later section. ‘These methods have the strict limitation that they offer no information about the rate at which evaporation will take place; in other words, the time required to bring about a given degree of drying. The foregoing discussion indicates qualitatively the effect of various factors on the rate of drying, but quantitative information can be determined only by experiment. At least nine separate factors have marked effects upon drying rates (45). These are: Variety of fruit or vegetable, shape and size of piece, method of pretreatment (blanching, etc.), method of support in the drier, thickness of layer of moist pieces on the sup- port, manner of exposure to the air stream, air velocity, air tem- perature, and air humidity. In a vacuum dehydrator still other factors appear. The number of possible combinations of these vari- ables is so enormous that the only practical way to investigate their effects is to conduct a series of controlled experiments; in a group of such experiments all conditions except one are kept constant, and several values of that one will be tried in successive experiments; then in other groups the effects of other variables will similarly be tried one at a time. After careful analysis of a long series of ex- periments of this kind it becomes possible to estimate with consider- able accuracy what the rate of drying will be under any combination of conditions. Detailed results of experimental work on drying rates are not in- cluded here, because such information is principally valuable to de- hydrator designers. The following general discussion is introduced for the value it may have in promoting intelligent understanding of what goes on in a dehydrator. Suppose that the following choice of conditions is made for a single experimental run: Russet potato, “julienne” strips five thirty-seconds inch square, steam-blanched 6 minutes, wood-slat trays, original load 1.5 pounds per square foot of tray surface, air flow across the trays, air velocity 500 feet per minute, air temperature 150° F., wet-bulb temperature 90°. If these conditions are maintained constant and the progress of drying is determined from time to time by weighing the tray, a drying curve such as that reproduced in figure 26 may be drawn. This curve is typical of hundreds that have been observed. Two characteristics are especially important: (1) The very rapid initial fall in moisture content, and (2) the very slow final approach toward equilibrium moisture content (in this case, from figure 26, equilibrium T=0.025, about 2.5 percent). Note that an entire hour is consumed in drying from 7 percent to 6 percent moisture, whereas three-fourths of the total original moisture is evaporated in only an hour and a quarter. 60 MISC. PUBLICATION 540, U. 8. DEPT. OF AGRICULTURE MOISTURE CONTENT (LBS./ LB. BONE- DRY) 8% 7% 6% (oe) 1 2 3 4 5 6 TIME (HOURS) Figuke 26.—A typical drying curve. Effect of Wet-Bulb Depression Other things being equal, drying will proceed faster if the wet-bulb depression of the air is high (relatively dry air) than if it is low (relatively moist air). Figure 27 compares drying curves for potato strips in three experiments; in all of them the wet-bulb temperature 7 Ce reed : 1 Ww z é | om 20 rea) Air Temperature a Dry-Bulb > 105° w (1.0 eee N co = Dry-Bulb ie 130° z -.50 FE, Dry-Bulb Wet-Bulb z 150° 90° e Wet-Bulb Depression ° 90° [40 tu .20 Peete ee iad ES EN SS Depression = 60° ” OF 0 Cees 05 O 4 5 6 tf TIME (HOURS) Figure 27.—Drying curves for potato strips at varying dry-bulb temperatures. of the air was 90° F., but dry-bulb temperature was 105° in one case, 130° in another, and 150° in the third, so that wet-bulb depressions were 15°, 40°, and 60°, respectively. The very great effect of this factor is apparent. 5 In this figure and the other drying curves which follow it, a logarithmic scale for mois- ture content is used, in order to show differences at low moistures more effectively. VEGETABLE AND FRUIT DEHYDRATION 61 Effect of Temperature Level At a given wet-bulb depression, drying will proceed faster if the temperature level is high than if it is low. Figure 28 compares the results of two runs, in both of which the wet-bulb depression was 40° F.; in one, however, the temperatures were 90° and 130°, while in the other they were much higher, 120° and 160°. In the latter the rate of drying was substantially faster. Note, however, that the effect was less marked than that of wet-bulb depression as shown in figure 28. That is, if dry-bulb temperature were the same in two runs, the one at the higher wet-bulb temperature would be the slower because of the predominant effect of wet-bulb depression. 5.0 2.0 1.0 coe oe = Dry- Bulb Dry- Bulb | ISOS ae| 160° Wet- Bulb | 908 20 Wet-Bulb . 1! 120 Depression ° Depression ; a : = MOISTURE CONTENT (LBS./ LB. BONE-DORY) TIME (HOURS) FicuRE 28.—Drying curves obtained with differing temperatures but with the same wet-bulb depression. Effect of Air Velocity The higher the velocity of air over a moist surface, the more rapid the evaporation from that surface. Figure 29 compares drying curves from experiments with potatoes. Air velocity across the tray was 500 feet per minute in one case, 675 in another, and 855 in the third. Drying was somewhat more rapid with higher air velocity. The effect of air velocity is much more substantial in drying cabbage and leafy vegetables in general than it is in drying potatoes. A change in the air velocity in a dehydrator has two quite inde- _ pendent effects. One is the effect on drying rate, which has just been described. The other will be considered more fully in a following section, and, in brief, is a consequence of the fact that the weight of air moved through the dehydrator rises in proportion to the velocity of that air. The greater the weight of air circulated, the less it will fall in temperature for a given amount of evaporation, and hence, 62 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE MOISTURE CONTENT (LBS./LB. BONE- DRY) TIME (HOURS) FIGurRE 29.—Drying curves obtained with varying velocities of air. the higher the wet-bulb depression that will be maintained in the dehydrator. Manner of Exposure to Air Stream In commercial dehydrators the pieces of vegetable are not suspended separately in the air stream, but must be piled on one another, or on a support. Contact with the air stream is thus hindered. The lower pieces in a deep layer will dry very slowly if air flows only across the top. In some types of dehydrator the air is caused to flow through the layer of pieces, which are held on a perforated support. This is known as through circulation. It exposes all pieces, even in a deep layer, to contact with the entire flow of air. If the air flow is across, or substantially parallel to, the surface of a layer of pieces, the arrange- ment is known as cross circulation. wo MOISTURE CONTENT (LBS/LB. BONE-DRY ) foe DISTANCE FROM WET END (FEET) Figure 39.—Drying conditions in a heavily loaded counterfiow tunnel drier. Time, 8.2 hours; relative capacity, 1.00; final moisture content, 5 percent. final dryness. Figure 42 illustrates the change in air temperature and moisture content through such a tunnel at high input of wet material. Under these conditions the product would not be dried pele about 10-percent moisture even if the tunnel were indefinitely ong. If the rate of input of wet material to this tunnel were decreased (as by loading trays lightly), so that the fall in air temperature would not be so great, drying conditions at the dry end would not be so unfavorable. In fact, it is obvious that the performance of a parallel-flow tunnel would be indistinguishable from that of a 80 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE counterflow tunnel if the air flow were so high or the amount of water evaporated were so small that little drop in air temperature occurred. 160 TEMPERATURE (°F.) rs) (@) ial 805 10 20 30 DISTANCE FROM WET END(FEET) ol w ff in) MOISTURE CONTENT (LBS./LB.BONE-DRY ) ) 10 20 30 DISTANCE FROM WET END (FEET) FIcureE 40.—Drying conditions in a lightly loaded counterflow tunnel drier. Time, 6.35 hours; relative capacity 0.71; final moisture content, 5 percent. Combination Arrangements The parallel-flow arrangement provides the highest possible rate of evaporation at the wet end of the tunnel, while the counterflow arrangement provides the highest possible rate at the dry end. Com- binations of the two have therefore come into increasingly wide use for the dehydration of vegetables. BLOWER HEATER INTAKE DRY MATERIAL OUT Ce ff a po TRUCKS EXHAUST-AIR STACK Fieure 41.—Parallel-flow arrangement in the tunnel-and-truck dehydrator. The simplest combination consists of two separate tunnels, as illustrated in figure 43. The parallel-flow tunnel may be relatively short and is designed to accomplish a large proportion of the total evaporation very rapidly, but to discharge a product which stili VEGETABLE AND FRUIT DEHYDRATION 81 contains as much as 50 to 60 percent of moisture. High air circu- lating capacity and high heat input are provided. The counterflow tunnel may be longer, but the air flow and heat input will be sub- Teo) WJ a@ = 160 : eee a ra} S w 140 lo ee 20 30 BET ANCE eon WET END (FEET) te Wo 4 5 ' O33 7:9 a eee > 0 Ee ea yn DQ | = so (@) 10 20 30 DISTANCE FROM WET END (FEET) Ficure 42.—Drying cotJtitions in heavily loaded parallel-flow tunnel: Time, 13.4 hours; final moisture content, 10 percent. stantially less, since comparatively little evaporation will be required there. In some designs the air exhausted from the counterflow tunnel, being warm and still comparatively dry, is used to make up all or a part of the “fresh” air required by the parallel-flow tunnel. Division of the drier into two separate tunnels necessitates an additional handling of the trucks, and therefore some increase in BLOWER HEATER FRESH-AIR INLETS HEATER FAN WET MATERIAL IN DRY MATERIAL OUT EXHAUST-AIR STACK MOVABLE PARTITION Figure 43.—Combination of counterflow and parallel-flow arrangements. labor cost. Consequently there are several designs of dehydrators that combine both parallel-flow predrying and counterflow finishing into a single tunnel structure. Figures 44 and 45 illustrate two such arrangements. They are generally termed “center-exhaust” 569074446 82 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE tunnels, since hot air flows into both ends and cool, moist air is exhausted somewhere near the center of the tunnel. ‘Inspection of the diagrams will make it evident that whatever gain in cost of operation there may be is at least partly offset by increased com- plexity of construction and control. EXHAUSTER AND FRESH-AIR INLET EXHAUST -AIR STACK FRESH-AIR INLET MI Id A WET MATERIAL IN DRY MATERIAL OUT Figure 44.—Center exhaust tunnel dehydrator that combines parallel-flow and counterflow drying. Whether the parallel-flow and counterflow sections are separate or combined in a single tunnel, the typical course of the air temperature and the moisture content of product in passing through the system are illustrated in figure 46. The maintenance of effective drying condi- tions throughout the entire time is reflected in rapid drying and in high WET. MATERIAL IN COUNTERFLOW SECTION Ficure 45.—Multiple-section tunnels. capacity secured from equipment of a given size. Suggested tempera- tures and humidities suitable for any of these combination arrange- ments may be found in later sections of this manual. Compartment Arrangement A less common arrangement of a continuous truck-and-tray dehy- drator is known as a combination compartment and tunnel. The characteristic feature is that air flow through the trucks is transverse to the long axis of the tunnel. This arrangement makes it possible to circulate the same air several times through each truck, and to reheat the air at each stage. For example, if several cabinet dehydrators (see later section on cabinet dehydrators) are placed end to end, with doors between units omitted, a truck load of wet material may start at one end of the group and progress through one compartment after an- other, emerging completely dry at the other end. Then if a blower is used to supply fresh air to the dry-end compartment, and air exhausted from each compartment is used to supply the “fresh-air” requirement of the next one, there is a general countercurrent flow of air through the whole group. Each compartment can be provided with a separate heater, which makes possible a wide freedom of choice with regard to air temperature at successive stages of drying. Since the air can be exhausted at high absolute humidity, good heat economy can be obtained without undue sacrifice of drying rate. VEGETABLE AND FRUIT DEHYDRATION 83 Against the theoretical advantages of this type of dehydrator must be set the fact that it is complex to build and operate. The trucks must fit very tightly into the tunnel to avoid short-circuiting of air through useless channels, and the numerous changes in direction of air flow increase the fan horsepower required and impose difficult problems of equalization of flow. The temperature conditions suitable for successive sections of this TEMPERATURE (°F.) Wet-Bulb Tempera- ture MOISTURE CONTENT (LBS./LB. BONE-DRY ) 0 10 20 30 DISTANCE FROM WET END (FEET) Ficure 46.—Drying conditions in a heavily loaded combination parallel-flow and counterflow tunnel. Time, 7.6 hours; relative capacity, 1.48; final moisture content, 5 percent. type of tunnel are similar to those in a single cabinet dehydrator dur- ing successive equal periods of time. (See section on cabinet dehy- drators.) For example, if there are six sections, the temperature in the first section must be maintained at conditions suitable for a cabinet near the end of the first sixth of the total drying time; conditions in the second section must be similar to those in the cabinet at the end of the first third of the drying time; and so on. | | tl 84 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Closed-Cycle Arrangement Any type of dehydrator can be modified so that water vapor is con- aensed from the exhaust air, which is then returned and used over again as “fresh” air. In this way it becomes possible to use an inert | gas, such as carbon dioxide or scrubbed flue gas, without an insup- | portable loss of the gas. While this process, as applied to vegetable dehydration, is being investigated in several quarters, its cost and | complexity have discouraged wide application. The dehumidifica- tion of the exhaust gas can be accomplished by passing it through pipes cooled externally by trickling cold water or a refrigerant, or, more simply, by direct contact of the exhaust gas with a spray of water at a temperature lower than the dew point of the gas. In the com- partment type of tunnel some of this dehumidification can be accom- plished in each compartment by means of a water spray in advance of the heater. Combined Blanching and Predrying Some development work has been done on very brief predrying at | very high wet-bulb temperature, for the purpose of producing a | blanching action simultaneously with the removal of a substantial part of the easily evaporated water. For example, exposure of cut vegetables to a current of air at a temperature of 250° F. or even 300°, and a wet-bulb temperature of 200° for from 6 to 10 minutes may evaporate as much as half of the total moisture, and at the same time blanch the material. While the process has interesting possibili- ties it has not yet reached commercial development. STARTING AND STOPPING THE TUNNEL DRIER Under normal conditions, the operation of the tunnel drier pre- sents few difficulties; the beginning and ending of a dehydration run, however, require certain modifications of conditions and these are dis- cussed below. Simple Counterflow Tunnel Unless corrective measures are applied, the first truckload of prod- uct introduced into a counterflow drier will be subjected to a hot, un- tempered blast of air during its entire time in the tunnel. Each suc- ceeding truck, until the tunnel is filled and stabilized, will be sub- jected to abnormal, nonuniform drying conditions. The variation from normal drying conditions will, however, decrease as the tunnel becomes filled. As a result, the product dried during the starting-up period may be scorched and ruined. In theory at least it is possible to schedule the drying in accordance with a predetermined time-temperature drying curve and adjust the recirculation of air to produce a constant wet-bulb temperature throughout the initial charging. The curves could be established from similar dehydrator runs or from published data or pilot tests. A typical time-temperature drying curve is shown in figure 47. This precise method is, however, impracticable, because it is impossible to adjust tunnel temperatures rapidly to the desired levels. Theoreti- cally, as many temperature adjustments should be made as there are cars in the tunnel. VEGETABLE AND FRUIT DEHYDRATION 85 Either of two approximate methods can be used satisfactorily. With one method trucks are introduced into the drier at a faster rate than is normal, and the time interval between trucks is progressively increased until normal operating conditions are reached. Although the method produces fairly uniform drying, it is limited in applica- tion. For instance, a plant with a large number of driers will be faced with a formidable scheduling and coordinating problem. In order to keep up with the initial demand, the processing line must 140 160 12 4 (VT 130 150 S19 Ww = TEMPERATURE ADJUSTMENT SCHEDULE a 9 3 FOR STARTING UP COUNTERFLOW = DEHYDRATERS (SEE TEXT) a S 120 140 8 TEMPERATURE OF AIR ENTERING TRUCK a NEAREST LOADING END TEMPERATURE DROP AGROSS TRUCK NEAREST THE LOADING END 10 130 6 2 CABBAGE POTATOES MOISTURE CONTENT (Z/ (LBS. WATER PER LB. BONE-DRY SOLIDS) fe) ie) ~ 0 2 3 4 5 CABBAGE w wo 2 4 6 10 POTATOES Coo aS TIME (HOURS) or zo oa Figure 47.—Typical drying curves for counterflow tunnels. be speeded up temporarily, or the prepared product must be accumu- lated over a period of time. If this first expedient is impossible, a second may be used. This method consists of the application of a modified drying schedule, and produces acceptable results. It is apparent from figure 48 that the greatest drop in dry-bulb tempera- ture occurs at the wet or loading end of the drier. Therefore, in modifying the temperature schedule, most of the adjustment should occur during the initial stages of the starting-up period. With this as the basis and by reducing the number of temperature adjustments to a minimum, a rough approximation of the time-temperature curve is established as shown by the dotted graph. Since the drying curves for all products are basically similar in shape, this simplified graph can be used in starting most counterflow dehydrators. The success of 86 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE this method is predicated upon accurate estimation of hot- and cold- end temperatures. In preparing a schedule, a suitable hot-end dry-bulb temperature is selected and a cold-end temperature is computed or estimated. (See section on principles of dehydration.) A normal retention time is estimated, based on past performance; this time is the same as that required to fill the tunnel under normal scheduling. The initial charging temperature is then located on the curve as equal to the ies \ -—Assumed Dry-Bulb Temperature \ for Shutting Down Parallel- Flow Stage | — 180 170 10 -\— 160 - Actual | Dry-Bulb | Temperature— 150 140 “Assumed Ory-Bulb Temperatur 4 eos for Starting up Counter- DRY-BULB TEMPERATURE (°F.) 130 MOISTURE CONTENT/7) (LBS. WATER PER LB. BONE-DRY SOLIDS) ron) 3 flow Stage ! Q Moisture | 120 Content | I = nie) SECOND || FINISHING STAGE STAGE PARALLEL |e FLOW) ee (BIN FINISHER) Figure 48.—Typical drying curves for multistage dehydration. expected cold-end temperature plus a third of the expected over-all tunnel-temperature drop. A second control point is similarly located on the curve in a position corresponding to a lapse of 10 percent of the retention time. The temperature at this point will be equal to the initial charging temperature plus another third of the difference between the expected hot-end and cold-end temperatures. Similarly a third point is spotted at a time position corresponding to a 30 per- cent lapse of the retention time. At this point, the tunnel temperature is brought up to full normal hot-end dry-bulb temperature. In operation this seemingly complicated program is easily carried out. For example, consider a 10-truck counterflow dehydrator being used to dry potatoes. Previous runs under similar conditions in- dicate that the drying time will be approximately 10 hours when the hot-end temperature is 160° F. and the cool-end 130°. The initial temperature should therefore be 130° + (160° —180°/3) or 140°. At the end of one hour—that is, after 10 percent of the estimated drying oC” ii Si a. ae VEGETABLE AND FRUIT DEHYDRATION 87 time has elapsed—the set temperature is raised to 140°+ (160°— 130°/3) or 150°. At the end of 3 hours, corresponding to 30 percent of the estimated drying time, the temperature is again raised to the normal hot-end dry-bulb temperature, 160°. During this starting-up period, loaded trucks are introduced into the drier at their normal rate. The wet-bulb temperature within the drier should be maintained at or near its normal operating value. This means that a large proportion of the air must be recirculated at the beginning, decreasing in amount as more trucks are introduced. Ending a run on a counterflow tunnel dehydrator normally does not present any difficult problems. The only effect of terminating the supply of raw product isa ee decline of the wet-bulb temperature. This condition will probably not result in injury but will carry the product to a lower moisture content than desired. To prevent this, the recirculation damper should be readjusted to maintain normal wet- bulb conditions. This may also be accomplhshed by speeding up the removal of the final trucks or slightly lowering the tunnel tem- peratures. Parallel-Flow Tunnels in Multistage Driers The parallel-flow tunnel is never used as a single-stage vegetable de- hydrator, because of its poor drying characteristics at the finishing end. Therefore it will be considered only as one stage of a multistage de- hydration unit. The product is usually dried in that stage to a mois- ture content of about 50 percent (Z7’=1.0). For various products, this would correspond to removal of 75 to 93 percent of the original mois- ture and hence, if we were to refer to the time-temperature drying curve as shown in figure 48, the range would be confined to the fairly steep part of the curve which indicates the relatively rapid drying range. In starting up a parallel-fiow tunnel, the hot-end tunnel temperature is brought up to normal operating temperature, and then the loaded trucks are placed in the tunnel on the normal operating schedule. For temperature-sensitive products and especially those that are susceptible to case hardening, care should be exercised by the operator to adjust the recirculation damper so as to maintain normal operating wet-bulb conditions. During the initial loading, the drying load is ght. Hence, greater than normal recirculation must be practiced to keep the wet-bulb temperature at a reasonably high level or else the product may be permanently injured as a result of excessive drying of the outer product surfaces. : When operation is to be stopped, care must be exercised to prevent damage due to excessive drying and scorching. When a run is ended and filled trucks are replaced with empties, the wet-bulb temperature will begin to decline if countermeasures are not taken. This is oc- casioned by the decrease in drying load. The dry-bulb temperature, being automatically controlled, will remain constant; hence, the last filled car will be subjected to a constant high temperature and a con- tinually falling wet-bulb temperature during its passage through the tunnel. The effect, unless controlled, will be to produce higher and higher drying rates within each successive car of the last tunnel load. Scorching, discoloration, case hardening, and other injury may result. At best the product will not be uniformly dried. Adjustment is carried out in the following manner. The hot-end 88 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE dry-bulb temperature is decreased, to simulate normal drying condi- tions. This is achieved by dropping the hot-end temperature in suc- cessive steps equal to the average temperature drop across a truck each time an empty truck is placed in the tunnel and until the unit is emptied of all products. The average temperature drop across the truck may be computed by dividing the difference between the hot-end and cold- end tunnel temperatures by the number in the tunnel. Similarly, the wet-bulb temperature can be maintained substantially at a constant level by adjustment of the recirculation damper as each succeeding truck is removed. By reference to figure 48, it will be noted that a straight line between the upper and lower temperatures in the parallel- flow drying range will not be at great variance from the curve that specs temperature. This fact suggests the reliability of the method. Counterflow Tunnel as Second-Stage Drier In multistage dehydration, the counterflow tunnel is almost invari- ably used as the secondary or intermediate stage. The entering product has been predried and usually contains about 50 percent moisture (7=1.0). There is little danger of case hardening; there- fore exposure to normal dry-end tunnel temperatures over prolonged periods of time will result only in drying to lower moisture levels, approaching the equilibrium point as a limit. Since the moisture removal in a secondary stage is comparatively small, the temperature drop in the tunnel is also small. Thus, when the predried product is first placed in the counterflow tunnel, which has been brought up to normal operating temperature, the product will be subjected to a temperature slightly higher than the average for its normal retention time. The result will be a satisfactory product that has been slightly overdried. In the event that excep- tional operating precautions are required with a temperature-sensitive product, the procedure previously outlined for the counterflow tunnel operation can be used. For most if not all products, this procedure will be found unnecessary. No special precautions are suggested for shutting down a counter- flow secondary drier. It is operated at normal temperatures until the product has been cleared from the tunnel. Some fuel may be saved by readjustment of the recirculation damper to maintain normal wet-bulb temperatures. Here again, it is possible to shorten the retention time of the last trucks as a means of preventing ex- cessive drying. CABINET DEHYDRATORS Cabinet driers serve two main functions: (1) In single or multiple units they are suitable for capacities such as 1 to 20 tons of fresh vegetables per day, and particularly when different vegetables are to be handled at the same time, and (2) they are useful for pilot- scale and experimental operations as a means of obtaining drying rates and other data applicable to tunnel drying. Batch dehydrators, of which cabinet driers are an example, thus have a special field of usefulness quite distinct from that of con- tinuous dehydrators. Batch driers can be built as small as desired, for community, institution, farm, or even family use. Since the drier can be shut down completely when a batch has finished drying, it is well adapted to daytime or one-shift operation. Continuous CA RES EE SEE I ee VEGETABLE AND FRUIT DEHYDRATION 89 driers, on the other hand, require a number of hours to start up and shut down, and hence are best adapted to continuous, three-shift operation. The purpose here is to describe several types of cabinet driers and to discuss in greater detail suggestions for the operation of these cabinets as single and multiple units. A typical cabinet dehydrator consists of an insulated structure, square or rectangular in shape, equipped with a fan that forces the drying medium (usually air) through a heating system and dis- tributes it uniformly through one or more stacks of trays loaded with prepared material. The walls and top can be constructed from a number of materials such as plywood, masonite, transite, brick, or hollow tile. Inflammable materials should be adequately pro- tected around the heating system, particularly if the latter is of the open-flame type. Duct turns, baffles, or other means are provided to insure proper distribution of the heated air and thus prevent uneven drying. Adjustable dampers are provided to admit fresh air and to exhaust the moist air. By proper adjustment of the damper a defi- nite percentage of the moist, hot air can be recirculated as a means of conserving fuel, and in addition a control is maintained over the degree of humidity in the cabinet. Dry-bulb and wet-bulb ther- mometers are mounted on the exhaust side of the cabinet and a dry- bulb thermometer on the intake side. Cabinet dehydrators differ in the manner used to heat the air and in the method used to circulate and distribute this heated air through the loaded trays. Some are heated by the combustion of natural gas directly in the circulating air, and under controlled conditions some grades of fuel oil can be similarly used. Others are heated by fin-type steam coils or 1ron-pipe radiators, whereas still others are equipped with one of several types of heat exchangers and are heated with gas, oil, coal, coke, or wood as fuel. With direct-burning gas or oil heat- ers, most of the heating value is utilized, whereas in the indirect heat- transfer systems, only 50 to 75 percent of the heating value is made available. In any case, the heating system should be of such capacity that it will supply to the circulating air from 1,200 to 1,600 B. t. u. per hour per square foot of active tray surface used in the cabinet. A common error is the installation of a heater of insufficient capacity. The actual capacity of the heating unit will depend on a number of factors, such as amount of insulation in the cabinet, air flow, tempera- ture ranges desired, degree of recirculation of the air, and the load to be dried. Most cabinet dehydrators are equipped with centrifugal or rotary fans, of which there are many types; others employ one- or two-blade propeller or radial fans. These fans should be of such size and should be driven at such speed as to deliver an air velocity, measured between the loaded trays, of 800 to 1,000 feet per minute. The prepared vegetables are spread on suitable trays made from either metal or wood with bottoms of hardware cloth. All-wood trays are also used. The trays are usually of two sizes—a two-man 3 by 6-foot or the one-man 3 by 3-foot size. The loaded trays are stacked one above the other, either by sliding them in on guide rails arranged in a metal frame on a movable truck or by stacking them directly on top of each other on the truck without any support. In the smaller cabinets, individual trays are placed on guide rails one above the other. The free opening between trays should be 2 to 3 inches. 90) MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Types of Cabinet Dehydrators To illustrate more clearly how these various parts are assembled to form a cabinet and how they function, brief descriptions of several types of cabinet dehydrators are included. Perhaps the most com- mon type of cabinet drier is one similar to the design in figure 49. This cabinet is equipped with two or three centrifugal fans mounted on a single shaft and propelled by one motor. Air is forced through the fin-type steam coils and distributed through the trays by means Dry- and Wet-Bulb Thermometers Dry-Bulb Thermometer FigureE 49.—Cabinet dehydrator equipped with centrifugal fans and steam-heated fin-type coils. of adjustable louvers. Unless duct turns, baffles, or adjustable louvers are provided in this type of cabinet, most of the heated air will be forced through the bottom row of trays. Dampers are provided to control the amount of recirculation of air and the degree of humidity in the cabinet. reli Ficure 50.—Cabinet dehydrator designed to reheat the air in stages. The cabinet dehydrator illustrated in figure 50 is designed to re- heat the circulating air in several stages, rather than all at once as in the design shown in figure 49. Fresh air is drawn through the in- take damper, picked up by the centrifugal fan, and forced through the left-hand steam coil in the first stage of heating. Partitions are so placed that the heated air passes through a few trays at the top of the truck, then through the heater on the right-hand side, and back through another section of trays as indicated by the arrows. The moist air comes through the bottom row of trays and is carried through a duct to the exhaust damper at the top of the cabinet. Recirculation and humidity control are maintained with this damper. Two ad- vantages are claimed for this design: (1) By repeated heatings fewer pounds of air are necessary to remove a given weight of water, and (2) more moderate dehydrating temperatures can be used, SS ee Ss VEGETABLE AND FRUIT DEHYDRATION OQ] More recently several cabinet dehydrators equipped with propeller- type fans have been designed. One of these designs, which was developed for use by State and Federal institutions, is illustrated in figure 51. Double-walled sides and top are made up in such a manner that the sections can be bolted to form a portable unit, and these sections are insulated to prevent excessive heat losses. A four-blade, laminated-wood propeller, 514 feet in diameter, is driven at 600 r. p. m. FieurE 51.—Cross-blower cabinet dehydrator employing airplane propeller. to deliver 800 to 1,000 feet of air per minute as a suction air flow. Four sets of stationary duct turns are installed to insure a uniform air flow through the trays. Steam-heated fin-type coils or iron-pipe radiators make ideal heating units for this type of cabinet, although direct gas heaters can be used also. An intake-exhaust damper is Air-iniet Damper Air- = a Air-Fiow Outlet Drying@ Zz A aS 4 RS a Figure 54.—Coal-burning, single-truck cabinet dehydrator. The first type, as one might expect, is inefficient and difficult to con- trol. It can, however, utilize a variety of fuels. Temperature regu- lation is difficult even when gas or oil is used. However, complete combustion is not essential for successful performance of the indirect- heated dehydrator. Some smoking can be tolerated, because the prod- ucts of combustion do not come in direct contact with the drying air. If coal or wood is used with this system, a fair degree of control can be obtained by either wasting some of the products of combustion to atmosphere before they come in contact with the heat-exchange sur- faces, or by diluting and cooling the flue gases with outside air prior to their flowing over these surfaces. Obviously these methods of control are wasteful. Although this type of indirect heating has its applica- tions, it is of little value in commercial use. A typical example of a small unit employing this system of heating is shown in figure 54. This unit employs an automatic coal-fired stoker. The second type of indirect system, on the other hand, offers practi- cally ideal control characteristics. The heat-transfer medium is usu- an steam, but may be hot water or oil. This method of heating offers a clean, even source of heat with any type of fuel, either liquid, solid, or gaseous. The boiler or accumulator is a source of heat that can be used instantaneously to cope with variations in demand. Regulation of VEGETABLE AND FRUIT DEHYDRATION 99 temperature within the dehydrator can be accomplished by a relatively simple method, which is applicable over a complete range of dehy- drator sizes. On the other hand the equipment is considerably in excess of that required by other methods, and the over-all efficiency is considerably below unity because of boiler and transmission losses and will in all probability be less than 60 percent. Initial cost, upkeep, and overhead are correspondingly high. Heat exchange surfaces are normally copper-finned coils. Rela- tively large amounts of heat can be transferred to air in motion by a compact heat-exchange surface of this type. However, because of wartime restrictions, copper heating coils cannot be procured at this time and, as an alternate, steel-finned coils are being manufactured. Their procurement is difficult, and the operator may find it expedient to improvise some other surface, such as steel pipes arranged to form a suitable coil. The amount of unfinned pipe required to effect a given heat transfer will be considerably in excess of that required for finned pipes. The size of coil thus formed might well exceed the normal space usually allotted to the heating unit, and may therefore be a limiting factor in the design of the steam-heated dehydrator. A second limitation would be an increase in static pressure caused by pipes or coils, which must be overcome by the fan or blower. To keep the face velocity of the air and hence the static pressure down, the pipes must be properly spaced. If a coil is to be built for use in a dehydrator, it 1s suggested that care be taken in the design to provide ample surface for adequate heat transfer, because the performance of the dehydrator is at stake. Rapid. and complete elimination of the condensate and air is also necessary _to promote coil efficiency. Some large steam-heated dehydrators have been successfully de- signed to use steam for driving the blowers and to utilize the exhaust steam in the heating system. The blowers are driven by bleeder-type turbines, operating at a back-pressure of from 10 to 50 pounds per square inch. Such an arrangement may reduce power costs very con- siderably. Each turbine may be designed and operated so that the steam flow through it will normally be less than the minimum steam demand of the corresponding heaters, so that additional high-pressure steam will always have to be bled into the heating system. Automatic eontrols will then adjust the auxiliary steam supply to satisfy the heater demand without affecting the operation of the blower. TEMPERATURE CONTROLLERS There are two general basic types of temperature controllers—the off-on controller and the modulating controller. The choice between these two is determined by the type of heating system used in the dehydrator. Essentially, the piping arrangement is the same and is independent of the type of controller used, as well as the kind of fuel or heating medium employed. (See figure 55 for a typical arrangement. ) As its name implies, the off-on controller operates a motorized valve in such a way that it is either fully opened or fully closed. There is no intermediate valve position. For this reason, all off-on controllers are essentially instruments with high sensitivity. This type of con- troller is relatively simple to operate, and is inexpensive. It is avail- 100 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE able as an air-, electric-, or direct vapor-operated instrument. Fea- tures such as indicating and recording devices can be included and are desirable for use on vegetable dehydrators. The modulating controller is more complicated than the off-on type and is available as an air- or electric-operated unit. Because of its comparative ease of adjustment and its ability to transmit a modulat- ing force to the motorized valve, the air-operated instrument dominates. In modulating controllers, action is imparted to the motorized valve in such a way that the valve-stem movement is proportional to the difference between the actual tunnel temperature and the set tempera- ture on the controller. Instruments vary in sensitivity—that is, they vary in the amount of movement of the indicating pointer required to cause the motorized valve to change from a fully opened to a fully closed position. Sensitivity is measured in degrees required for maxi- AIR-OPERATED TEMPERATURE CONTROLLER AIR SUPPLY AIR-OPERATED MOTORIZED VALVE FLAME-FAILURE THERMO- STATIC SHUT-OFF VALVE THERMAL ELEMENT OR BULB LOCATED IN DEHYDRATER FLAME - FAILURE SHUT-OFF er, THERMOSTATIC ELEMENT FUEL SUPPLY SS MANUALLY OPERATED =D BURNER GLOBE VALVE FOR LOW-FIRE ADJUSTMENT VicurRE 55.—Typical simplified burner control diagram. mum valve opening. Thus, if the temperature required for full open- ing of the valve is at wide variance with the set temperature, then the instrument is said to have low sensitivity. Conversely, if the vari- ance is close, the instrument is said to have high sensitivity. If the sensitivity of an instrument were carried to its ultimate, it would - become an off-on instrument. Sensitivity should not be confused with instrument response. Re- sponse of an instrument is the ability of the thermal element to trans- mit temperature variations to the controller. It is assumed in sub- sequent discussion that the thermal-element temperature response 1s the same regardless of the type of controller used. Most instruments incorporate means to permit change of sensitivity. The need for this variable-sensitivity device will be discussed in more detail. Every heating system, whether it is for a tunnel or a cabinet dehy- drator, a bin finisher, or a lye vat, has an inherent characteristic that must be fully recognized before an automatic temperature controller can be properly selected. Generally, if the temperature response of the system is rapid, the controller should be of the modulating type; if it is sluggish, the controller can be either the off-on or sensitive modulating type. In the latter case, the off-on controller, being the less costly of the two, is generally used. In order to illustrate these general rules, the temperature-control problems of several different heating systems will be analyzed below: Suppose that a counterflow tunnel is being used as a finishing drier for onions and that it is being heated by an open gas flame. What type of controller should be provided? Temperature regulation for onions is critical. Excess temperatures may cause discoloration of the ed ' wo wo TS ST A kee VEGETABLE AND FRUIT DEHYDRATION 101 finished product, whereas low temperatures retard the drying. For optimum conditions, the dehydrator should be operated at the highest permissible temperature that can be controlled within close limits. Because of the small heat inertia of the open-flame method of heating, the use of an off-on controller would be unsatisfactory. Up- ward variations in flame intensities are immediately manifested by increase in tunnel temperatures. With extreme care in operation, the manually operated globe valve (fig. 55) can be opened to maintain a low-fire condition, so that the resultant flame, without action from the temperature controller, will produce a temperature within the tunnel only a little lower than the set temperature. The off-on con- troller may then be relied upon to admit periodically additional fuel to bring the average tunnel temperature to the desired condition. However, because of the low heat inertia of the heating system, even a diminished fluctuation of fuel flow may permit the temperature to fluctuate so widely as to scorch the onions at the dry end of the tunnel. An effort can be made to improve this condition by adjusting the manually operated globe valve to admit more fuel, so that the low-fire temperature level within the tunnel is raised. The flow in the modu- lating valve branch can be restricted, so that variations in flame in- tensity will be minimized. However, constant vigilance will still be necessary to prevent excessive tunnel temperatures during tunnel charging and under other varying load conditions. When a modulating controller is placed on this system, the opening of the modulating valve will be related to the heat demand. Thus a comparatively even tunnel temperature will be realized. Obviously, the globe valve can be used to desensitize the action of the controller. However, care should be taken in its adjustment for low-fire condi- tions so that the resulting temperature is not in excess of that desired in the dehydrator under minimum-load conditions—that is, when part of the tunnel is shut down for the changing of cars. As a second example, let us suppose that the same dehydrator em- ploys a confined flame unit and a combustion tube similar to that shown in figure 53 of the preceding section. The same close regula- tion of temperature is required. Let us analyze this example and as- sume that an off-on controller is used. The manually operated globe valve shown in figure 55 is adjusted so that the low-fire temperature level is approximately 10° F. below the set temperature of the con- troller. A small drop in temperature below the set temperature will open the motorized valve completely, and the burner will be under high-fire conditions. The immediate reaction will be to store up heat energy in the refractory lining of the combustion tube. There will be a small time lag before full flame intensity is manifested by a maxi- mum increase in heating effect on the drying air. Thus the large heat inertia of the heating tube has a dampening effect on the fluctuations of temperature produced by the off-on controller. Variations in tem- perature regulation will, in all probability, be within acceptable limits. Further dampening of the maximum amplitude produced by the off-on controller can be obtained by a fixed throttling device placed in series with the motor-operated valve. As a third example, let us assume that the same counterflow finish- ing dehydrator used with onions is steam-heated. Close temperature regulation is again essential. The problem is, again, to,select a suit- able type of controller. Admission of large quantities of steam to the 102 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE heating coil will produce a delayed heating action and the tunnel temperature may exceed the set temperature. If the steam-supply piping and controls were similar to those shown in figure 56, the man- ually operated valve might again be used to throttle the steam in order to maintain a temperature level within close but safe approach to the set temperature. If we assume that 125° F. is the safe tempera- ture level for a set temperature of 135° under this condition of opera- tion, the sensitivity of the modulating controller must be adjusted so that at a given temperature, for example 120°, the motorized valve would be fully opened. At 135° the valve would be fully closed. It will be seen, then, that at some point between 125° and the set AIR-OPERATED TEMPERATURE ——7 CONTROLLER AIR SUPPLY AIR-OPERATED MOTORIZED VALVE yO] O —/ THERMAL ELEMENT OR BULB LOCATED IN DEHYDRATER SHUT-OFF Oe) STEAM SUPPLY — MANUALLY eect GLOBE VALVE FOR BYPASS FIguRE 56.—Typical simplified steam heater control diagram. HEATING COIL temperature, the system would come to equilibrium, even though this temperature is obviously lower than the set temperature. The opera- tor must recognize this fact, and readjust the controller indicating point and recording arm so that they indicate and record the new equilibrium temperature; otherwise his records and the true tunnel temperatures will be at variance. If the modulating controller is of the simple type, this action must be carried out as often as the load in the tunnel changes. Instruments are available, however, which perform this function automatically. When this feature is included on a modulating controller, it is said to have re-set features. Other special features are obtainable on temperature-control in- struments. One type deserving special mention is the cam-operated time-temperature controller. This unit includes a clock-driven cam that moves the control arm in accordance with some prearranged temperature schedule. The controller can be of either the off-on or modulating type. This type is especially suited for use on a cabinet dehydrator in reproducing scheduled temperature variations. In addition to the control of dry-bulb temperature, specially adapted sensitive elements fitted with wicks or other wetting devices can be used for indicating, controlling, and recording wet-bulb tempera- tures. An important matter is the care of wicks and wet-bulb devices. Care should be taken to keep these surfaces clean. It is preferable that the water used in the appliance be distilled, since water contain- ing calcium and other salts will leave a residue after the water has evaporated. ‘These salts contaminate the wick and cause the instru- ment to give a false reading. If hydrant water is used in wetting the bulb, frequent attention should be given to cleaning or renewing the wick. Multiple-pen units can be obtained for such purposes as indi- cating, controlling, and recording dry-bulb temperatures and wet- bulb temperatures simultaneously. Many motor-operated devices, besides valves, are available for use with temperature controllers. These can begused for the operation of dampers and other devices necessary in the automatic operation of a dehydration unit. Sr VEGETABLE AND FRUIT DEHYDRATION 103 MECHANICAL MOVEMENT OF AIR IN DEHYDRATORS Fans generally used for the mass movement of air in dehydrators can be divided into two classes: (1) Propeller, axial-flow, or disk fans, and (2) centrifugal or rotary impeller fans. Disk fans are generally limited to delivery of air against relatively low static pres- sures, although some special designs are capable of developing a pres- sure of 10 inches of water or higher. Centrifugal fans, on the other hand, can be used against either high or low static pressure. Instal- lations in which low-static-pressure disk fans are suitable include cabinet sdriers, low-performance tunnel driers, and belt-conveyor driers. Centrifugal fans can be employed in addition, on high- performance tunnel driers, bin driers, and in general on installations where the static pressures are substantially higher. A centrifugal fan consists essentially of an impeller or fan wheel and a casing or housing. The design of these parts can be varied to suit peculiarities of the load. The casing has either of two functions or both; one is to transform velocity pressure into static pressure and the other is to collect and conduct the air to the point of discharge, or fan outlet. Centrifugal or Rotary Impeller Fans Centrifugal impellers can be roughly divided into three classes: (1) Those with forward-curved blades, (2) those with straight radial blades, and (3) those with backward-curved blades, sloping away from the direction of rotation. Impellers thus formed are termed slow-speed, moderate-speed, and high-speed types, respectively. Their ranges may be overlapping; therefore, each of these types may be used for the same work. Fan characteristics in each group may, however, be varied by changing the depths of the blades. Thus, fans with deep blades will develop relatively high static pressures, whereas fans having short or shallow blades will handle large volumes at low pressures. This latter type is sometimes designated as a volume fan. Although the forward-curved-blade centrifugal fan is widely used on tunnels originally designed as fruit dehydrators, they are not best suited for that purpose. The probable reason for their original adoption lies in the fact that the relatively low shaft speed required for a given discharge was applicable to use with steam or gasoline engines. The forward-curved fan has many characteristics that are un- suited to dehydrator service. First, the fan power increases when the static pressure of the fan is reduced and therefore the fan absorbs maximum power at free discharge. However, at the normal operat- ing static pressure, which is usually a condition near the point of maximum mechanical efficiency, the fan power may be less than one- half of free discharge power. Second, the fan has an unstable capacity or volume characteristic when operating at static pressures approach- ing the shut-off condition. Third, the slow speed of rotation necessi- tates an appreciable speed reduction if a standard electric motor is used to drive the fan. Systems having variable static pressure loading as found in tunnel dehydrators and bin driers are best served with fans having back- ward-curved blades. This type of fan, which is sometimes known as a limit-load or nonoverloading fan, has operational characteristics 104 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE which, at any given speed and specific volume, prevent it from absorbing more than a limited amount of power. This is of special importance when the fan is used in dehydrating systems where the static pressure in the tunnel drier may be suddenly decreased as a result of opening of doors, or, in the case of a bin drier, where the static load can be changed by removal of the product from the bin. Propeller, Axial-Flow, or Disk Fans Unless very special designs are used, the disk-type propeller fan is generally suitable for use only where the static pressure 1s less than 1 inch of water pressure. There are two general classes of disk-type pro- pellers: Those with thin, steel, curved blades, and those with air-foil sections similar to those used on airplane propellers. The former type is available commercially and the latter can be readily built of wood. Irrespective of the type used, the entrance and exit cones should be streamlined and the propeller shrouds should be built with minimum clearances in order to secure the highest efficiencies. High-speed disk fans are likely to be very noisy, and this factor may restrict their use. When properly applied, they are efficient and con- stitute a relatively cheap means of obtaining a high volume of air movement with a minimum of equipment. Although simple in appearance and operation, propellers are intri- cate in design. An improperly designed propeller may be extremely inefficient, wasteful, and sometimes very dangerous. Properly de- signed, however, they can be readily constructed and are efficient and safe. Those who contemplate their use should make use of publications that contain adequate design information (4, 8, 17,23, 25). Application of Fan Laws in Choice of Fan The proper choice of fan size and the prediction of its performance under specific conditions requires a knowledge of basic fan laws. These laws are applicable to all geometrically similar centrifugal or disk- type fans. It should be understood that two fans are geometrically similar only if they are alike in all proportions and in all details re- gardless of size. B=barometric pressure. D=diameter of fan wheel. hp.=fan power, horsepower. Ce ae or volume of air per unit of time, cubic feet per minute C.-M)’. Ps=statie pressure of fan, inches of water. r. p.m.=fan speed, revolutions per minute. T= absolute temperature of air (°F.+460) or (°C.+ 278). V=specific volume of air, cubic feet per pound (specific volume of air at standard conditions for fan tables is approximately 13.3 cubic feet per pound of air). 1. For a given fan speed and a constant specific volume of air when the size of fan varies: a. Qa D3 b. Psa D? ce. hpa D> 2. For a given fan size and a constant specific volume of air when the speed of the fan varies: Ae ce (Tae psstln) Deeescc (np: min) e c. hpa(r. p. m.)3 eee VEGETABLE AND FRUIT DEHYDRATION 105 3. For a given fan size and a constant static pressure when the specific volume of the air varies: (r. p.m., Q, hp.) (VT, ae v7) 4. For a given fan size, capacity and speed when the specific volume of air varies: (hp., Ps) (e B, n) 5. For a given fan size and a constant amount of air by weight when the specific volume of air varies: ae (@ ors ps me, Pyx(V, * r) Sia | b. (hp.) (V2, BR’ oe) In order to apply these fan laws it is necessary to understand the nature of the static pressure load of the fan as governed by the re- sistance of a given dehydrator system. It is therefore well to add the following laws: 6. For a constant specific volume of air, the static pressure of a given dehydrator system varies directly as the square of the air velocity. Ps « U?*, when V=constant. Ps=static pressure of the dehydrator system and fan, inches of water. U=the air velocity either in the dehydrator or at the fan outlet, feet per minute. V=specifiec volume of air, cubic feet per pound. 7. For a given dehydrator system and a constant amount of air by weight when the specific volume of air varies, the static pressure of the system changes directly as the specific volume. Ps x YV, when W=constant. W=total weight of air per unit of time, pounds per minute. A comparison of laws 6 and 7 with the fan laws shows that Nos. 2 and 5 may be directly applied to any given dehydrator system. ‘Their use is best demon- strated by specific examples as follows: Heanple 1—Assume that a fruit-drying tunnel is to be converted into a vege- table dehydrator. The measured air velocity in the drying section is 400 feet per minute. The observed fan speed and power are 700 r. p. m. and 10 hp., respectively. If the same fan is used and the specific volume of air remains un- changed, what fan speed is needed te furnish an air velocity of 600 feet perminute in the drying section? How much power will the fan absorb at the new operat- ing condition? Applying law 6 the static pressure of the dehydrator system and fan both vary with the square of the air velocity. Therefore (from 6 and 2b) the static pres- sure at this new condition will be (600/400)’?=2.25 times the original static pressure. The new fan speed must be 700% (600/400) =1,050 r. p. m. since (from 2a) the capacity is directly proportional to fan speed. Likewise, the fan power for this condition (from 2c) is10 X (500/400)*=33.75 hp. Hzample 2.—Suppose 4,000 pounds of air per minute is necessary for the removal of moisture from a given dehydrator; the specific volume of air at the fan is 16.7 cubic feet per pound and the static pressure of the dehydrator system using standard air conditions is 1.45 inches of water. If standard fan tables are used, what size fan shall be chosen for this dehydrator? What is the correct fan Speed and how much power will the fan absorb? Assume there is no air leakage. The specific volume of standard air is approximately 13.5 cubic feet per pound of air. Therefore the rated capacity of the fan will be 4,000 X 13.3, or 53,200 ce.f.m. The corresponding rated static pressure will be 1.45 inches of water. Using the standard fan tables, assume it is found that a fan of suitable dimen- sions operates at a speed of 475 r. p.m. to deliver the rated volume of air against the specified static pressure and the table lists fan power as 19 hp. = 4 ' 106 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Applying law 5a, both capacity and fan speed are directly proportional to the : ay specific volume of air. Hence the proper fan speed is 475 X 13.3096 r. p.m. The power absorbed by this fan (5b) will vary directly as the square of the specific volume, 19 X (16.7 */13.3) =29.8 hp. It is now possible to predict the actual operating capacity and static pressure of the fan by applying law 5a, which shows that both vary directly as the spe- = 16.7 cific volume. Therefore, the actual capacity is 53,200 X 13 366,500 clfom: 7 and the actual static pressure ofthe fan is 1.45 X 1337 1-81 inches of water. BIN-TYPE FINISHING DRIERS The bin finisher provides an efficient, relatively inexpensive means of finally drying vegetables to low moisture content. It is compact and has a comparatively high heat use. If it is used as final drier, better use can be made of the preceding dehydrator, whether it is a tunnel, cabinet, or belt type. All of these types use large areas of loading space for the reduced volume of thinly spread, partially dried product. If a bin drier is added to a system and, further, if additional heat- generating capacity is available in the preceding drier, then the plant capacity is potentially increased. The shortened retention time re- quired by the preceding driers in reducing the moisture of the product only to a condition satisfactory for bin drying (approximately 10 to 15 percent moisture) permits the system to handle additional tonnage. This additional tonnage of course requires additional heat. Very little is known of the performance characteristics of bin driers. Their use as an expedient for increasing plant output or for carrying the dryness of the product to low moisture limits is, however, of unquestionable value. As a secondary aid, the bin driers can be used to supplement the plant’s finished-product storage facilities. Being relatively simple and easy to build and operate, finishing driers are becoming standard equipment in most dehydrating plants. Types of Bin Driers There are two basic types of bin driers—the batch-process bin drier and thé continuous bin drier. The continuous bin drier usually con- sists of a single unit, whereas the batch-process drier is almost in- variably a multibin drier. The continuous bin finisher, because of its operating characteristics and its size limitations, is generally confined to use in small plants. The unit consists of a large storage bin designed for working depths ranging approximately from 3 to 8 feet. Drying air is introduced through louvers at or near the bottom of the bin. The bin must be of airtight construction except for an exhaust port located in the top or at some other position above the active bin level. A gate is provided in the bottom of the bin to permit gravitational removal of the prod- uct in progressive layers. To facilitate removal and to prevent arch- ing, mechanical or electrical agitating or shaking devices are some- times provided. Figure 57 shows a typical design for a continuous bin finisher intended for use with onions. Note the location of the agitator. ‘When the bin is in operation, the partially dried material is allowed to enter in a steady stream, at such a rate that it keeps the bin com- — % VEGETABLE AND FRUIT DEHYDRATION 107 pletely filled. A draft of heated dry air is forced through the louver inlets at the bottom and the product dries progressively upward; there- fore, the dried product at the bottom is removed at the same rate as the bin is loaded from the top. The batch-process bin finisher is somewhat similar in design. A gate is located at the bottom to permit dumping of the entire bin. Three or more bins are usually required, as will be explained later. In operation, the entire bin is filled with the partially dried product; the PRODUCT LOADING fs DOOR {— PARTIALLY DRIED PRODUCT HOT DRY AIR SUPPLY DUCT PRODUCT UNLOADING DOOR FIGURE 57.—Continuous-bin finisher. material dries progressively upward, and, as soon as the material near the top is dried to the required moisture content, the entire bin is dumped and a new cycle is commenced. Operation of Bin Driers During normal operation, the product, as loaded into the bin drier, contains not over 15 percent moisture. This partially dried product is of sufficient stiffness so that it will not crush, and will form a solid - mat. Therefore care should be taken not to tamp the product during loading operations. The charging depth may range between 2 and 6 feet. Under this condition of loading, air can be readily forced through the material by standard-design blowers. As a product loses its moisture, its rate of drying becomes slower and slower. The latent heat necessary for evaporation will be required at a proportionally slower rate. Since the air is the source of heat supply and transfer medium, the required quantity and velocity of air become correspondingly smaller. The air bathes the product and carries away the diffused moisture as it is released. The principles involved are discussed in publications by Furnas (15) and Gamson, Thodos, and Hougen (16). The depth of bed will determine to a large degree the batch retention time. Usually bins are operated at the upper limit of the usable temperature range. If excessive bed depths are used, the dried product on the bottom may be injured by prolonged exposure to maximum temperature. Therefore, to reduce the batch-retention time, relatively high velocities should be used. An undesirable condition that may arise is illustrated by the follow- ing example: Let us assume that a deep-bed bin drier is being used to finish potatoes. Assume further that the potatoes have been scraped from drying trays of the preceding stage in a room having a temper- 108 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE ature of 60° F. It is probable, then, that when the bin is filled with these potatoes, the average bin temperature will be in the neighborhood of 60°. Assume that air at a relatively low velocity and temperature is circulated through this bed. The air will pick up moisture and gradually drop in temperature until, at a certain point in the bed it can no longer dry the product. From this point, the cool product will eradually reduce the air dry-bulb temperature until, at a point within the bed, the air becomes saturated. Further penetration of the air, while in contact with the cold bed, will reduce the air temperature PRODUCT LOADING DRAG CONVEYOR AIR-EXHAUST => ae ie ai DUCT SV A S Ws / SN N ge] BRAN s Rett ARTIALLY RIED PRODUCT aera HOT DRY AIR Nef SetetN=—T SUPPLY DUCT ov UNLOADING GATES SS Figure 58.—Multibin finishing drier. below the dew point; thus moisture will be condensed upon the product. If this condition arises, the wetted product may be injured. The condition can be remedied by increasing either the temperature or the velocity or both, by decreasing the bed depth, or by providing auxiliary internal bin heating devices. An operator can detect faulty bin operation by taking samples from the uppermost surface of the product periodically throughout the dry- ing cycle and analyzing them for moisture content. The time intervals should be fairly close because the region of condensation, which is hkely to be fairly shallow in depth, might otherwise not be detected. Regardless of the velocity and temperature of air employed, a bin drier will ultimately carry the product through the full depth of the bed to a condition approximating equilibrium with the entering air. This, of course, is based on the assumption that heat loss due to radia- tion, convection, and conduction from the outer surfaces of the bin are relatively small. Batch drying is intermittent in operation; therefore several bins are usually provided to permit continuous drying operations. At least three bins are usually employed, and while one is being loaded the others may be used in drying or unloading. Figure 58 shows a suitable multibin finisher design. Any number of units in addition to the three shown can be provided to suit plant capacity. Determining Needed Capacity To illustrate a method of determining required capacity, we may consider a small onion-drying plant producing approximately 225 pounds of flaked onion per hour. Laboratory tests indicate that a 5-foot depth of product, can be dried from 12 to 4 percent moisture VEGETABLE AND FRUIT DEHYDRATION 109 in approximately 8 hours. The plant will accommodate a bin not over 10 feet wide. What should be the bin proportions? The bulk density of nearly dried flaked onions is found to be ap- proximately 15 pounds per cubic foot. Since one continuous bin is to be installed, it must have sufficient volume to accommodate the entire output of the plant for an 8-hour period. Its capacity can be com- puted by multiplying the retention time in hours by the plant output per hour and dividing by the bulk density of the product. Thus the volume of the bin equals 8X 225/15 or 186 cubic feet. With a bed depth of 5 feet, the cross section of the bin must be slightly over 27 square feet. Since the bin is to fit into a 10-foot-wide space, a bin 4x75 feet deep may be satisfactory. Let us suppose that a unit of this size has been built, and that sub- sequent tests show that the 8-hour drying time is too short. If the highest permissible air temperature for drying onions is already in use, the bin capacity can be further increased by resorting to higher air velocities. If, however, the fan is operating at full capacity, the only remaining alterable condition is to reduce the moisture of the entering product by increasing the retention time in the preceding stages. Wee oaceil drying in the preceding stage affects the bin capacity in either of two ways. First, the longer retention period in the pre- ceding stage decreases the input to the bin drier. Second, the reduc- tion in total moisture to be removed by the bin drier shortens retention time here. Hence, under a particular ratio of primary to secondary retention time, a balance will be reached. Although predictable with a fair degree of accuracy, the optimum ratio can best be found by trial and error. The balance established under one set of drying conditions will not necessarily hold for all other drying conditions. For example, the operating balance established with daytime initial drying and night- time bin drying may not be suitable if the drying cycle is reversed. This change in retention ratio from day to day is obviously undesir- able. However, a safe working ratio can be established, based on the most unfavorable conditions, and thus a workable solution can be ob- tained. The effect is to produce extra drying some of the time. How- ever, the ease of operation on a fixed schedule will in all probability offset the expense of the additional heat. The loss of product weight due to extra drying will be negligible. As an example of multibin driers, let us assume that the manage- - ment of a medium-sized plant is considering the installation of a mul- tibin finisher to increase the plant capacity. The plant is to be able to process 300 pounds of nearly dry potatoes per hour, having a mois- ture content of 12 percent. Packaging is to be done only during one 8-hour shift. How large a multibin finisher will this plant require? The bulk density of partially dried potatoes is found to be 15 pounds per cubic foot. Material will then be delivered to the bins at a rate ot 300/15 or 20 cubic feet per hour, or 160 cubic feet per shift. Assume that previous experience indicates that 12 hours are required to re- duce the moisture content of potatoes from 12 to 4144 percent. The bins, therefore, must have a minimum total volume of 1220 or 240 cubic feet if packaging operations are to be continuous. The bins selected for use are to have a working volume of 80 cubic feet. The 110 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE minimum number of bins required would then be 240/80 or three bins. However, since the product is to be packaged only during the 8-hour- _ day shift, three bins are inadequate. For purposes of computation, let us consider the period immediately after the 8 hours of packaging. At this time, at least three bins are filled or partially filled because of the 12 hours required in the bin drier. In addition to these three bins, four more are required to dry and store the product that will be produced during the coming 16 hours when no packaging is done. One additional bin must also be provided for use during the time the first bin is being emptied the fol- lowing morning. Therefore it is obvious that at least eight bins are required for this operation. Little is known about drying rates of various vegetables in bin driers, but, as a basis for determining the approximate number of bins required, it is safe to assume that the retention time will be from 6 to 9 hours for an air velocity equivalent to 80 or 100 cubic feet of air per minute for each cross sectional square foot of active bin capacity. The maximum Incoming air temperature should not exceed 10° F. below that recommended for the dry-end temperature in a counterflow de- hydrator for the same vegetable. It may be necessary with vegetables that are very sensitive to heat, to keep the temperature even lower | than this. If final results indicate that the capacity of the finishing unit is below the capacity expected, the operator need only increase the re- tention time in the preceding drying stages until a working balance is reached. This condition was illustrated by the previous example. Air Desicecators Bin drier operation will vary considerably between humid and arid regions. If high relative humidities are prevalent in the plant locality, it will be almost essential to employ some means of drying the air before it is introduced into the bin finisher. A unit for this purpose is called an air desiccator or air dehydrator. The former is the pref- erable designation, because the other may lead to confusion. One of the most satisfactory means of drying air is through the use of a chemical adsorption or absorption drier. Units of this type are capable of delivering air at fairly high temperature and low ab- solute humidity. This method obviates the necessity of reheating the dehumidified air, as would be required by a dehumidifying unit using refrigeration. ‘This factor may be of considerable importance in the selection of an air-desiccating method. When an air desiccator is used with a bin finisher, it may become desirable to recirculate all or part of the drying air. In multibin oper- ation, the air expelled from each bin will have a different absolute humidity, because the products in the bins are at various stages of dryness. In practice, however, the air from each bin is discharged into a common return duct. If the absolute humidity of the mixed air in the return duct is less than that of the outside air, then it should be recirculated; if not, it should be entirely wasted to atmosphere. On “borderline” installations this condition may vary from day to day; hence repeated daily checks should be made. memes peer 2 VEGETABLE AND FRUIT DEHYDRATION Wi Sources of Heat The air supply to bin driers is preferably heated by steam. Direct-fired units are not recommended, because the product may acquire a disagreeable taste from the flue gases. There is also a fire hazard with the open flame, which may cause dust explosions. The amount of heat required by a bin drier is relatively small; hence, the heat wasted by not. recirculating it is negligible. The problem of recirculation should be decided on the basis of absolute humidity only. OTHER TYPES OF DEHYDRATORS AND THEIR USES Patent files and other publications describe numerous designs of food dehydrators. The truck-and-tray tunnels, the truck-and-tray cabinet, and the belt conveyor are described in previous pages of Figure 59.—Truck carrying trays of sliced onions enters a tunnel dehydrator. this manual and their operation has been discussed in detail because of their extensive use in the dehydration of vegetables and fruits (fig. 59). Other types of dehydrators and their uses are described below. Spray Driers Spray drying is based upon the general principle that moisture is removed rapidly from a finely divided substance suspended in an atmosphere capable of absorbing the water vapor and of supplying 112 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE the heat necessary for its evaporation. Under these conditions it may be assumed that rapid evaporation maintains the particle tem- perature at or near that shown by a wet-bulb thermometer until dry- ing is substantially complete, after which the powdered product is removed from the heated zone so quickly that thermally induced damage is avoided even when high dry-bulb temperatures are em- loyed. F The necessary situation is attained in practice through the use of a large drying chamber, usually cylindrical in form with a cone bot- tom, but sometimes in the shape of a box. The cylindrical form ranges in commercial practice up to about 20 feet in greatest diam- eter and 40 feet in over-all height. Collectors that accumulate the powder are attached to, or built into, the drying chambers. Fans, heaters, and baffles induce a rapid whirling flow of hot air or flue gas through the drying chamber, collector, and exhaust duct. The product to be dehydrated is introduced into the hot-air stream near its inlet in the form of a fine mist produced by a spray nozzle. Moisture is removed in the space of a few seconds and the resulting fine powder is deposited in the dust-collection system, from which it is removed, cooled, and packaged, usually in a continuous operation. The spray drying process is particularly adapted to the production of dried milk and eggs. Certain vegetable purees are readily dehy- drated for use in soups, baby foods, and similar products. Tomatoes and most of the fruits have not thus far been successfully spray-dried on a commercial scale, presumably because of their high content of low-melting, hygroscopic sugars which impart to the dried product a tendency to stick to the equipment rather than to flow smoothly through it. Vacuum Driers Water can be evaporated from moist substances at relatively low temperatures in a vacuum, and substances that are extremely sensitive to heat, such as serums, are accordingly dried under these conditions. If the vacuum is sufficiently high the temperature may be lower than the freezing point. The process has the additional advantage that there is little or no contact with oxygen during the drying. Equipment for vacuum processing must be of heavy construction to withstand the pressure of the atmosphere, and all closures must be fitted with extreme precision to avoid leaks. For this reason equipment is expensive. Most types of dehydration equipment can be built to operate under vacuum. Rotary-drum driers and rotary-kiln driers have been so used for some time. A type of cabinet dehydrator adapted for vacuum operation is perhaps used more extensively for food products than others. It consists of a cast-iron shell in which shelves internally heated by steam, hot water, hot oil, or electricity are placed. Trays loaded with material to be dehydrated are placed on the shelves. Heat transfer is slow and inefficient in the absence of a vehicle such as mov- ing air, and time for complete dehydration is therefore relatively long. A recent development in this field employs a specially designed fan to circulate the attenuated water vapor present in the dehydrator over a heater and then over the charge. In this way evaporation is greatly accelerated and the equipment, though operating batchwise, may prove to have a substantial capacity in terms of dried product. Another recent development is a continuous tunnel operating under 2 a ae Se eee Sir VEGETABLE AND FRUIT DEHYDRATION 1S vacuum. Neither of these devices has as yet been extensively used in commercial dehydration. The principal handicap to practical use of vacuum driers for dehy- drating fruits and vegetables, namely, the difficulty of transmitting heat to the product rapidly enough to evaporate a large quantity of water effectively, can be largely overcome if the drier is used only for the finishing step in a multistage dehydration process. Prelim- inary steps may remove 95 percent or more of the moisture originally present in the material; from that point on the vacuum drier is able to operate very effectively. A combination process of this nature is being used commercially for the final dehydration of certain fruits, which are first dried in hot air to the usual moisture content for dried fruits—about 20 to 25 percent. Dehydration of fruits or vegetables to final moisture contents of only 1 to 2 percent can be accomplished in one of two ways—by the use of highly desiccated air as the drying medium or by the use of a vacuum drier. Choice between these two methods may be determined by availability of the equipment or by the sensitivity of the nearly dry product to oxidation by warm air. Rotary-Drum Driers Drum drying provides means for rapid dehydration of solutions, slurries, purees, and the like, spread in a thin layer on a heated sur- face from which the dried product is removed continuously. The equipment consists of an internally heated revolving drum, upon the outer surface of which the substance being processed is spread at one point while the dried product is removed by means of a blade at another. ‘The temperature and rate of revolution of the drum are so chosen that drying is completed in the time required for the material to travel from the point of spreading to the blade that removes the product. The operation can be carried out in air or in a vacuum. In the latter case heat-sensitive products are better protected since lower operating temperatures can be maintained. The material being de- hydrated can be spread on the surface of the drum by spraying it, by a “doctor” blade, or by use of two drums placed parallel with a small clearance between. In the latter case both drums can be used as driers, in which event they will be similar in size, or one may be small and used only for the purpose of providing a small clearance through which even spreading on the large drum can be attained. Many products are removed from the drum in the form of con- tinuous sheets which are conveyed to a mill operating in a suitably conditioned atmosphere where the dry product is reduced to a powder and then packaged. The drum-drying process is widely used for the production of dried milk and to a lesser extent for certain fruit and vegetable purees. The form of the product is confined to thin flakes and powders. Rotary Driers Rotary driers are rotating drums inclined at a slight angle from the horizontal. The wet substance is fed in at the high end and is conveyed with constant tumbling toward the low end, from which 569074—44—_8 114 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE the dried product is continuously removed. Drying is facilitated by a current of air, either counterflow or parallel flow, induced by suit- ably situated fans, and heat may be simultaneously supplied on the outside of the drier. Construction is of either the single- or double-- shell type. The latter affords greater protection of the product from damage by burning. Double-shell driers are sometimes provided with louvers in the inner shell through which air is admitted in direct con- tact with the material. Rotary driers are efficient and extremely useful in drying many substances, but they have found little application in the field of fruit and vegetable dehydration. The constant rolling or tumbling of the wet product destroys the characteristic form of the pieces (slices, strips, or cubes) and may result in production of a large proportion of powder. Rotary driers have been used successfully for the drying of precooked ground meat. Apple and Prune Kiln Driers A distinctive type of kiln drier finds extensive use in dehydra- tion of apples and prunes in the Pacific Northwest. The drier con- sists of a two-story building in which the lower story houses the cen- tral heating system, from which heated air is circulated through wooden ducts to the kilns above. The kiln proper consists of a series of stacked, shallow, wooden bins having slotted floors on which the prepared fruit is placed in layers from 6 inches to 24 inches in depth. Warmed air from the heaters below passes upward through the fruit and is removed through a duct at the apex of the V-shaped roof. In some modern kilns the air circulation is speeded up by an induced- draft fan at the roof, and in some cases provision is made for re- circulation of part of the exhaust air. When apples are being dried, it is common practice to turn the material with shovels or forks after 4 to 6 hours of drying. Initial temperatures of 150° to 165° F. are usual. Final drying should be carried on below 160° F. Driers of this type are built in ca- pacities ranging from a few tons to 120 tons of raw material per day. Their initial cost is relatively low but difficulty of control is not con- ducive to the highest quality of product and drying costs are likely to be high as a result of large heat losses. Kilns are not well adapted to the drying of cut vegetables because vegetables do not stand the necessary rehandling so well as fruits, and because the relatively low air velocity makes rapid drying impossible. MULTISTAGE DEHYDRATION It should be apparent that each of the types of dehydrators de- scribed in foregoing pages has its peculiar advantages and disadvan- tages. No one of them is “best” under all conditions. No one of them is necessarily the best for all of the stages of dehydration of a single lot of moist fruit or vegetable. The advantages of a com- bination of parallel-flow and counterflow tunnels over either of them separately have been pointed out. Such a combination is one exam- ple of a multistage dehydration process. While a complete discussion of the factors that determine the ad- visability of combining different kinds of dehydrators belongs in the VEGETABLE AND FRUIT DEHYDRATION 15 realm of dehydrator design, rather than operation, the following summary may assist the operator of such a system to understand what can be done with it. Changes in Material as Drying Progresses The fundamental factor at the base of all combination systems is the enormous change in properties of the material as it dries. A freshly blanched piece of vegetable is soft and easily crushed, is in- deed a mass of water held together by a tenuous cell structure: its vapor pressure is nearly equal to that of pure water. Tn contr ast, the same plece of vegetable after drying to 5 percent moisture is gr eatly shrunken in volume and distorted in shape, is hard and may be very brittle, and its vapor pressure may not be more than 10 or 15 percent of the vapor pressure of water. These properties all change as the drying progresses, but not all at the same rate. Hence the conditions which would be “ideal” for drying change as the piece dries. When the piece is very wet the water will evaporate very rapidly if a great deal of air is supplied, and that air may be very hot without burning the piece. At the same time the piece will be too tender to stand more than a minimum amount of handling. Essentially similar conditions prevail while at least the first half £0 three-quarters of the weight is being lost through evaporation. As the moisture is still further reduced the piece (which will have already undergone most of the shrinkage in volume that it will experience ) becomes tough or leathery. The rate of evaporation may not be more than a half or a third of what it was at first, even though drying conditions are kept the same; increasing the air velocity past the piece has no perceptible effect on the rate; the temperature of the piece rises almost to the dry-bulb temperature of the air. “Ideal” conditions now demand a lower temperature than at first, and the use of a high air velocity would be a needless waste of power. However, the piece will now withstand handling, tumbling, or deep piling without damage. Conditions substantially similar to these prevail down to a moisture content of about 10 to 15 percent. Below that point, and more and more clearly as the moisture content falls further, conditions change again. ‘The piece becomes hard and brittle. The rate of further evaporation is determined almost entirely by the thickness of the piece and the dryness of the air? The rate will perhaps become less than 1 percent of what it was at the beginning of the drying. “Ideal” conditions now call for a slow current of dry air, not too warm, and very little mechanical disturbance of the product. The usefulness of different kinds of dehydrators at different stages of drying will depend not only on these physical factors but also on economic ones. It is an economic waste to use a large and expensive piece of equipment to perform work that a small, cheap one will do as well. Consideration of the labor or complications involved in rehandling the product may favor one kind of transition over others. Examples of Multistage Systems Out of the very great number of possible combinations, the following may be mentioned. Not all of these are now in commercial use. 12 More exactly, by difference between the vapor pressure of the piece and the partial pressure of moisture in the air. 116 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 1. Three-stage conveyor-belt drier, followed by a drying bin. The bin itself may be continuous in operation, in which case it is essentially a fourth stage of the conveyor, arranged for very deep piling. _ 2. Three-stage conveyor-belt drier, followed by a continuous vacuum-finishing drier. 3. Counterflow tunnel, followed by a finishing bin. This combina- tion has been used in commercial operations for some years. __ 4, Cabinet drier, followed by a finishing bin. Use of the bin re- leases the cabinet from the part of the run on which it is normally most inefficient. 5. Rotary drier, followed by vacuum drier. The rotary may be used for most of the drying of some products which are tough enough to stand the tumbling action, but it tends to reduce a nearly dry product to powder. | 6. Parallel-flow predrying tunnel, counterflow secondary tunnel, and finishing bin. A combination which is being widely adopted for large commercial operations. 7. Parallel-flow predrying tunnel, followed by two-stage conveyor- belt drier. The tunnel takes the part of the cycle for which the con- veyor drier is least well adapted. The second stage of the conveyor drier can be designed for deep piling, as in a finishing bin. 8. Center-exhaust tunnel followed by finishing bin. The combina- tion is equivalent to that of No. 6 above, except in the manner of handling the product between the first and second stages (fig. 60). FINISHING AND PACKAGING The equipment required in finishing and packaging dehydrated products ordinarily includes the following: A bin drier (if used), a picking belt, one or more shaking or jogging stands (if used), placed at the end of the picking belt, gassing equipment in plants that dehydrate carrots and cabbage, scales, a can seamer or bag and carton closer, space for boxing and labeling, a clean dry space for the storage of cased dehydrated products and for empty cans, bags, and cartons, and finally a loading platform. The floor area required for finishing and packaging in a plant that handles carrots at the rate of 30 tons daily has been estimated as shown below. Area in square feet BB Ve ae No a ae Cae ef ra re 12 Picking pelt.6:x 2146 feet.@2 women) = ee ee 125 Gans. ‘shakers 22 6 12a eh al eR re La Sah Na apne Aaa 10 Gassing unit “(vacuum chamber and pump) 2222s Se ee eee 50 Semleg eo 2 on A 1 tee a ele ae 10 Can-lid closing machine or carton and bag-sealing unit__________________ 15-75 space for boxing, Jabeling, and«Steneiim gs: es Oe eee 50 Storage of empty 5-gallon cans and boxes, 1% carloads________-___________ 750 Storage of filled 5-gallon cans, boxed, 1% carloads__________________-__ 750 AISIO@S 2 6 Sal So A ee ie ee 640 Car shipping’ platform et. 222 2G se es ee ee eee 400 4 Nt): 3 (ane ee OR ee et al ie sat ed AE Pe eT Oe 3, 000 The floor space required for this department in a 25-ton plant is estimated to range between 2,400 and 4,100 square feet. (See p. 10.) This estimate is based on diced carrots, packed as follows: 14 pounds in 5-gallon cans, 2 cans per case, 1.75 cubic feet of gross volume per case having outside dimensions of 2014 x 984 x 1434 inches. A boxcar with a volume of 3,150 cubic feet will hold approximately 1,800 cases : VEGETABLE AND FRUIT DEHYDRATION erate fel EF of 2 cans each and a net weight of 50,000 pounds of dehydrated car- rots. It is good practice to store these cans prior to shipment, 8 high on their sides, which will require an area of 490 square feet per car- load exclusive of aisles. The space used for finishing and packaging should be between the Figure 60.—Truck carrying trays of dehydrated, diced carrots leaving a tunnel dehydrator. ; dehydrator and the loading platform, and should be a separate room, : partitioned off from the rest of the plant where the humidity will . be higher. The doors and windows should be screened to keep out ; flies. It is desirable to have the bin drier in this room, but the air p discharge must be vented to the outside. Air in the room must be ! 118 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE sufficiently dry so that as little absorption of moisture as possible occurs during packaging. In a dehydration plant with a capacity of 30 tons daily, one man does all the work from the end of the picking belt through the stacking of the cases. Dehumidification of the air in the finishing and packaging room may be necessary. When exposed to air any dehydrated fruit or vegetable tends to increase or decrease in moisture content until it finally reaches an equilibrium content which is typical for that product and that relative humidity (table 9). Operators should con- TABLE 9.—Selected data’: Equilibrium moisture content of various dehydrated foodstuffs * Moisture | Tempera- Relative Product content ture _ humidity | | Percent | oF, | Percent 3 ) | | 29 Apples == ----=——= ~ = == == == -- = === ~~~ ~~ ~~~ =~ ~~~ -==~-~~---~-=---- a 99 51 10 | JApTIGO tS ae am Ags eel Se hal A ee eke dace ease | 15 | Room 4 Cabbage. icto= io. tee De Eee ree eae Bes Se AS ee oR } 6 | 5 | @arrotsa Se See en ee ee ES, eee ee | 10 —I ~I gS Z or Ee iS Bree fe ae as es eee wre 00 He OD 2 | Eggs, wholesss) 2? 22-205 ee =a He NEB aca ee 2 ene ma nar ece Pe | 4 | | 6 | Flour, wheat, 83 percent patent_____ pra ation At Cosas Sree ey eel, 10 | ~I ~J IM OMe Aedtolta. GSi OCA MATERA ee a ee ee | Nilke skimmed, Ispercent fats =o 5.22 same eee ey eee | Peaches se 2 vee 5 ae oe oie Sh ae i ape Bee erie { Room Room TR OLALOCS willl te eerste ee ee ee ee [se eral 99 Potatoes, sweet = cote see os See ae | 99 Room | EAT UTI CS en eet Te ns ere at aE ep ca Ag ce ge 8 ae | Room 99 | 99 | 1 See Anker, Geddes, and Bailey (3); Makower and Dehority (30); Schwarz (40); and Supplee (42). 2 Data on dehydrated milk are significant in the storage and packaging of some soup mixtures, and data on eggs and flour are of interest for the sake of comparison. sider carefully the advisability of dehumidifying the finishing and packing room in all cases where the moisture content at shipment corresponds to a relative humidity which is less than that which will occur in the packing room at any time of the year or of theday. This is important in handling leafy products and is most important when products of 3 percent moisture content or less are ground. The United States Weather Bureau publishes data on dew points which may be expected in various parts of the United States. If the prob- VEGETABLE AND FRUIT DEHYDRATION 119 able highest dew point can be estimated, the corresponding relative humidity in the packing room at any desired temperature may be determined from a humidity chart. (See p. 52.) Some dehydrators in dry areas find that they can raise the air temperature in the packing room enough to avoid moisture absorp- tion by the product. Warm air has a lower percentage of relative humidity than cold air of the same dew point. Besides the method just indicated, there are three methods commercially used for de- humidifying air, all of which actually remove a part of the water present. Silica and alumina gels are both used to absorb water vapor from air to be dried. The gels are reactivated by the use of closed steam pipes or electric heaters and a stream of air.- Refrigeration is also used to condense moisture, which is separated thus from the incoming air. Another method consists of absorption of moisture in a spray chamber by special solutions, such as lithium chloride, which are regenerated after use. Finally, one must bear in mind that when the finishing and packing room is dehumidified by any method which actually removes water vapor from air, it may be practicable to remove some air from that room as the air feed to a bin drier. FINAL INSPECTION OF THE DRY PRODUCT Purchases of dehydrated vegetables for the several Government agencies are inspected by the Fruit and Vegetable Branch of the Food Distribution Administration, United States Department of Agricul- ture. Processing procedures are observed and recorded and the fin- ished product is inspected for quality according to the specifications under which the purchase is made. Certificates are issued only when inspections are made on the sealed containers representing the shipment. In order to facilitate inspection and as a direct aid to himself, the manufacturer should follow certain steps. The packaged mate- rial should be coded and warehoused by coded lots. The coding sys- tem used can follow any system desired but should impart the fol- lowing information: Product, type, year, month, day, shift. Thus, if 100 five-gallon cans of Julienne potatoes were produced on February 1, 1948, on the swing shift, they could be stamped PJ3B1A and ware- housed together as a lot. This code can be stamped on the can, at the time the product is labeled, with a stamp pad and a water-insolu- ble canner’s ink. Coding entails very little actual expense but yields a great deal of valuable information. It enables the operator to keep an accurate running inventory of his production, and affords a basis for compari- son of the efficiency of personnel and of the yields obtained from dif- ferent lots, varieties, or sources of raw materials. It also enables the operator to find the source of trouble quickly, and affords a means of segregation, without jeopardizing the entire block, in the event that a particular part of the pack fails to meet specifications. In- spections are customarily made on carload lots. | Samples are drawn at the rate of approximately one container per hundred. The rate may vary according to the number of containers per individual code, the variations within the lots, and other factors. It is the inspector’s responsibility to satisfy himself that any given 120 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE lot meets the specifications and if any great variation is encountered more samples must be drawn. When an inspection is made, representative samples are taken to a place in the plant where the containers can be opened and the opened product can be protected from moisture pick-up from the atmosphere. To facilitate inspection a table approximately 3 feet high with a top 6 x. 3 feet and a hole 6 inches in diameter at one end is very ad- vantageous. The top should have a light surface and be well lighted. The condition of the container, the net weight, and the packing medium (such as an inert gas) are noted. The entire contents are removed from the can and mixed thoroughly. From the resulting mixture, a cross-section is taken to make a composite sample, which is sealed in previously dried jars. One to two such samples may be taken for such laboratory determinations as are required by the speci- fications. Examinations for defects, uniformity of size, presence of fines, and color of dry product can be madé while the product is being weighed, and most of the material can be returned to the packer for repackaging. The inspector may also examine the raw product, plant sanitation, and packing procedures. Laboratory analyses are made to determine the moisture content, enzyme inactivation, and reconstitution; other analyses, as outlined in the specifications under which the product is being graded, are also made. Upon completion of the inspection the results are forwarded to the contractor and purchasing agency. Loading for shipment will be supervised if such supervision is specified by the purchaser. Official certificates are issued and dated as of the day on which the analysis was completed. These certificates serve as a basis for payment when the merchandise is received and accepted. Grades and Specifications for Fruits and Vegetables United States grades for dried fruits are available through the United States Department of Agriculture, War Food Administration, Office of Distribution, Washington, D. C. Federal specifications for dried fruits can be purchased through the Superintendent of Docu- ments, Washington, D. C. At the present time United States grades and Federal specifications for dehydrated vegetables have not been issued. Purchases are made on Quartermaster Corps Tentative Specifications, which are obtain- able through the Chicago Quartermaster Corps, 1819 West Pershing Road, Chicago, Ill., or Tentative FSCC Specifications obtainable through the Office of Distribution, War Food Administration, United States Department of Agriculture, Washington, D. C. TEMPERATURE REDUCTION TO MAINTAIN QUALITY IN DEHYDRATED PRODUCTS The quality of dehydrated vegetables can readily be injured before shipment by failure to reduce the temperature of packaged materials and to keep them cool. Holding dehydrated cabbage at 120° F. for a week, for example, results in almost complete spoilage. Measures must be taken to insure that such products will be cooled to 90° or less within 24 hours after drying has been completed. While they are on the inspection belt, freshly dehydrated vegetables cool to a con- VEGETABLE AND FRUIT DEHYDRATION All siderable extent. Table 10 indicates the rate at which dehydrated potato strips cool when exposed to still air. TABLE 10.—Rate of decline in temperature of dehydrated potato strips exposed to air—single layer, 0.4 pound per square foot Time of exposure in minutes— 0.0 0.5 1.0 2.0 4.0 ouRF 10, a ie Sie Temperature of potatoes________-_______- elas a rae e eilbyl 123 117 106 94 PRempenratUTe mails = Het eee ee ee) ee ee 74 74 74 74 74 | Bitlerencomees asd ue ert seh oe) | 77 | 49 43 32 20 As an example let us assume that the inspection belt for dehydrated | potatoes is 12 feet long and moves 8 feet per minute. The tempera- | ture of the potatoes is 160° F. when they drop to the belt; that of the air is (0°. Time on the belt will then be 4 minutes. From table 10 we obtain an estimate of the temperature of the dehydrated potato leaving the belt. In this case the temperature would be 90° to 100°. With air at 85°, potatoes dropped on the belt at 150° would leave the belt after 4 minutes at 105° to 115°. TABLE 11.—Hstimated cooling times for dehydrated julienne potatoes in 5-gallon square cans, standing alone—packed 15 pounds to the can Ln Qe Time required to reduce differences in Initial difference in temperature (° F.) between temperature to (° F.)— AIPOUtSId eran dephercentersO fat Cia ees peg el ere ee em | | | 40° 30° 20° 10° 5e Minutes | Minutes | Minutes | Minutes | Minutes (ee ee rae Wee gees pe ered Ae Nate oY eae a ES 70 160 290 510 730 A (Barret nae gerry eS ey Ne ied pets ep pa GTi Ms ip Ns eg [Inti 90 220 440 660 cS Oe ties Rca eae eyelet an eh AD a vee PN eae eA Le a 130 350 570 Dye fede JET NOt SS 7 Bee es eee age by bie Ui cause Oe (Sie erate aceite cea, Le nee 220 440 I HL (ae eee rte nie eM a a Ne er, A ge ny PL Pig nk de ee pd ren ee eae ee Geo Re ee eo 220 | Measurements of the temperature of vegetables should be made after | they have cooled on the belt. Such measurements can be made by | thrusting a thermometer into the center of a filled carton or can and | by allowing 10 minutes before the reading is made. Measurements on rate of cooling of potato strips in cans have demenstrated that | the center of a full 5-gallon can when filled at 105° F. and placed ! in air at 85° will cool to 90° in 7 hours. Estimated cooling times for | dehydrated potato strips in 5-gallon square cans, standing alone, are | shown in table 11. | The estimated time required to cool the same potato strips in 5-gal- lon-size lead-foil packages placed in outer cartons approximately 1314 x 71% inches in size is the same as for the metal cans (35). Ifthe cartons are stacked back to back on their sides, in a stack two cartons thick, the estimated time required to cool the contents from 105° to 90° F. at the central plane of the stack with air at 75° is 7 to 8 days. Similarly, for a stack that is four cartons thick the estimated time is 4to 5 weeks (47). Cartons and cans should be kept apart or unstacked, as far as pos- 122 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE sible, until they are cooled to 90° F. or lower. A period of several hours may be needed. Temperature of the air is of course a factor. Canners commonly make use of cooling rooms. When dehydrated vegetables are shipped from a cool area through a hot one, close pack- ing will be helpful provided the cases when shipped are close to the air temperature at the point of shipment. The heat-diffusing and heat-conducting properties of various de- hydrated foods have been measured recently at the Western Regional Research Laboratory. The measurements on strips of dehydrated white potatoes, which are representative, approach the corresponding properties of sawdust, which is used commercially as a heat insulator. Cooling rates are directly proportional to the diffusivities. When the containers are cans, the average diffusivity is increased to about three times that of dehydrated vegetables in foil-protected cartons. This is because the steel sheets are highly conductive and will aid in the removal of heat from the stack. No such increase in the rate of cooling occurs in unstacked, single cans, since the steel is on the outside and at once comes to room temperature. The peaks of storage rooms should be ventilated in summer and cases should not be stored next to the roof. In a warehouse thus venti- lated. a pronounced increase in temperature from the ground level up- ward has been observed. The outside air was 85° F., that at the floor level was also 85°, at a point 8 stacks high it was 93° and 6 feet above it was 98°. STANDARD TYPES OF PACKAGES Government specifications define several standard types of packages for use in the exportation of dehydrated vegetables and fruits to the armed forces and to lend-lease nations, but for domestic use “commer- cial packages” are specified. Export-containers, particularly for mili- tary use, are designed to be used under average temperatures of as high as 90° F. at a relative humidity of 90 percent, and at occasional temperatures as high as 120° and as low as —15°. The package and contents may be exposed to rain or even immersed in water on being landed from ships. Dropping of cases during handling is three to five times as severe during export for military use as in domestic use. The cases must be capable of withstanding 120 to 200 drops of from 1 to 3 feet without damage. The package must not allow the contents to increase more than 2 percent in moisture content per year even when the humidity of the surrounding atmosphere remains constantly at 90 percent. Packages that contain dried vegetables packed in an inert gas must allow no air to enter nor gas to escape; that is, they must be hermetically sealed. Ten types of packages are described below and the approximate numbers of containers required for dehydrated vegetables are shown in table 12. Types 1 to 5 are tin or steel cans, of designs that can be hermetically sealed, excluding moisture vapor, water, air, and insects. Types 6 and 7 are laminated lead-foil, heat-sealed packages. In order to free steel for other purposes and to lessen the demand for tin, sub- stitute containers of high moistureproofness have been developed. For the less hygroscopic dehydrated vegetables and fruits, for ex- ample dry, shelled beans and evaporated apples, standard packages for export (type 8) consist of heat-sealable laminated cellophane, or Sete chees: cee pene reer ere VEGETABLE AND FRUIT DEHYDRATION 123 TABLE 12.—Estimated container requirements for dehydrated vegetables * 5-gallon con- tainers re- | -wx793 Over-all : a Weight per : quired per ton Vegetable Form 5-gallon Shae Sinan itie | trimmed vegetables Pounds Number IB CCUS Rearend a 346-inch slices___________________ 10.0 | 13-16 tol 12-15 1D) Oberon Nea ek 38-inch CUDeS a= 8 7.0 | 13-16 to 1 7-9 @abbavewseue. seen nek ee: STO GS asain o 7.0 | 13-22 tol 13-22 CATO ES Sree eee eae eae Bet J8-1N Chs CUES es ss He 17.5 | 14-16 tol 7-8 ONION Sisters eee Se a RAKES: te ticni Ce erat tea 12.0 | 12-15 tol 12-15 ROLALOCSsWinite= senses ees 36-In Che CUD CS eee ee 16.0 5-8 tol 12-25 1 Da yse Coe ete ee Sah SUH EN) One ats ee eee ee a ieee 10. 0-15. 0 5-8 tol 17-40 Ritabacas see cee Oe IAIN CheSli CCS ss ee 12.5 6-10 to 1 16-27 Sweetpotatoes________...-__.---_| --.- GODS see ee ee 12.0 4-6 tol 30-40 1 No standard loadings per container have been established. Among the vegetables, sweetpotatoes re- quire more containers per ton of raw material than are required by the other vegetables; however, a 50-ton plant requires only 1,500 to 2,000 5-gallon cans or cartons per 24 hours, or 60 to 80 per hour. Because of this low requirement, highly mechanized packaging lines have not been used with 5-gallon containers. laminated cellophane-to-glassine bags in which a degree of moisture- vaporproofness results from the use of lacquers and a plastic laminat- ing agent. These packages are less likely to be attacked by most tropi- cal insects if no food particles are left on the outside of the package. Food particles stimulate boring by the insects and perforation of the hning of the package may result. For cut sulfured fruits with a moisture content of approximately 25 percent, strips of paper are used to line the boxes (type 9). For pasteurized fruits, such as prunes, the largest standard package is the vacuum 5-pound can (type 10). Type 1.—The 5-gallon square can is approximately 9% inches in width and 13% inches in height, and weighs 2.4 pounds. The tin plate used in these cans consists of 107 pounds of steel and 1.25 pounds of hot-dipped tin per base box. (A base box consists of a bundle of 112 sheets 14 x 20 inches.) Provision is made in current specifications for the use of electrolytic tin plate containing 0.5 pound of tin per base box for the entire can or for the side walls. The stud hole is 6% inches in diameter. Where hermetic seals are required for gas packing, lids may be soldered or the newer compound-coated lids may be used in conjunction with double or single seamers. These must be of approved design. Type 2.—Thirty-pound frozen-egg or fruit cans were permitted by the United States Army Quartermaster Corps for some vegetables until replacement foil containers came into use. Type 3.—The 5-gallon round steel can with a clamping ring has been used as a temporary package. Its relative cost and weight are high for single-trip packages. Type 4.—Some use has been made of No. 10 round, sanitary, hermetically sealed cans with a tin-plate body (107 pounds of steel and 1.25 pounds of tin per base box) and ends of black plate. Other combinations that reduce the tin require- ment are also used. The side seam is soldered, while the end seams are closed by compound and double seaming. Type 5—The No. 2% round can, hermetically sealed, contains the same types of materials as type 4. ; Type 6.—This is a foil-protected package consisting of (1) 5-gallon inner carton that is 13 x 6144 x 14 inches in outside measurement with taped joints, and consists of solid sulfite, kraft, manila, or corrugated kraft, (2) sealed lami- nated envelope consisting of kraft, asphalt, lead foil, and heat-sealing cellophane, which covers the inner carton, (3) a protective strip of chipboard, and (4) a weatherproof solid-fiber outer carton. Type 7.—This is a smaller foil-protected package consisting of (1) various permitted liners in contact with the food, (2) a laminated sheet containing lead foil, made up 4s a heat-sealing bag outside of the liner, and (3) a chipboard earton. An alternate package consists of an inner bag of amber glassine waxed at the rate of 25 to 55 pounds per ream, a kraft-lined chipboard carton and a 124 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE double overwrapping of paper with a basic weight of 20 pounds plus 30 pounds of wax perream. These are packed in solid fiber cartons. Type 8.—This is one of the optional soup packages and resembles a cereal container. It consists of an interlining bag of amber glassine with 30 pounds of wax per ream and an outer carton of bending chipboard. The carton is double, overwrapped with paper that has a basic weight of 20 pounds and is coated with 30 pounds of wax per ream. Type 9.—In normal times and when hazards are not severe, evaporated apples, dried prunes, apricots, and figs are exported in paper-lined wood boxes containing 25 pounds net weight, and dates are shipped in flat boxes containing 15 pounds. Domestic boxes hold 10, 25, and 50 pounds of dried fruits. Uncut fruits, such as prunes, raisins, and apricots, are graded into various sizes, which are packed separately. Federal packaging specifications are available for most dried fruits. Type 10.—Dried fruits are shipped abroad for the armed forces in No. 10 cans, hermetically sealed, and sometimes evacuated. Evaporated or dried apples, apri- cots, pitted dates, figs, peaches, prunes, and raisins are packed at rates varying from 4 pounds per can for evaporated apples to 7 pounds per can for prunes and figs. Six cans are packed in each wood case. PACKAGING EQUIPMENT AND METHODS After final inspection and culling, dehydrated vegetables and fruits are packaged. If they are to be powdered, a hammer mill will be needed, and also a dehumidifier if the atmospheric humidity in the room ishigh. Vibrators are often used in packing to produce a higher bulk density. A simple type consists of a low platform, set diamond- shaped with the pivot placed diagonally. Strips along the two back edges form a stop for the cans. Power is supplied by a 14-horsepower motor operating at a few hundred revolutions per minute. An ec- centric is mounted on a low-speed shaft and this is connected to the back corner of the platform by a rod. Im another type, a spring- mounted platform is vibrated by a cam in the motor shaft. The vi- brator can be equipped with a weighing scale. Weighing scales should have a 50-pound range, an attachment for indicating the amount over or under, and a tare weight. Gassing is required for cabbage or carrots that are to be sold under Government contract (fig. 61). Flavor, color, and vitamins are retained through the elimination of air and its replacement with ni- trogen or carbon dioxide. Vacuum packing is not used because 5-gallon cans require a very solid fill if they are to be sealed under vacuum. A maximum limit of 2 percent of oxygen is recommended for sealed, gas-filled containers, and the analysis should be made at least 12 hours after the cans are filled and sealed. Air can be dis- placed to a point at which the oxygen content is below 2 percent by the use of (1) the cylinder-and-meter method, (2) the vacuum- chamber method, or (8) the carbon-dioxide-snow or “dry ice” method. Equipment and methods for the analysis of atmosphere in cans are described on page 157. Cylinder-and-Meter Method The most common method of removing air has been by the use of gas run from a cylinder through a reducing valve, a rubber tube, and a metal purge tube thrust to the bottom of the can. The amount of gas is controlled by the pressure setting of the reducing valve, the duration of the gas flow, and the occasional testing of the gas at the top of the can with a burning match. If the match goes out, the can is considered to be sufficiently low in oxygen content. While this VEGETABLE AND FRUIT DEHYDRATION £25 method commonly results in analyses below 2 percent of oxygen, the time is not accurately measured and the cans have been found to con- tain as much as 8 percent of oxygen. Tests have shown that the introduction of an iron-case dry-gas meter, of stock design, between the cylinder and purge tube improves the method. The stock meter used in testing had one dial that meas- ured 1 cubic foot per revolution. In the displacement of air by this FIGURE 61.—Filling 5-gallon cans with diced dehydrated carrots. method, the lid of the can is placed over the opening; the can is gassed and, after removal of the tube, the lid is attached to the can by seaming to form an hermetic seal. Soldering is an effective method of closing cans but is a slower method than seaming. The following results have been obtained in tests of the cylinder-and-meter method of gassing: Time Ozygen content of Cubic feet of gas: (seconds) can (percent) emer Me ee SENS Salas 3s Ee De ei es GO 0.8-1.0 [See a ee Ee ea ee ee ee 30 1.2-1.6 1 bots ROR AE Sa ny A ele TE BE Eee ae nop ea lea SS Sel 10 1.82.4 These results were obtained with dehydrated cabbage and carrots at typical loads per can, eee eee ee eee 126 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Vacuum-Chamber Method Another process of removing air from 5-gallon cans and replacing it with carbon dioxide or nitrogen can be termed the vacuum-chamber method. This method requires equipment similar to that required in vacuum canning. One type of equipment has a flat plate with a space on it for one or more cans. Cans are placed on the plate and are covered by a heavy metal chamber or bell provided with a counter- poise. For the evacuation of the air in a chamber, a vacuum pump is provided. The effectiveness of the equipment depends largely on the closeness of the joint between the chamber and the plate, which coverns the leakage of air inward, and on the degree of vacuum to which the pump can exhaust the air in the chamber and the can or cans thereunder. 3 For a single-unit chamber the pump should have a capacity dis- placement of 100 cubic feet per minute, and should produce a vacuum of 29.5 inches of mercury. The highest degree of vacuum can be attained by the use of any of a number of rotary pumps. The next type of pump, in order of attainment of vacuum, is the two-stage steam ejector. At least 75 pounds per square inch of steam pressure is necessary for efficiency. The first cost 1s lower than those of rotary or reciprocating pumps, but the demand on steam between evacuations diminishes the utility of this device. Single-stage reciprocating pumps will not produce the required vacuum, but double-stage reciprocating pumps will. Some double-acting reciprocating pumps can be easily and inexpensively converted to double-stage pumps. Modern gassing chambers of the bell or horizontal type hold the lid away from the can during the operation by means of an ex- ternally operated chuck or by an electromagnet. The procedure is carried out as follows: The can is filled with a weighed amount of dehydrated vegetables, and a lid is set in place. The chamber is closed. A vacuum of at least 29.5 inches is drawn on the chamber and this will take 20 to 40 seconds. The valve to the vacuum pump is then closed, and a valve to the cylinder of gas is opened, releasing the vacuum. A pressure of 1 to 2 pounds per square inch is built up in the bell, which will prevent the entrance of air when the chamber is opened. The chamber is opened and the can is removed and hermeti- cally sealed immediately. Cans should be handled by the corner edges, since any bellows action of the sides caused by handling will draw air into the can before it is sealed. In practice, the time for evacuation is 20 to 40 seconds, for breaking the vacuum about 20 seconds, and for a cycle including opening and closing of the chamber and sealing a pair of cans, about 90 seconds. In tests oxygen content of the atmosphere in cans of carrots and cabbage sealed by means of a double-stage reciprocating pump, with a 29.5-inch vacuum, and one chamber, was found to be 0.5 to 1.6 percent. Three to four cans may be evacuated, gassed, and sealed per minute in modern-type machines (fig. 62). Gassing by Means of “Dry Ice” The solid carbon-dioxide snow or “dry ice” method has been de- veloped at the Western Regional Research Laboratory as a means of gassing carrots or cabbage with a minimum amount of equipment. The method has not yet been tried out commercially. Unlike water VEGETABLE AND FRUIT DEHYDRATION Lae ice, dry ice changes directly into a gas when it is heated. It is shipped in 50-pound blocks, packed in quadruple corrugated cartons. Losses in shipment and storage are approximately 10 to 6 percent per day, respectively. Suitable storage bins can be made with hardwood frames, sheet metal, or plywood walls for the lining, 6-inch layers of cork or other insulation on the bottom and sides, and a 4-inch-thick kapok, duck-covered pad as the cover. ne-fourth pound of ground dry ice is placed in the bottom of a 5-gallon can and distributed uniformly. Then the vegetable is FIGURE 62.—Replacing air with carbon dioxide in a can of dehydrated carrots. Operator is placing a 5-gallon can in a vertical-type evacuating and gassing chamber at left. Sealing machine is at right. weighed into the can, which is placed in a water trough equipped with a conveyor, and the lid is placed on the opening in the can. At once an evolution of carbon dioxide takes place and the air, and later 5 the gas, escapes around the lid. Two cubic feet of gas is formed in the sublimation of the dry ice. After 6 to 12 minutes in water all the solid carbon dioxide will have gasified and the can is capped after a | short period of standing in the air. ; The atmosphere in 5-gallon cans of cabbage or carrots has repeatedly ! been brought to 0.8 to 1.0 percent oxygen by this method. Care must be taken to avoid the sealing of cans in which ungasified dry ice is present ; dry ice in a sealed can will cause cans to bulge later. The ordinary | precautions for handling dry ice should be observed. i 128 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Sealing Dehydrated vegetables and fruits that are packaged in tin cans are hermetically sealed, even when packed in air. Formerly the stud hole was covered with a lid and a tight joint was made by soldering, but more recently, because of the shortage of tin and the lower percentage of tin in solder, the trend has been toward the use of square cans with mechanically formed seams. These seams are of two types, double seam and single seam. With both a sealing compound is used on the surfaces subject to closure, in order to produce an hermetic seal. An hermetic seal may be tentatively defined as one that is gasproof, moistureproof, and bacteria-tight, both initially and throughout a common period of handling and storage. Shipments of 5-gallon cans of dehydrated products are frequently made across mountain passes 7,000 feet high; in a trip of this sort a sealed can filled at sea level would be subjected to 314 pounds per square inch of internal pressure. Tests show that when a leaky can is packed with carbon dioxide at sea level, shipped over a 5,000-foot pass, and returned to sea level, the oxygen content may be expected to increase 4 percent. Seaming machines for 5-gallon square cans have been developed by several companies and can be leased. Machines will seal at rates be- tween 6 and 18 cans per minute (fig. 63). Packaging in Cartons The equipment and methods recommended for cartons vary some- what from one supplier to the other. An inner carton of banding chipboard must be assembled and taped at the bottom, and the carton is then filled and the top is taped. A lead-foil bag is expanded into shape on a mandrel and the filled carton is slid into the bag on an inclined chute. The package is placed in an erect position and passed on a roller conveyor through a heat sealer, where the envelope is deflated by suction or other method. ‘The “ears” of the envelope are folded over, a strip of folding U-shaped chipboard is placed over the bottom and the sides, and the unit is thrust into the outer carton, which is then sealed, and the seams at the end are covered with cloth tape. Labeling Packages and Cases The labeling of packages is described in Government contracts, which in general require the name and type of product, the net weight in pounds, the month and year of dehydration, name of packer, loca- tion of processing plant, and specific directions for rehydration. Part of the foregoing is repeated on the packing cases. It is important that finished boxes be stenciled daily ; if for example inspection reveals too high a moisture content, the number of cases involved will be fewer if only 1 day’s product has been grouped and stenciled together. Cases Materials for cases and methods of packaging are included in Gov- ernment specifications. For overseas shipment, packing cases undergo 3 to 5 times as much handling as for domestic shipments. A loaded case that withstands 40 drops of 1 to 3 feet in a tumbling drum 7 feet in diameter is safe for domestic use, but must withstand 120 to 200 adidas thas ih cama alah iil VEGETABLE AND FRUIT DEHYDRATION 129 drops for export shipment. Exposure to rain and immersion of cases in salt water during landings are 2 more conditions that frequently must be withstood. SUBSTITUTES FOR TIN-PLATE CONTAINERS Wartime shortages of steel and tin have stimulated a search for containers that will successfully replace tin plate in the packaging of dehydrated foods. The substitute container must be one that ap- Figure 63.—Sealing machine which forms a 20° angle, hermetic seal on cans. 569074 _44—_9 130 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE proaches or meets the standards set by tin-plate containers. ‘These standards are (1) hermetic sealing, (2) durability, and (3) prevention of transmission of light. Glass containers can be sealed hermetically but fail to meet other requirements. Flexible materials, such as heavy treated papers, moistureproof cellophane, simple laminations of paper and cellophane, and compound laminations of paper, cellophane, and metallic foils, have certain advantages and disadvantages as packaging materials and, if handled appropriately, can be used satisfactorily. Functions of the Package Since all unprotected dehydrated foods absorb moisture from the atmosphere, it is essential that the package be moistureproof. ‘The speed of absorption of dried vegetables varies considerably, because of differences in composition of products and also differences in the extent to which the food has been dehydrated. For example, cabbage dehydrated to 5 percent moisture content is more hygroscopic than potatoes containing the same amount. Also cabbage that contains 5 percent of moisture will absorb moisture more rapidly than cabbage that contains 10 percent. Excessive absorption of moisture by dehy- drated foods causes chemical changes that quickly impair appearance, palatability, and nutritive values. A suitable high standard for long storage, one that is commonly used, is this: Sufficient moisture-vapor resistance to prevent a maximum absorption of 2 percent of moisture during a storage period of 1 year. In a consideration of substitutes for the metal or glass hermetically sealed container, a number of factors that affect moisture-vapor per- meability must be evaluated. The principal factors are as follows: The nature and type of the protective materials—Some materials show a greater moisture-vapor resistance than others. Laminated or compound-lami- nated sheets are more moisture-resistant than single sheets. The ratio between the area of the package and the weight of the food contained in it—The greater the package surface exposed per given weight of hygroscopic food material, the greater the absorption; that is, well-filled packages of hygro- scopic food materials permit the absorption of less moisture per given weight of contents than partially filled packages. Length of time the package will be exposed to humid atmospheres.—The amount of moisture absorbed through a material is in direct proportion to the time of exposure. Leakage of moisture through package wall due to defects.—Thin spots or holes in coatings on materials permit the passage of moisture vapor. Leakage of moisture vapor through breaks in the protective films caused by rough treatment.—Some moistureproof films are inelastic and their efficiency is impaired by abuse, which causes breaks in the continuity of the film. Leakage due to poor sealing properties or careless sealing.—Some thermo- . plastic coatings do not make a firm bond after heat sealing, and as a result the seams open. Improper sealing temperatures or technique may result in faulty seals. ; Leakage due to destruction of the coating during heat sealing.—Heat sealing at excessively high temperatures may impair the adhesive properties of thermo- plastic coatings or cause sufficient decomposition to affect moisture-vapor resistance. The moisture-vapor differential between the inside of the package and the storage atmosphere—Since dehydrated foods are hygroscopic, the relative humidity within the package is usually low. Packages are stored in atmospheres of high relative humidity. The attraction for moisture within the package is quite marked and the amount of moisture absorbed is directly proportional to the difference between the internal and external relative humidities. Rate of circulation of air in the storage space.—The circulation of humid air across the surfaces of packages increases the rate of moisture absorption through the packaging material. VEGETABLE AND FRUIT DEHYDRATION 131 Single Sheets, Double Laminations, and Compound Laminations A single-sheet substitute material to take the place of tin-plate or other hermetically sealable containers has not been discovered. For example, a close-textured paper heavily waxed on both sides satisfies all of the requirements with the exception that rough treatment ser1- ously impairs or destroys its efficiency. Laminations of two or more sheets of similar or different characteristics will frequently offset the weaknesses of single sheets. Such laminations as glassine paper to cellophane, cellophane to parchment paper, cellophane to cellophane, and other combinations show increased resistance to the passage of moisture vapor as compared with single sheets. With some laminations the increase in resistance to moisture is due to the thermoplastic adhesive used to cement the materials together. If the membranes are subject to “pinholing” or other imperfections during their manufacture, the adhesive seals the imperfections during the process of lamination. Far superior to double laminations is the recently developed com- pound lamination that includes a metal foil, which is an extremely moisture-vapor resistant material. Kraft paper is laminated to the foil with asphalt on one side and cellophane with thermoplastic adhe- sive on the other side (fig. 64). This compound structure protects ft: > WAX Dtesieb vs WAX HHH LACQUER Figure 64.—Construction of laminated packaging materials. the lead-foil sheet against pinholing or rupture due to flexing and other stresses. The finished laminated sheet has strength, waterproofness, resistance to moisture vapor, lightproofness, and exceptional heat-seal- ing properties. Its heat-sealing properties are due in part to the lacquer coating on the surface of the cellophane, and the seal strength is further enhanced by the thermoplastic coating applied to the inner cellophane surface. Its unusual moisture-vapor resistance is due to the multiple barrier that it presents. In order to pass through this struc- ture, moisture vapor must penetrate kraft paper, asphalt, lead foil, thermoplastic adhesive, and moisture-vaporproof cellophane. In use, this material is first fashioned into flat envelope-type bags. Before these bags are filled they are shaped by the use of a mandrel. After they are lined and filled, a partial vacuum is created in the con- tainer by means of suction and the bags are quickly and securely heat sealed. The sealed package is placed in an outer shipping container. This package has been found to approach closely the standards set by the hermetically sealed tin-plate container and, when the closure has been carefully completed, it complies with virtually all of the re- quirements necessary to protect dehydrated foods against deterioration caused by absorbed atmospheric moisture. The use of the less efficient packaging materials is always fraught with danger. However, certain of the less hygroscopic products can be packaged in less efficient materials, especially if they are to be held only a short period. With the less hygroscopic foods, such as certain of the dried fruits, and especially those with moisture contents 10 or | | | 132 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 15 percent above those permissible with vegetables, the high standard of efficiency is not required. None of the membrane or laminated-membrane packages will pre- vent the penetration of all insects, nor are they proof against the at- tacks of rodents. But properly dehydrated vegetables are too dry to support insect life. Eggs will live in the packages, however, and will hatch if moisture becomes available. Dried fruits, on the other hand, commonly contain sufficient moisture to maintain insect life as well as microbial activity, and special sanitary precautions are re- quired during packaging. Glaze Packaging for Compressed Dehydrated Foods Current investigations have shown that the packaging of compressed dehydrated vegetables by the application of a glaze of moistureproof thermoplastic material is entirely feasible. After the food is pressed to the desired density, the dehydrated product is tightly wrapped in heat-sealable membrane, such as waxed paper or heat-sealing cello- phane, and is then dipped into a molten mixture of thermoplastic waxes, which upon cooling solidify to a firm continuous film that pro- tects the food against absorption of moisture and contact with air. COMPRESSION TO HIGH DENSITY FOR PACKAGING In addition to the saving in shipping space that results when foods are dehydrated, futher saving can be achieved by compression. When a ship is loaded with cargo at the rate of 1 ton per 40 cubic feet, the holds are filled and at the same time the ship carries a full load. The objective in the compression of dehydrated foods should therefore be 55 to 60 pounds per cubic foot as the minimum density. This calcu- lated objective takes into account the fact that the finished package is a little less dense than unpackaged cakes. Investigations have shown that such densities can be attained without loss of quality in the recon- stituted product. Even higher densities are to be preferred if an oxygen-sensitive product is compressed and packaged in hemetically sealed containers. Dehydrated cabbage, carrots, and tomato-juice cocktail are examples. The denser the pack, the lower will be the ratio of oxygen in the can to the dry fruit or vegetable. A further advantage of compression is the diminished tonnage of steel and tin required when cans are used, since doubling the density halves the metal required. The percentage of saving in cans result- ing from compression is numerically equal to the percentage of re- duction in bulk (table 18, column 9). In addition to increased density, rehydration to original size and shape is an objective, with a low percentage of fines or small particles. In tests, fines that pass through a 4-mesh screen after rehydration have been kept under 5 percent. Weighed samples of blocks were rehydrated and screen analyses made of the cooked product. The low percentage of fines prevents a mushy texture and maintains palatability equal to that of unpressed, rehydrated products. The table shows the conditions under which fruits and vegetables have been compressed with 5 percent or less of fines. The time required for reconstitution is not increased over that required for unpressed foods. High densities in the pressed block require higher pressure when VEGETABLE AND FRUIT DEHYDRATION TABLE 13.—Compression of dehydrated fruits and vegetables Densities (pounds per Ap- Pres- cubic foot) 1 g@ | proxi- Tem- | Sure mate = for con- | Reduc- Mel thre of | Dlock tent of | tion in Fruit or vegetable ing Com- | pack- | bulk (per- | press- (Ibs. After pres- ages (per- cent) ane. per | Initial | CO™ sion (Ibs. | cent) 2 (© FB) square ee ratio per inch) si (tol) | 5-gal- ons) 1 2 3 4 5 6 ) yaa) oe? | SpplewmUece hse ee ee 2 17 450 12.5 64 5.1 12.0 | 71 1D (Up pea ee et eee 2 75 850 12.5 53 4.2 £20) | 64 IA PricotG HAalVveSs == fn ae et eee 75 150 42.0 79 1.9 28.0 | 44 COG CUDCS aaa ne eS a Ee 4.1 160 650 25.0 62 2.5 17.0 57 IB CC & SNCOSE) Mess oS ES See 5.7 | 120 650 ZED 64 ak 8.0 80 [BCCE SUELDS eee ae see ee 4,2 | 120 650 15.0 57 | 3.8 10.0 | 72 Warrot cubes 2228s oe Bo a 5.2 160 650 19.0 | 62} 3.3 17.5 | 50 Carrotisheese == sane oa be oe 4.8 160 650 6.0 | 56 | 9.3 8.0 | 75 @ALEOUSEEIDS oe ae we Sn! 160 650 | 10.5 AT 4.5 10.0 | 62 Omniontilakesssewte ee Pe 3.0 140 650 6.0 6h. 1021 12.0 | 60 PE REES aN Ole ese ate | 28.0 75 | 150 48.0 78 1.6 36.0 | 27 Rutabaga slices 3_______. ________--_ bogs 140 850 | 11.0 59 FS ea 68 ‘Tomatoes, spray-dried juice #________ | 4.0 78 1, 500 30.0 60 2.0 27.0 | 31 | | 1 Densities before compression were measured on shaken but unpressed material (column 5). Densities of commercial shipments are frequently higher. 2 The percent reduction in bulk equals the difference between the weights of a 5-gallon can of compressed vegetables and the corresponding weight of the uncompressed vegetable divided by the former weight. In computing the net weights of packages of the compressed foods, allowance is made for 15 percent of unused space in cans and for 5 percent in cartons. ; : 3 The tentative moisture content of rutabagas is 5 percent, at which blocking will produce a lower density than that found. 4 Spray-dried tomato-juice cocktail is packed 4 pounds per No. 10 (3 quarts) can. the dried fruits and vegetables are low in moisture content. Further- more, with less moisture content any given pressure will produce a higher density and a more cohesive block if the dried product is hot instead of cold. The densities shown in the table were measured on disks one-half to three-fourths inch thick. These disks were kept in cans without wrapping while they cooled, with the exception of onions and carrots, beets, and rutabagas, which were pressed into 1-pound blocks 214 inches thick and were cooled in a holding press. With onions, beets, rutabagas, and carrots it has been found that blocks tightly wrapped in cellophane, with the wrapping sealed, need not be kept in a press to cool. During the cooling the densities under such conditions decrease from 61 to 59 pounds per cubic foot for onions and from 62 to 58 pounds for carrots. Sun-dried apricots, cut and pitted, can be compressed by hand at room temperature to a bulk density of 42 pounds per cubic foot. A pressure of 150 pounds per square inch at room temperature has resulted in a density of 79 pounds per cubic foot. Higher pressures and an elevated temperature are required for high densities when fruit products are dehydrated to as low as 2 or 3 percent moisture content. The table shows data on apple nuggets for which, because of a 2 percent moisture content, a pressure of 450 pounds per square inch and a temperature of 170° F. were required to produce a block density of 64 pounds per cubic foot. At 75°, 850 pounds of pressure was required to produce a density of 53 pounds per cubic foot. For the K rations used in the Army, fruit bars with 20 percent mois- ture and containing glucose are made by extrusion from machines of the sausage-making type on a belt where the bars are cut to length. 134 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE These machines use a worm screw to force the fruit paste through a tapered nozzle. Determination of a suitable taper for a particular product requires some experimentation. The bars are wrapped after cutting. If they are to be pasteurized, a suitable wrapping consists of a layer of greaseproof paper for use in handling, a cellophane bag that is heat-sealed after insertion of the bar, and a light chipboara box. A typical pasteurized fruit bar packaged in this manner has a density of 80 pounds per cubic foot. The maximum attainable density of dried fruits and vegetables can be estimated from (1) existing analytical data on their pulps and (2) data on the densities of their constituents. For dried peaches the esti- mated attainable density is about 75 pounds per cubic foot at 20 per- cent moisture content, or 85 pounds if free from moisture. Presses and Processes Hydraulic presses have been used in Germany for the compression of mixed vegetables, herbs, carrots, cabbage, and dried sauerkraut. Continuous tile presses are reported to be in use, in which pressure is imparted by two cams acting at different stations in the machine. Knuckle-joint presses are used in the United States to compress hops, | and screw presses are in use for compressing dehydrated-egg powder. Some free-flowing powdered soups have been smoothly handled in in- dustrial tableting machines on an experimental basis. The latter are not well suited for use in forming blocks of dehydrated vegetables because: (1) The vegetables are not free-flowing, and therefore auto- matic and uniform charging of the molds cannot be accomplished in typical machines; and (2) such tableting machines ordinarily operate at compression ratios of 2 to 1, or less. ‘The compression ratio is the ratio of the volumes of the filled mold before and after compression. It can be computed by dividing the density after compression by that before compression. Column 7 in the table shows such ratios. Press makers must know the compression ratio in order to determine the length of stroke of the ram (fig. 65). The capacity of a press is usually stated in terms of the total safe working pressure between the platens. Molds may have one cavity or multiple cavities. Dried fruits and vegetables have been formed into blocks 6 x 83x 1 inches in size. Blocks 214 inches thick have been formed and a brick approximately 614 x 414 x 214, inches with 1 edge rounded has been proposed, since it lends itself well to packing in standard 5-gallon cans at the rate of 16 blocks per can. Such a block weighs 2 pounds and can be reconstituted into 50 servings of carrots (fig. 66). Military recipes are in terms of 100 servings (43) for companies, and probably the most suitable blocks should contain weights corresponding to this number of servings. Blocks can also be formed with indented scorings like those on milk-chocolate slabs to facilitate breaking the blocks into smaller units of known weight. To reduce friction, the surfaces of molds must be very smooth, a condition which can be produced by a surface grinder. It is fre- quently the practice to follow this by nickel or chromium plating and polishing. The surface of new dies must be lubricated with salt- free, moisture-free, edible oils or fats such as commercial hardened shortenings or special edible lubricants. Continuous use of lubricants may be required to reduce the friction as the fruit or vegetable moves VEGETABLE AND FRUIT DEHYDRATION 135 Ficure 65.—Hydraulic press used in compression and packaging research on dehydrated foods at the Western Regional Research Laboratory, Albany, Calif. by the plunger past the sides of the mold. Friction resulting from unpolished, inadequately lubricated steel surfaces diverts part of the pressure from the foodstuff to the sides of the mold; thus the effective pressure is decreased at the bottom of the mold, and poor cohesion and a laminated, leafy brick may result. ; Sometimes very fine, dried materials or even powder are com- pressed. Examples are soup mixtures and dehydrated tomato-juice cocktail. Most dry powders flow readily and uniformly and such conditions are essential in the automatic feeding of presses. De- hydrated soup powders have been processed satisfactorily on indus- trial tablet-compressing machines. Dried tomato juice of 4 percent moisture content has been readily compressed hydraulically by 1,500 136 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE pounds per square inch at room temperature to a density of 60 pounds per cubic foot. The cakes were reconstituted to a 6.5 percent Juice simply by beating with an egg beater and then adding boiling water and heating 1 minute. The dry compressed cakes crush easily. Dehydrated vegetables of diced, sliced, stripped, or flake form yield high-density blocks that readily rehydrate and in which the content of fines is low. Hydraulic presses have been used in obtaining the results shown in table 13. The maximum pressure has been main- tained for a period, or “dwell,” of 1 minute. Blocking has been ef- fected ordinarily at temperatures from 140° to 160° F. The purpose of an elevated temperature is to produce a phable state of the vege- table for compression so that it will reach a high density without breaking into small pieces to any considerable extent. Firm blocks cf somewhat lower densities have also been produced at ram speeds of 1 to 2 inches per second followed by a “dwell” of 15 seconds. Fine material passing a 4-mesh screen, caused by compression under the conditions shown in the tabulation, did not exceed 5 per- cent of the rehydrated vegetables for any form of beets or carrots, or for onions or rutabagas. In general, the cohesion of the block was only fair at lower pressures and the same temperatures as those listed in the table. Higher pressures were not required for good cohesion nor were they necessary for adequate ,compression. A shght expansion of blocks one-half inch thick occurs in that dimension when they are removed from the hot molds. This ex- pansion is slight because these pieces cool rapidly. An estimate of FIGURE 65.—Sixteen 2-pound bricks in arrangement shown at right fill a 5-gallon can with 800 servings of carrots. VEGETABLE AND FRUIT DEHYDRATION 37 the time required for cooling thick blocks of carrots in air is afforded by these test data: A block 214 x 13 x 634 inches of compressed, diced earrots cooled at the center from 140° to 90° F. in 3 hours, in air at 80°. The block was cellophane-wrapped, confined in a holding press, exposing the narrow sides and ends only. Removed at 90°, it held its shape. - Two-pound blocks of diced carrots were formed in a mold 61145 x 43% inches of a suitable depth. The conditions were 160° F., 1,200 pounds per square inch, and a 30-second period at that pressure. The blocks were removed, wrapped in paper and cellophane, and sealed. After compression the dimensions over the wrapping were 6174, x 41% x 234, inches. The expansion in a direction perpendicular to the movement of the ram was 549 inch on a 614-inch length, with allow- ance for thicknesses of paper and cellophane. The immediate expan- sion parallel to the movement of the ram was three-eighths to one-half inch. An additional expansion resulted before it was stopped by the wrap and by air cooling. Holding presses prevent the latter expansion. Actual practice will be a compromise between the goal of most de- sirable density, on the one hand, and, on the other, the obtainable equipment and its economic adaptation to use. A diagrammatic sketch of a packaging press 1s shown in figure 67. Blocks should be wrapped and sealed at once in kraft paper or cello- phane. If the packing case is large enough to permit slight swelling of the blocks, they may be packed immediately but if very close ad- herence to the dimensions of the mold is required, the blocks must be cooled under light pressure, part way to room temperature. If compression is carried out with the wrapper inserted in the mold, complete cohesion of the block is not required. With this latter method of compression and wrapping, the wrapper must maintain its integrity as a moisture-vapor resistant sheet without tears, pinholes, or other breaks. Holding Presses The use of holding presses for the purpose of fixing the shape and dimensions of tobacco plugs and blocks of dried hops, catnip, and sage is well known. The time required for tobacco is 5 days; for hops, 12 hours. Such holding presses are not heavy or expensive, because little pressure is used. In one form a hardwood frame of rectangular shape is used. It has a solid bottom end and a loose top end. Plugs are stacked in it nearly to the top, the loaded frame is placed under a small screw press, the top pressed down, wedges are inserted above the top and against the top of the frame, and then the pressure is released and the loaded frame is removed. This type of press, with spaced blocks for ventilation, is one suggested way to cool under pressure. : SANITATION A primary essential in a dehydrating plant that is endeavoring to make products of the highest quality is the maintenance of suit- able standards of plant sanitation. Among the important factors affecting plant sanitation are building construction, equipment, access cf rodents and other pests, storage facilities, water supply, waste disposal, and, most important of all perhaps, the operating personnel. 138 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE A plant should consist of well-constructed buildings with concrete floors capable of being satisfactorily cleaned, and free from crevices. The floors should be sloped to drains that are tightly joined to the sewerage system. All preparation and drying equipment that comes Conveyor Feed Hopper Pre ss Cakes Za ELEVATION Figure 67.—Packaging press. : into direct contact with food should be washed thoroughly at fre- quent intervals. There are definite possibilities of a build-up of microorganisms, such as bacteria, molds, and yeasts, that may con- taminate and even spoil the material that is put through such a piece of equipment. Stored raw material may contain fungus or bacterial ts He VEGETABLE AND FRUIT DEHYDRATION 139 diseases which often spread rapidly. The storage bins or containers should be washed after the removal of each lot; otherwise infection will spread to other raw materials brought in later. Water Supply A most essential matter is a good water supply, which is both pure and abundant. It should be analyzed frequently because the character of water changes. Many times the waste from the plant | _ itself contaminates the water supply. In general, hot water does a better cleaning job than cold water. The use of a steam hose can be very beneficial, but the steam must be applied under pressure directly to the area to be cleaned. Detergents added to water are valuable in removing secretions of organic material from plant and equipment. Soap and cleaning compounds are excellent detergents. The efficiency of the cleaning operations may be enhanced by the use of chemical compounds con- taining chlorine. The use of a germicide alone without the cleansing operation is valueless, however. It should be associated with or im- mediately follow cleaning. Waste Disposal The selection of a dehydration plant site should include careful consideration of the waste-disposal problem. The capacity of the intended place of final disposal (creek, treatment plant, disposal bed, etc.) must be carefully evaluated. If the permissible loading is found to be less than that transported by the plant wastes, it will be necessary either to treat the wastes to lower their organic loading to the permissible value or else to move the plant to some other loca- tion where treatment will not be required. . The waste-disposal system is another matter of great importance, especially if an adequate municipal sewerage system is not available. Unless the wastes are properly conducted away from the plant, there is likelihood of pollution of the plant water supply, of odor nuisance, and even legal suits by property owners in the vicinity. Waste ma- _ terials from vegetable-dehydration plants may be classified as “solid wastes” and “liquid or water-borne wastes.” Solid wastes, such as trimmings, are in most cases readily disposed of as garbage, and do not enter into the liquid-waste disposal problem. Liquid wastes from vegetable-dehydration plants are derived from the various washing operations. Most important are the wastes from the peeling units and the wastes resulting from washing the vege- tables after they have been cut into small-sized cubes, bars, slices, or other shapes. Of lesser importance are the wastes derived from washing the raw product and the intermittent plant wash-up waters. Those liquid wastes are difficult to dispose of not only because of their volume, but also because of the large quantities of soluble and sus- pended organic matter which they transport. The organic material contained in the wastes from a potato-dehydration plant of 20 tons (unprepared basis) daily capacity, for example, may be roughly equivalent to that contained in the sewage flow from a town of 8,000 people. This organic matter will combine chemically with oxygen, when it is present, so that if the wastes are dumped into a small creek, drainage ditch, or other small body of water, the dissolved oxygen resources of that body of water will be quickly depleted. 140 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE When all the dissolved oxygen has been used up, the remaining (un- oxidized) organic materials will begin to decompose anaerobically (ferment), and odoriferous and unsightly conditions will result. If the wastes are dumped into a sewer which flows into a sewage-treat- ment plant, trouble can again be expected. Most treatment plants are designed to handle only a limited loading of organic matter, and if this amount is exceeded the entire operating balance of the plant will be upset and objectionable conditions will exist. It is usually desirable to separate the dehydration plant-food wastes from toilet wastes. The relatively small volume of the latter can be treated separately by approved methods, which involve little difficulty. If a dehydration plant is located in sandy terrain and there is suf- ficient area available, the waste water can be drained into the soil. It may be necessary to remove all suspended solids prior to the final disposal of the waste. This may be accomplished by first running the wastes from the plant through mechanical screens having a mesh of from 40 to 60 per inch. After this, the effluent is treated with a chemical agent which will change the pH of the product. Lime is one of the agents most commonly used for this purpose. By the ad- dition of sufficient lime so that the pH is approximately 9.0, the waste — material is rendered alkaline. Iron sulfate is another agent that can be used. ‘These chemicals will facilitate the precipitation of finely suspended materials so that the waste water can be run into a settling tank, and the suspended solids removed by gravity. Even after such treatment, further treatment may be necessary when the rela- tively clear water is removed from the upper levels of the tank. This may involve the utilization of trickling filters or spray devices which permit an aeration of the wastes. In some instances, the waste material can be run directly into shal- low lagoons or closed areas of land diked on the edges, from which the liquids drain into the soil. The addition of small amounts of sodium nitrate has been found beneficial in controlling bacterial changes at this stage of waste treatment. The best solution of the waste-disposal problem for any particular plant is a complicated one, and will generally require the services of a competent sanitary engineer. Careful consideration must be given to the possible means of final disposal, the degree of treatment re- quired by each, and the funds available for building and operating treatment works. Fossibly the most satisfactory scheme of disposal for general use is that comprising primary treatment by settling, with the effluent from the settling tank disposed of on irrigation beds, the liquid percolating into the ground. This scheme of disposal avoids any use of public sewerage systems and is therefore recommended whenever suitable irrigation beds are available. Operating Personnel No matter how good the plant, the equipment, and the water supply, if the employees have sanitary habits that are open to question, the products which they handle may be contaminated. If workers happen to have disease organisms on their hands, there will be disease organ- isms on the products. Employees cannot be expected to keep clean unless proper facilities are available, Dirty wash rooms and dirty latrines, with no towels VEGETABLE AND FRUIT DEHYDRATION 14] or soap, are not conducive to habits of cleanliness on the part of the employees. Some of the best food plants in the country have their washing facilities outside the rest rooms and the toilet room. The employees must wash their hands where they can be seen by the super- visor and the other employees in the plant. No one is permitted to go back to his work unless he has washed his hands in that place. Hot water, sanitary soap dispensers, and individual towels are provided. CONTROL OF INSECTS AND MITES Food manufacturing or processing plants, including dehydration plants, have the almost universal problem of insect and mite control. The most effective means of preventing the access of such pests to both the raw and finished materials is by the use of tight construction, ratproofing, screens, and thorough sanitation. Unceasing vigilance is necessary in removing the waste food materials upon which these pests thrive. There should be regular and thorough inspection of the premises and sampling of the products. Scalding water or vapor heat in the form or live steam will kill all forms of insect life on direct exposure. Fumigation A common method used in the control and extermination of insects and mites is fumigation. It can be used to treat suspected products, to check known infestations, and to clean out infestations and the premises housing them. It is especially suitable for areas that can be tightly enclosed. Fumigation is quick and effective and if properly handled leaves no complicating after effects. It should be done by trained or experienced personnel, with proper equipment, and at the right time and temperature. The chemical to be used is determined by the product and insects to be treated, and by the storage conditions under which the treatment is to be made. The absorption and adsorp- tion of the gases by the infested materials must be considered. After - treatment, the product must be properly aerated to allow the escape of contaminating gases. : Specially constructed, tight, ventilated fumigation rooms are extremely useful in treating sacked and packaged material. Steel vacuum fumigators are up-to-date devices for the treatment of insect-infested products. Some of these are large enough to enclose one or more freight boxcars at a single treatment. If possible, all fumigants should be applied from the exterior. Gas masks equipped ‘with an unsaturated canister of proper type for the fumigant used should be worn when handling or applying toxic chemicals. All fumigants which are effective in killing insects are also toxic to humans. Some gases, such as hydrocyanic acid gas, methyl bromide, and others, are extremely dangerous and should be used only under expert supervision. The correct length of exposure varies with the concentration of the gas and the temperature. The temperature for the best results is usually 70° F’. or higher. Sprays A spray composed chiefly of 10 to 15 percent oil emulsion, in which ‘the viscosity of the oil is 90 or above, combined with an emulsifying agent, is of great value in disinfecting empty bins. Water-white — ee Se 142 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE kerosene alone or combined with oil extracts of pyrethrum or lethan is excellent also. These sprays may be applied by hand or power sprayers for small areas, or by regular orchard power sprayers for lange and extensive structures. They should be applied to the ceilings, walls, floors, under the floors, and, in fact, everywhere insects or mites are likely to find hiding places. a The addition of 1 percent of creosote will kill the wood-boring cadelle and the lesser grain beetle in wooden containers. While there is no great fire hazard with the careful use of these sprays, proper precautions should be taken to prevent fires and to secure the sanction of insurance companies and underwriters involved. Temperature and Moisture Proper temperature and moisture control of storage facilities may effectively prevent infestation or retard the activities of insect pests indefinitely. Lack of such control may favor the insects and mites by providing ideal conditions for their development, feeding, and breeding in the foodstuffs. Even and mild to warm temperatures combined with a high moisture content in the product (10 percent and above) will favor — pest development by making the food more attractive to pests and the living conditions ideal. Thus, in the milder temperate regions and in the tropics, it is difficult to store dried fruits and vegetables because of the destructive work of mites, insects, fungi, and bacteria. Regions of great extremes of temperature offer a certain amount of protection to stored foods. Low temperatures and ordinary cold storage of 40° to 50° F. will prevent insect development in ware- houses. Temperatures of 30° will kill all stages of the insect de- velopment if maintained constant throughout the bulk of the material. High temperatures of 120° to 130° F. will prove effective in killing all forms of mite and insect life. The heat may be supplied by steam pipes and has proved effective where practical. Fans are invaluable in sterilization work. The effectiveness of temperature is determined by the extent to which it is made to operate throughout the sacks, bins, or entire storage facilities. Unless the materials in storage are arranged so that temperature can penetrate evenly and effectively throughout the bins and sacks, it is necessary to provide proper ventilation or to bring the material into direct contact with the temperature. This will add considerably to the handling and shifting of the produce unless the material is stored in bulk in relatively small bins or units. THE CONTROL LABORATORY Every food-dehydration plant, large or small, must have a control laboratory. ‘This laboratory may consist of minimum essentials, such as equipment and personnel for moisture testing, testing for adequate blanching, and testing of quality in raw and finished products, or it may be much more elaborate, even sufficient for research work. Regardless of size, it 1s an important feature in any plant, and it should be regarded as a unit worthy of separate consideration and not as an incidental matter. The man or woman in charge should have chemical training and plant experience and should be free to VEGETABLE AND FRUIT DEHYDRATION 143 devote as much time as possible to the work of the laboratory without too many additional duties. Equipment is important. The laboratory should have well-lighted, roomy space, shut off from noises and odors. It should be free of dust, steam, and excessive vibration. Some of the necessary facili- ties and important items of equipment are work benches or tables, water, gas, electricity, and a sink. Although not essential, a refrig- erator is very useful. Cupboard space for equipment, glassware, and supplies, and shelves for sample storage should be provided. Other items will be mentioned in subsequent discussions of the various types of tests and inspection work that are ordinarily carried on in plant-control laboratories. Analysis, however carefully made, can do no more than give the composition of the sample. It is essential therefore that the sample be taken in such a manner that it will represent the lot under test. It must be remembered that variations in composition can be found in almost any lot of material. Portions of the material must be taken from various parts of the lot under test, carefully mixed, and either quartered by hand or, if the material is suitable, portioned with a rifle sampler. In this way a representative sample can be obtained. The remainer of the collected material can be returned to the lot. After the sample has been collected it should be treated in such / a manner that it will have the same composition when analyzed as when collected. Refrigeration can be used to hold samples for short periods of time. Samples to be analyzed for moisture must be kept tightly sealed until the analysis is made. Prompt analysis of samples after collection is always desirable and is almost a necessity when the sample is fresh material. : Examination of Raw Product Since the quality of processed foods is greatly influenced by the stage of maturity of the commodity when harvested, canners and freezers give special attention to the maturity of raw products, and no less attention is required in dehydration. Other factors, such as color, uniformity, insect infestation, dirtiness, mechanical damage, and wilted condition are important and some attention has been di- rected toward the development of objective tests for evaluating some of these factors. Materials which would be rejected for canning or freezing are equally unacceptable for dehydration. Unfortunately few satisfactory tests for quality of the raw product have been devised and reliance must be based largely on general appearance as interpreted by an experienced operator. A few commonly employed tests are briefly described below. TENDEROMETER TEST (PEAS) One of the most recently developed methods for peas measures the force required to press a definite volume through a standard grid; the force necessary to shear the peas is directly proportional to toughness and inversely proportional to tenderness. The instrument used for this purpose is known as the tenderometer. Procedure: The tenderometer value is determined for each load of shelled peas as it comes to the packing plant. Care must be taken to obtain a representative sample of the load. 'This is usually accomplished by filling a No. 10 can from lug - boxes at several points. The peas are then cleaned and thoroughly mixed. 144 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Before the tenderometer reading is taken it is desirable to bring the peas to a temperature of about 60° F. by immersion in water for 3 to 5 minutes. At least three readings should be taken, and the average used to represent the tender- ness of the load. For first-grade peas the tenderometer value should probably not exceed 105; for second-grade, 115; and for third-grade, 125. It may be expected that third-grade peas will yield a very poor quality of dehydrated product. COLOR TEST (GREEN LIMA BEANS) The maturity of green lima beans is generally estimated by deter- mining the percentage of white beans in a well-mixed, representative sample of the raw material. Procedure: The sample should be obtained by subsampling various parts of each load. Usually each packer sets up his own percentage limits of white beans for different grades ; however, it is generally required that first-grade green lima beans contain less than 10 percent of white beans. PRESSURE TEST (SWEET CORN ) The maturity of sweet corn is usually determined by the thumbnail test and observation of the character of the expressed juice. Procedure: The pressure required to break the hull of several kernels on several cobs selected from a representative sample is noted. At the stage of | maturity desirable for dehydration the expressed juice is milk white in color and has acreamy consistency. If the juice is slightly cloudy the corn is too immature, and if it is thick and sticky the corn is overmature. Obviously this method is subject to considerable variation due to the personal factor. MOISTURE CONTENT (SWEET CORN ) The maturity of sweet corn is sometimes estimated on the basis of moisture content in a representative sample. For dehydration, first- grade corn should have a moisture content of between 70 and 75 percent. Corn with a moisture content greater than this range will probably be too immature, and with a lower moisture content it may be too mature. ‘Procedure: Use the rapid distillation method (p. 146) on a carefully chosen sample obtained by careful removal of several kernels from several cobs selected from a representative sample. Moisture Determination Moisture content is probably the most important criterion of keep- ing quality of dehydrated products. a EL <= | 7 152 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE er Procedure: Five grams of finely divided sample material is weighed, placed upon a Biichner funnel and washed with warm water (50° C:) under gravity flow (about 200 ml. total). The filter paper and the softened washed sample are transferred quantitatively to a blender and disintegrated in the presence of 200 ml. of 90-percent acetone.“ Five grams of Filter-Cel is added during the last 30 seconds of desintegration and the suspension is filtered under reduced pressure directly into a 500-ml. volumetric flask through a sintered-glass funnel. The cake formed upon the filter by means of a tamper and the application of vacuum is successively washed with a 50-ml. portion of 90 percent acetone, resuspended in an equal portion of acetone, reformed, washed, resuspended, ete., until the residue and filtrate are colorless. The filtrate is then made to volume (500 ml.) with 90-percent acetone. ‘In the analysis of fresh samples, 25 gm. of thinly sliced plant material (50 em. if weakly pigmented) is disintegrated in a blender in the presence of 200 ml. of 90-percent acetone.” Subsequent steps are identical with those given above for dehydrated materials. A 100-ml. aliquot of the 90-percent acetone extract is mixed with 75 ml. of diethyl ether in a 500-ml. separatory funnel and the acetone removed by the con- tinuous washing method of Hubert (22) and LeRosen (26). In this method water is continuously introduced below the ether layer in the separatory funnel through a glass tube bent upward at the end. The tube is held in a rubber stopper which closes the upper opening of the funnel. The stream of water introduced through the tube and emitted from the lower stopcock washes the acetone from the ether hyperphase. Traces of water remaining in the hyperphase are forced out with the addition of 20 ml. of petroleum ether and the water drawn off through the lower stopcock. The hyperphase is then made to volume in a 100-ml. volumetric flask with petro- leum ether. Triplicate 25-ml. aliquots of this solution are placed in 50-ml. round-bottom flasks, and the solvents are evaporated off under reduced pressure with a small stream of nitrogen in a constant-temperature bath held at 40° C. Each of these samples is redissolved in a few cubic centimeters of petroleum ether and transferred quantitatively to a calcium phosphate (CaHPO:) chromato- _ graphic column as described by Moore (32). Carotene is washed through the col- umn with petroleum ether and caught in a 100-ml. volumetric flask. Other pig- ments are retained on the column. After dilution to volume, the carotene is determined spectrophotometrically by the use of wave length 486 uw (a=199) as suggested by Beadle and Zscheile (6) or colorimetrically with the use of a blue filter. In the latter method, the colorim- eter can be calibrated by the use of solutions of known concentration of pure crystalline beta-carotene dissolved in petroleum ether. ASCORBIC ACID The following procedure for the determination of ascorbic acid (27) has been found particularly useful in handling fresh, frozen, and de- hydrated food materials. Apparatus: Photoelectric colorimeter of the direct-reading type with a filter in the 520-millimicron range, blender, calibrated pipettes of 1-ml. and 9Q-ml. capacities. . Reagents: Metaphosphorie acid, 1 percent (freshly prepared) ; indophenol dye solution, approximately 13 mg. per liter. If metaphosphoric acid is unavail- able, oxalic acid (0.25 percent) can be substituted, provided the extracts are tested promptly. Procedure: Blend 25 to 50 gm. of fresh fruit or vegetable tissue with 500 ml. of 1-percent metaphosphorie acid in a blending machine operated for 5 minutes at high speed. If the material is of high ascorbic acid content, such as leafy vegetables, raspberries, strawberries, or asparagus, use the smaller quantity. Fifty grams is used with foods containing less ascorbie acid, such as potatoes, carrots, peaches, plums, and apricots. If a dehydrated fruit or vegetable is 14This washing process is unnecessary for many dehydrated vegetables, such as carrots, spinach, chard, and others. In these cases 25 ml. of hot water (50° C.) is added to the d-gm. Sample in the weighing bottle and allowed to stand 1 hour. The sample and excess water are then transferred quantitatively to a blender with 175 ml. of acetone, and the sample is disintegrated and extracted as described. 18 Some difficulty has been experienced in obtaining complete extraction of raw carrots by the procedure described. Steam blanching for 2 to 5 minutes prior to disintegration has been found to facilitate the extraction. Since carotene is stable in steam blanching of this duration, the inclusion of this step is recommended with raw carrots. VEGETABLE AND FRUIT DEHYDRATION 53 being analyzed, 5 to 10 gm. of sample is sufficient, in accordance with this same classification. These may require a half hour of soaking in the acid before blending. Centrifuge or filter the extract through fluted filter paper. Moderate turbidi- ties do not interfere, since the instrument is calibrated with proper blanks. Ex- tracts of starchy vegetables, such as potatoes, should be centrifuged or suction- filtered, if difficulty is encountered with the ordinary filter. Pipette 1-ml. portions of the filtrate into three matched tubes from the photo- electric colorimeter. Ajdd 9 ml. of distilled water to one tube and adjust the colorimeter to read 100 with this tube. To each of the other tubes add 9 ml. of a previously standardized indophenol dye solution from a calibrated rapid-delivery pipette. Take a reading in the colorimeter 15 seconds after the beginning of the addition of dye. This read- ing is G., from which the corresponding ZL, value is obtained from the calibra- tion table provided with the instrument or calculated from the following formula: L=2—log G or log 1/T. Ascorbie acid, mg. per 100 ml. of filtrate=K (I,—Zz). K must be determined for each colorimeter by preparation of a curve with pure ascorbic acid. K=C/D. C=concentration of ascorbic acid in mg. per 100 ml. D=density= (L,—IL2). The equation for fruit and vegetable tissue becomes: Ascorbic acid, mg. per 100 gm. tissue= ml. acid extractant+ml. water in sample — is 2) gm. of sample id With dehydrated vegetables, the moisture content (if below 5 percent) can usually be neglected. The formula then becomes: ml. acid added gm. sample Ascorbie acid, mg. per 100 gm. product=K (L£,—TLz) Add the dye to the tube outside the colorimeter and agitate the tube slightly © before putting it into the instrument. If an automatic 9-ml. pipette is used, it must extend to near the surface of the liquid in the tube to avoid splash- ing and must be calibrated to drain uniformly in less than 5 seconds. The dye is standardized by noting the 15-second reading given by a tube containing 1 ml. of 1-percent metaphosphorie acid and 9 ml. of the dye solution. This value is G:, from which L: is obtained on the calibration table or from the formula above. This standardization is very much easier and faster than titrimetric procedures. THIAMINE AND RIBOFLAVIN Because of the relatively infrequent determination of thiamine and riboflavin in dehydrated fruits and vegetables, details are not presented here. It has been found that the method of Conner and Straub (22) is useful for both fresh and dehydrated products. This is a fluorometric method in which the vitamins are extracted by dilute acid and separated from one another by adsorption procedures. Determination of Sugar (Vegetables) Determination of sugar content is valuable in examinations of raw products for maturity or loss of quality in storage. It is often used on dehydrated products, such as carrots, sweet corn, and potatoes, to indicate quality, loss of solubles in blanching and washing, and causes of difficulty in drying, and also to obtain information useful in the compression of the dried materials. The method described here * 16 Essentially as developed by J. P. Nielsen of the Western Regional Research Laboratory by modification of the method of W. Z. Hassid (29, 20). 154 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE has been found rapid and very satisfactory for both fresh and de- hydrated products. Apparatus: Blender, analytical balance, volumetric flasks (50 ml.), burette (10 ml.). Reagents: Ethyl aleohol, saturated solution of neutral lead acetate, saturated solution of disodium phosphate, dilute hydrochloric acid (mix 100 ml. concen- trated HCl with 1,100 ml. water), sodium hydroxide solution (24 gm. NaOH made to 100 ml. of solution with distilled water), alkaline potassium ferricy- anide solution (8.75 gm. K;Fe(CN). plus 10.6 gm. anhydrous sodium carbonate made to 1,000 ml. with distilled water and stored in refrigerator), sulfuric acid solution (5 N), standardized ceric sulfate solution (0.01 N), phenolphthalein (0.1 percent in 90-percent alcohol), Setopalin C indicator (0.1 percent in water), Mohr’s salt solution (0.01 N), dextrose solution (1 percent in 0.25-percent aqueous benzoic acid). Procedure: Before the test is applied dehydrated materials must be rehy- drated by heating in distilled water at 50° to 60° C. for 2 hours. An amount of dehydrated material corresponding to 60 gm. of fresh carrots, 75 gm. of fresh corn, or 150 gm. of fresh potatoes (or approximately equivalent amounts of other products, based on their estimated sugar content) is added to 150 ml. of water plus an additional amount calculated to bring the dehydrated material to the fresh basis. With fresh products only the 150 ml. of water is added. The mixture, fresh or rehydrated, is thoroughly broken up by agitation for 2 to 4 minutes in a blender. A 15-gm. portion of the resultant suspension (keep well mixed to maintain uniformity) is transferred to a 50-ml. volumetric flask with a minimum amount of ethyl alcohol. Add 2 ml. of a saturated solution of neutral lead acetate, mix, and add 4 ml. of a saturated solution of disodium phosphate. Make up to volume by addition of water and alcohol in such pro- portion as to yield a final mixture of about 50-percent alcohol. Mix thoroughly and filter through a folded filter paper. For total sugars, transfer 5 ml. of filtrate to a 50-ml. volumetric flask. Add 10 ml. of the dilute hydrochloric acid. The sample can then, according to con- venience, be heated in a boiling-water bath for 10 minutes, cooled, made up to 50 ml. with distilled water, and mixed; or it can be allowed to stand 24 hours at room temperature (approximately 25° C.), made to mark with distilled water, and mixed. Transfer 5 ml. of the solution to a 22 x 175-mm. test tube; add 1 drop of phenolphthalein and the NaOH solution until the pink color develops. Add HCl until pink color disappears. Use a dropping tube for additions of NaOH and HCl, as only a few drops of each are required. Add 10 ml. of the alkaline potassium ferricyanide solution and heat in a boiling-water bath 15 minutes. Cool and then add 8 ml. of 5N H.2SO;. Add 7 to 10 drops of Setopalin C indicator and titrate with ceric sulfate solution from a 10-ml. burette. The end point is indicated by the sharp change from greenish yellow to golden brown. Ceric sulfate may be standardized against 0.01 N solution of Mohr’s salt. An exact dextrose equivalent can be obtained with 5-ml. aliquots of dilute dextrose solution made up from the 1 percent stock solution by dilution so that 5 ml. contains 2.5 mg. of dextrose. It is advisable to standardize the ceric sulfate with the dextrose solution at least once a month. If it is more convenient, pure sodium oxalate may be used for the standardization instead of dextrose. In this instance the equivalent weight of invert sugar is 33.8 gm. and of sucrose 32.1 gm. It is usually advisable to run titrations in duplicate and to take the average. Generally the agreement is within 0.05 ml. of the ceric sulfate. A blank run should be made on alcohol and the reagents added as in a regular determination. This value, usually 0.1 to 0.2 ml. of ceric sulfate, is subtracted from the values for the samples to obtain true values. By means of the dextrose value of the ceric sulfate as found by titration of pure dextrose, and the amount of aliquot as run, the percentage of sugar, expressed as dextrose, is readily found. The aliquots titrated should not have more than about 7.0 mg. of dextrose. VEGETABLE AND FRUIT DEHYDRATION 1 Or Determination of Starch (Vegetables) Starch determinations are often useful in evaluating the quality and maturity of starchy vegetables and may be valuable in considering their drying characteristics. Nielsen’s method (37) has proved rapid and useful and is described as follows: Apparatus: Photoelectric colorimeter with test-tube adapter, a dozen or more matched test tubes, red filter, blender, balance (analytical or torsion sensitive to 0.01 gm.), air oven. Reagents: Perchloric acid (2.7 parts by volume of 72 percent reagent-grade Solution plus 1 part water), sodium hydroxide (2 N), acetic acid (2 N), potassium iodide solution (10 percent), potassium iodate (0.01 N), sodium thiosulfate (0.1 N), ethyl alcohol, ether, phenolphthalein (0.1 percent in 90 percent alcohol). Procedure: A 100- to 200-gm. sample of the fresh, frozen, or canned vegetable (or equivalent amount of dehydrated material rehydrated by heating one-haif hour or longer in an amount of water necessary to give the 100-gm. sample) is placed in the disintegrator cup with an equal weight of water, and the instrument is allowed to run at high speed for 3 to 4 minutes. Two or 8 grams of the ground sample, depending upon starch content, is weighed directly into a 380-ml. beaker on a torsion balance. One ml. of water (or none if 3 gm. was taken) is added, and then exactly 3.7 ml. of perchlorie acid, prepared as directed above, is slowly added with thorough stirring with a glass rod, so that there will not be momentary high concentrations of the acid in any portion of the sample. The mixture is allowed to stand with occasional stirring for about 10 minutes. If the product is not viscous, the simplest method of stirring is to whirl the contents of the beaker during the addition of the acid as well as in the later stirrings. After standing, the mixture is made up to 25, 50, or 100 ml. with distilled water, depending upon starch content, and then poured into a Suitable test tube to settle. A 1-ml. aliquot of the supernatant liquid is pipetted into a 100-ml. beaker and 6 ml. water added. A drop of phenolphthalein is added and the solution is brought to a pink color with a few drops of 2 N sodium hydroxide. Now 2.5 ml. of 2 N acetic acid, 0.5 ml. of 10 percent potassium iodide, and 5 ml. of 0.01 N potassium iodate are added accurately, and the solution is allowed to stand at least 10 minutes with occasional stirring. Dilute to 25, 50, or 100 ml., depending upon starch content. The color is estimated in a photoelectric colorimeter with a red filter having a transmission range from 640 to 700 milli- microns. The colorimeter should be set at zero absorption or the readings cor- rected with a blank containing all of the reagents. If the filtrate used in developing the starch-iodine color is turbid, an extra blank reading should be made if precise results are desired. This can be done by discharging the blue color with a few drops of 0.1 N sodium thiosulfate and comparing the turbid solution against the first blank with its iodine color discharged by thiosulfate. The percentage of starch is calculated from a curve prepared from the colorimeter readings of a known range of starch concentration. For solution depths of about one-half inch, the best range of starch concentrations is from 0 to 3 mg. per 50 ml., preferably 1 mg. Soluble starch cannot be used for standardization. If accurate results are desired, starch prepared from raw unblanched material similar to that which is to be analyzed should be used. Equal weights of starch from different prod- ucts, such as potatoes and peas, do not give the same amount of color with iodine; therefore one type of starch cannot be used as a standard for all products. Since factors have now been established among the various starches, a single standard, such as potato starch, suffices. If a series of analyses are to be carried out on a given product and only relative results are desired, then potato starch, which is easily prepared, can be used to prepare a standard curve. Disintegrate the raw product in the mixer with an equal weight of water. Separate out the fibrous material by washing the ground pulp through a 60- mesh screen. Now place the material that passed through the screen in a large beaker or pan and stir with a large volume of water. Allow the starch to settle and decant off the water. Repeat this process until the starch is free of extra- neous materia]. Now wash the starch with alcohol and ether and dry in an oven 156 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE at 70° to 80° C. for 30 minutes. This should give a Starch that is reasonably pure. Analyses can be made to establish its purity if such accuracy is desired. Note: If approximately half of the water is added to the Sample in the blender and the instrument run at low speed controlled with a rheostat until the sample is well disintegrated, and then the remainder of the water is added and the blender gradually run faster until maximum speed is reached, a more nearly homogeneous product is obtained. The volumes of the first and second dilutions depend upon the starch content of product and the internal diameter of the colorimeter tubes. Determination of Sulfur (Dried Fruits and Vegetables) It isnow common practice to treat cabbage with a solution of sodium sulfite or a mixture of the normal sulfite and bisulfite prior to dehy- dration. Future specifications may provide for similar treatment of other vegetables (carrots, potatoes, and possibly others) with sulfite solutions and control of this operation involves the determination of the sulfite in the dehydrated products. Dried and dehydrated fruits are commonly exposed to sulfur-dioxide gas in sulfurmg chambers before sun-drying or dehydration. The Monier-Willams procedure (4, pp. 463-465) can be used satisfactorily for both sulfured fruits and vegetables. It is a distillation method and is time-consuming and not especially well adapted to field work where a minimum of equipment is desired. Recently a direct titration method has been devised by Prater and others (39a) especially for use on dehydrated foods. The method as applied to dehydrated cabbage follows. Little modifica- tion is necessary for other vegetables or fruits but reference to the discussion in the paper by Prater and others (39a) is desirable before undertaking the assay of such materials. Apparatus: Blender, analytical balance, burette (50 ml.), flasks (1—liter Erlen- meyer), graduated pipettes (10 ml.). - Reagents: Sodium hydroxide (5 N), hydrochloric acid (5 N), acetone, starch solution (1 percent soluble), iodine solution (0.05 N). Procedure: The shredded sample should be ground dry in the blender im- mediately before the test is begun. From the ground material two 8-gm. samples are weighed (to 10 mg.) and labeled A and B. Each sample is transferred to a 1-liter flask and 400 ml. of water and 5 ml. of 5 N sodium hydroxide added. The mixtures are allowed to stand for 20 minutes with occasional shaking. Hach is acidified with 7.5 ml. of 5 N hydrochloric acid, which should reduce the Solution to a pH of about 2.0. To sample B, 40 ml. of acetone is immediately added and the mixture let stand for 10 minutes. To sample A, 10 ml. of 1-percent soluble starch solution is added and the sample titrated to the starch end point with 0.05 N iodine solution. After the required 10 minutes, sample B is titrated in the same manner. In order to obtain the blank values for the reagents, titrations A and B are repeated without the cabbage and the respective blanks subtracted from the first titrations for A and B. Blank determinations need be run only once a day or when fresh solutions are employed. The sulfur dioxide in parts per million is obtained by multiplying the corrected titration for A, minus that for B, by 200. The inexperienced analyst will find it desirable to run a preliminary titration, adding the iodine rapidly, about 0.5 to 1.0 ml. at a time to determine the approximate end point. In titration A, with a titer of 4 to 15 ml. a double end point may be observed, the first where the whole solution becomes blue but fades rapidly. The second is permanent for several minutes, and is obtained on addition of 0.5 to 1 ml. more iodine solution. A double end point is also observed with sample B, but both are much less permanent, owing to slow liberation of SO2z from the acetone. It is recommended that in each case the first or flash end point be taken. ‘Titration A yields total SO, both free and bound, and other reducing substanees. Titration B yields only the other re- ducing substances, because a complex is formed by the acetone and SO: which does not immediately react with the iodine, but only gradually, and on standing. The acetone concentration must be controlled, because an unsatisfactory end VEGETABLE AND FRUIT DEHYDRATION 156 point is obtained if it is too high. The acetone complex is unstable, and the titration must be conducted in the range of pH 1.8 to 2.6, where, in these eabbage tests, the complex shows greatest stability. Therefore both the 5 N NaOH and the 5 N HCl should be added by pipettes rather than from graduated cylinders. Analysis of Atmosphere in Cans of Dehydrated Vegetables In plants where dehydrated vegetables are gas-packed in accordance with purchasing specifications, a method of analyzing the atmosphere of the packed cans is useful. Figure 68 shows a gas-sampling assem- bly, a can-puncturing device, and an Orsat gas-analysis apparatus. The base of the gas-sampling assembly is a steel plate 12x 12x 1% inches. Two metal rods of 14-inch diameter (B) are screwed into the base. A wooden base of hardwood can be substituted, with lock nuts on the rods to fasten them firmly tothe base. The metal rod (C@), which carries the puncturing device, is held in position by the clamp holders (D) and can be raised or lowered to any suitable height. The can-puncturing device consists of a threaded tee (#), which supplies the force required to puncture the can, and the puncturing device proper, (/). These are also shown in detail. Glass capillary tubing (2-mm. bore) connects the can-puncturing device to a gas- sampling tube (G@) of approximately 300-cc. capacity. ‘This tube is connected by rubber tubing toa leveling bulb (#7) containing mercury. All necessary stopcocks are shown in the diagram. As a substitute for mercury, a solution made up in the ratio of 10 gm. of anhydrous sodium sulfate, 40 gm. of water, and 2 cc. of sulfuric acid, with the addition of some methyl] orange indicator can be used. Figure 68 also shows (upper left) a portable Orsat gas-analysis apparatus, consisting of a gas burette (A) enclosed in a water jacket (LB), absorption pipettes (C and Y) for carbon dioxide and oxygen, respectively, and a leveling bottle (/#) filled with a confining liquid of the same composition described above as a substitute for mercury. All necessary stopcocks are shown, and the apparatus is enclosed in a wooden case (/’). Other types of gas-analysis apparatus are also suitable. A commonly used stock solution for the preparation of absorbents for carbon dioxide and oxygen consists of 800 gm. of potassium hydrox- ide per 1,000 ce. of solution. For carbon dioxide, the stock solution is diluted with water in the ratio of 100 cc. of stock to 60 cc. of water. Ordinarily the absorption pipettes have a capacity of 160 to 180 cc. of solution. To make up the alkaline pyrogallol solution for oxygen determinations, ascertain how much solution will be required to fill one leg of the pipette and about a quarter of the other leg. In the - oxygen absorption pipette (DP), allow for the use of a half-inch layer of mineral oil (U.S. P.) to be used on the surface of the open side of the pipette. Now add the calculated amount of caustic potash solu- tion from stock to the open side of the pipette. On top of that solution add a half-inch layer of the oil. The pyrogallol required is in the ratio of 15 gm. of pyrogallol to 100 cc. of stock caustic solution. Dis- solve the pyrogallol in two-thirds of its weight of hot water, place a long-stemmed funnel in the pipette with the tip below the layer of oil, and run the pyrogallol solution into the caustic. The oil pre- vents rapid oxidation of the alkaline pyrogallol solution, by contact 158 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE with air. This protecting device is used in place of the rubber bal- loons commonly used in the past for this purpose. The process of sampling is carried out as follows: A can is placed in the stand, lid uppermost. The bulb (@) is filled with fluid to the cock nearest the puncturing device, and all three stopcocks are closed. Rubber Stopper IN ORSAT APPARATUS wernt ae Tube Screwed _ and Welded "7 (TEE) GAS ~ SAMPLING ASSEMBLY PUNCTURING DEVICE Figure 68.—lquipment for the analyses of the atmosphere of 5-gallon cans filled with dehydrated vegetables. The puncturing device is forced down against the can. Contact is first made between the can and the stopper, followed by the puncturing of the can. The three cocks are now opened to permit some of the gases in the can to flow into G when the bulb H is lowered. The stop- cocks are then closed. For analysis of the gas, a 100-cc. gas burette is used with an en- largement between the 0 and 50-cc. marks, with no intermediate graduations in that range. The rest of the scale is divided into 0.2-cc, VEGETABLE AND FRUIT DEHYDRATION 159 units. Before the sample is transferred to the Orsat apparatus, a 100-cc. sample of air is analyzed for oxygen. The result should be close to 20.9 percent. The purpose here is to remove all oxygen and carbon dioxide from the apparatus and to test for leaks. It is essen- tial to have the gas burette buffered against temperature changes by a water jacket, since otherwise changes in volume from one absorption to the next will be affected by an unknown error. When gases low in carbon dioxide are to be analyzed, the residual gas (nitr ogen) in the burette is ejected to the zero mark; the sample is then drawn in and analyzed. When the gas is high in carbon dioxide, 50 cc. of the nitrogen is retained in the burette in order to reduce the carbon dioxide concentration of the sample to be analyzed. The gas sampling bulb G is connected with the intake of the Orsat, a sample of gas (approximately 50 cc.) is taken into the burette A and the reading is recorded The carbon dioxide is absorbed by passage of the gas contents into end out of the caustic potash pipette until constant volume is reached. The burette reading is recorded. The volume of gas absorbed, multi- plied by two, is the percentage of carbon dioxide present when a sample of exactly 50 cc. is analyzed. Next the residual gases are contacted similarly with the alkaline pyrogallol. This second loss in volume, multiphed by two, is the percentage of oxygen. REHYDRATION TESTS Rehydration tests should be developed in each plant so that a daily evaluation of the quality of the product can be made. The specific FIGURE 69.—Equipment used in rehydration studies. Left to right: Accurate weighing, precise control of heating, and complete draining. procedure for these tests must be developed in each plant in order that the equipment available, the amount of material to be tested, and the complications of plant organization and management can be 160 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE duly considered. There are, however, certain fundamental principles which should be considered if these tests are to yield reliable informa- tion for plant control (fig. 69). Methods An amount of dehydrated material suitable to the equipment that is available is weighed directly into a glass beaker. Distilled water is added and the beaker is placed immediately on a heater. The content is brought to the boil at a fixed rate of heating and boiled for the scheduled number of minutes. In many instances soaking at room temperature before boiling produces a product of higher quality. The soaking time may vary from 1 hour to overnight. At the end of the boiling period the dish is removed and the contents quickly filtered. As soon as draining is complete the moist sample is weighed. The following procedure is used at the Western Regional Research Laboratory: Weigh two 10-gm. samples of dry material on a torsion balance. Place in 600 ml. pyrex beakers, add 86 to 150 ml. of distilled water, cover each with a watch glass, place on electric heaters, bring to the boil within 3 minutes, and boil 5 minutes. The precise amount of water will vary with the material, time, and rate of boiling; excessive amounts of water should not be used. Remove from the heater and dump into a 75 mm. Biichner funnel which is covered with a coarsely porous filter paper. Apply gentle suction and drain with careful stirring for one-half to 1 minute, or until the drip from the funnel has almost disappeared. Do not dry by long suction. Remove from the funnel and weigh. Set the drained sample aside in a covered porcelain evaporating dish for quality tests. Repeat this test, and then rehydrate six other 10-gm. samples, boiling two for 10 minutes, two for 20 minutes, and two for 30 minutes. It will be necessary to use 20 to 30 ml. more of water for the last two tests than for the shorter boiling tests.. Only small pieces will rehydrate in 5 minutes. Samples requiring more than 380 minutes of boiling are not likely to pass in- spection for Government purchase. A skillful technician will be able to operate four heating units and by staggering the time the filtering can be accomplished with one Biichner funnel. These dry materials are “water hungry” and i minute of variation in the time it takes to bring the sample to the boiling point, to remove the sample from the heater, or to complete the filtering, will cause significant errors in the final drained weights. Calculations that can be made will vary with the data available. The rehydration ratio (table 14) can always be calculated. When the percentage of water in the dry sample is known, the percentage of water in the rehydrated sample can be calculated. When the per- centages of water in the dry sample and in the undried sample are known, the coefficient of restoration of weight can be calculated. Calculations are illustrated in table 14. VEGETABLE AND FRUIT DEHYDRATION 161 TABLE 14—Wethods of calculating ratios and coefficients of rehydration Determination made on sample Wo 9.45 (weight of original sample). W 1.25 (weight of dehydrated Rehydration ratio________- sample). WR 7.50 (rehydrated weight) __| Coefficient of restoration | Ratio that may be cal- culated Equation and calculation ID Ryo TA IOs =e Wo_ 9.45 pounds _7.56 W 1.25pounds 1 Wer _7.50 pounds _ 6.00 W 1.25 pounds 1.00 ase Wr/W___ (6.00)100_ _, of weight. WalW os 2756 =79.4 A 87.5 (percent water in the | Rehydration ratio________- C_7.50_ 6.00 original sample). 7 IDTV OEE Sl B 5.3 (percent water in the dry sample). C 7.50 pounds, (drained weight of rehydrated sample). D 1.25 pounds (weight of dry Coefficient. of restoration of weight. Cc 7.50 X (100—87.5) (D—BD)100 1.25—(1.25X0.053) _ 100—A (7.50) (12.5) _ 93.75 __ >, 4 9.4 sample used for rehydration (1.25—0.07) 1.18 — (Rehydration ratio) 100 _6X100 Coefficient 79.4 7.50—1.18 632 7.50 7.50 | DEANS AOR se =7.56 Percent of water in the = =84.3 rehydrated material. x0 Sources of Error Needless to say, the technique of performing each test must be care- fully standardized if comparable data of value are to result. ‘The con- trollable factors of rate of heating, time held in the water, temperature of the water, ratio of water to sample, and the composition of the water can be readily standardized. It is not too simple to standardize the number of duplicated tests, the number of points on a given curve, or the number of curves that must be developed in order to gain ade- quate information concerning a sample. Effective heat control and accurate timing are of major importance. The results shown in figure 70 (curve @) were obtained at room tem- perature (78° F.) with 14-inch sliced carrots, which regained approx1- mately 41 percent of their original moisture in the first 20 minutes in the water. In the next 20 minutes they gained another 12 percent, and in 50 minutes they had regained 58 percent. Other samples (curve 0) at boiling temperature regained 69 percent of their moisture in the first 20 minutes, 10 percent more in the next 20 minutes, and at the end of 50 minutes they had regained a total of 81 percent. When soaked at room temperature before boiling (curve c) for a half hour, 75 percent of the original amount of water was regained during the first 20 minutes of boiling after soaking, and when soaked for a full hour, 82 percent was regained. In other words, soaking 1 hour and boiling 20 minutes was as effective in rehydration as was 50 minutes of boiling without soaking. From this it is obvious that comparative tests are valuable only when time and temperature at which the material is kept in water are carefully controlled. Since all calculations of results are based on the drained weight of the rehydrated sample, only careful weighing and adequate draining will yield drained weights that are worth while. Draining through strainers, cheese cloth, or by gravity through filter paper yields rough approximations, but the Biichner funnel, with ae suction and careful timing, has given the best results obtained thus Tar. 569074—44—_11 162 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 90 30 | | } a | - | | | ) | fe eee | ! | | | | 80 | 7 . | | t b T 80 | | | I | - 70 sara is 2 70 za | | | | | {o) | } ; | a | a | | | | | © 60 | —— (ee —+ 60 > | | | { | a | | } a ac | | } | | | we ro} 50 50 = | 2 ws < © 40 40 L | | | w | Ww (2) (o) 30 _—— + | 30 20 20 | te) ie) 0 5 10 15 30 35 40 45 50 20 25 MINUTES FIcuRE 70.—Effects of soaking and boiling for various periods on the amount of water imbibed by dehydrated carrot slices. (See text for details.) Composition and Amount of Water In order to maintain effective rehydration throughout the test, ade- quate water must be supplied to the sample. It is impossible to main- tain proper ratios of water to sample during the test if the rate of boiling varies from one test to the next. The amount of solution to use for a given weight of material in any test may depend upon the immediate purpose of the test. Enough must be used to insure complete wetting of all parts of the sample. It may be desirable to shake or stir during the test period, in order to insure uniform wetting. When a sample is prepared for palatability tests, or for the purpose of demonstrating the preparation of the foods for homes or institutions, it may be desirable to use relatively small amounts of water and thus conserve taste, flavor, and nutrients, even though a lower coefficient of rehydration is attained. The nature of the solution is also highly important, because a variety of reactions may occur as a result of variations in composition. ‘Taste, flavor, texture and color, one or all, may be affected. There are four groups of pigments in fruits and vegetables which determine color: anthocyanins, flavonols, carotinoids, and chloro- phylls. Red, purple, and black products, such as beets, plums, grapes, and red cabbage or onions, contain the anthocyanin pigments, which are highly soluble in water, and when excessive amounts are used and drained off, the color becomes poor. When acid is added to the solution, tue color becomes pleasantly brighter and lghter, but in VEGETABLE AND FRUIT DEHYDRATION 163 alkaline solution the colors become unpleasantly weak or dull and sometimes a complete change in hue, as from red to green or gray, may occur. In the presence of salts of tin and iron, these anthocya- nins develop metallic lusters, such as occur in canned sour cherries or berries when the lacquer on the can is imperfect. These are un- pleasant, although usually harmless. White vegetables are colorless because the pigments are largely flavonols, and these are present in the colorless forms. When heated in mildly alkaline solution, the brilliant-yellow forms are produced. Thus yellowish-white or pale-yellow colors are typical of cooked cauli- flower, potatoes, cabbage, and onions. These flavonol pigments are also soluble in water. The orange or orange-yellow vegetables, such as corn, carrots, and rutabagas, contain large quantities of carotinoid pigments. These 120 —— COEFFICIENT OF REHYDRATION 10) 100 200 300 400 500 600 700 800 300 1,050 MINUTES AT 78° F, _ FicurE 71.—Effects of slice thickness on coefficient of rehydration of carrots: a, one-eighth-inch slice; 6, three-sixteenths-inch slice. are not soluble in water and they are little affected by either acid or alkali solutions. However, the vegetables containing carotinoid pigments also contain flavonols and a minor change in color in alkaline solution, due to these latter pigments, will occur. The dominant colors in green vegetables are the chlorophyll pig- ments. Chlorophylls are generally insoluble in water. The rich green color is destroyed in acid solution and the vegetable becomes a drab dull yellow or olive green, while in very mildly alkaline solu- tion a lighter, more brilliant green is produced. Again, these vege- tables contain flavonol and carotinoid pigments, which are invisible because of the green but which may contribute to the color when the green is destroyed. Since both drainage and well waters may be mildly alkaline, the use of these in reconstitution tests may produce a light cream or yellow color in white vegetables, a brilliant green in green vegeta- bles, a purple, green, or gray in red vegetables, but will cause little change in the orange or red of carrots and tomatoes. These mildly 164 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE alkaline waters have little effect on texture, but strong alkalinity, such as is produced by adding sodium bicarbonate or sodium sulfite, causes an abnormal softening of the tissue which results in some in- stances in an unpleasant sliminess in the reconstituted product. Water supples high in calcium and magnesium salts are common. Such water is likely to harden the skins on peas and lima beans. Taste and flavor are affected only by relatively strong acids and alkalies, which are not encountered in ordinary usage. Variation in the composition of drainage and well waters is an important matter. Indeed, the lack of uniformity from day to day or month to month may be a crucial difficulty in comparative testing in dehydration plants and should not under any circumstance be over- looked or disregarded. The cost of supplying a uniform quality of water, such as ordinary distilled, may be negligible as compared with the cost in time, material, and poor results obtained when tap water is used. The size and shape of the individual pieces determine in part the amount of water that a given weight of dehydrated material will im- bibe, in unit time and at unit temperature. For example, from one lot of carrots (fig. 71) two samples were dried, one sliced one-eighth inch, the other three-sixteenths inch thick. The former slice reached its original water content in approximately 314 hours and the latter after 16 hours at room temperature (78° F.). These results show the im- portance of specific directions for home and institution use. In gen- eral the thick pieces require some soaking before boiling. Serial Tests No generalization can be offered here as to the number of duplicated tests that should be performed or the number of points on a rehydra- tion curve that must be found. From the data presented in figure 72 it is obvious that single tests have little value. Figure 72 shows a set of six pairs of carrot samples of one variety dried at various tempera- tures until they had attained finished moistures of approximately 5.0 and 2.0 percent. They were sliced three-sixteenths inch thick and blanched 4 minutes. The problem was to determine whether or not the rate and effectiveness of rehydration had been influenced by the drying conditions and to correlate these conditions with optimum quality in the rehydrated sample. From preliminary studies it had been found that good samples of carrots were soft and tender after boiling 10 to 20 minutes, and also that by soaking overnight and then boiling 10 to 20 minutes a plumper, smoother slice with a higher co- efficient of rehydration was obtained. It seemed advisable to try serial rehydration tests, in which time and temperature of rehydration varied from 10 and 20 minutes of boiling, without soaking, to soaking overnight (1714 hours) and then boiling 10 and 20 minutes. As a means of determining extent to which failure to rehydrate might be caused by surface hardening of the pieces, samples were ground through 20-mesh sieves and then boiled without soaking for 10 and 20 minutes. Duplicate tests were made at each time-tem- perature interval. The average drained weight from these duplicate tests were used for calculating the coefficients reported in figure 72. This plan gave 14 rehydration tests on each sample. 165 VEGETABLE AND FRUIT DEHYDRATION 44 7) (=) fesse ol ores] oa ouenee je a ooer es [ened | wees] ees ee | ee ee Seo7 ree) ans) \ 1 ob ao q [ses rom ONS 2°n oON 5) Ovi ot? N-o |] a—o | S a we f o oy >} fol oor ous D —_ — yt S| oO Sra omy a?) q cy onn own |} oS on! oom! a Q| a Ol gy ao SH ae ; wud a REY, ig [ese [ace] a | a0 8 e pial onee os) a 2 = Sa On y o|> 2 eS Ow a 2 & 7) ese pone sty =|@ N10, @ ° BoK%q |! Os Ys oon! cay 3 s a cee ‘ol< ol, Om oi a© = a pea =/0 = © =—— Sy, | x o |X sans eae CEC eal Cle uu 25 9 rs) © 2 [——— gas ® $ rs ore ras “ rs ; . ais © % ET a ”3 3| Clas F + 30 a: al;s | 2 i ; a) 2 2 119) wo a ny}. £ SSA OOS | nN] ® 0) = ane RI EE | ey Ven s 2 fe Siete s >? > wool s o o 9 |G = 5| 2 3/8 Sle ale os E} © S 12 E 2 € 9 & fs) ale =| eli %|o oe ole ees f= o/s 5, 8 a a OF OR lala alo ule = a a Ta SR ema | GAPS a} = | = ino o <4 a = 8] = qkO) <|O Oo ary : eae | ‘ oe a! SH iS wo oS eee |e ee O-O! oS S n ' “a LO & ES Re (ES OSA ST SURE ET io ing wu | e 2s Eo ee oe [SoS Ie RE ORE Pa a [eS Dw a fo) mon On | OO SI Eon Ean Ea EN 7 =) fo} fo) 0 SiG OL MOI NOM O LOMO Tae whee ao] o =s338 3300 aann anan : NOILVYOLS3SY LHOISM JO SLN3I0ISS509 pose NOILVYOLS3SY LHOISM JO SLN3I9IS43509 poll & oOzF=r fa PP a 166 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE As shown in figure 72 there was a definite relationship between the temperature of drying, the finished moisture, the time in the drier, and the rate and effectiveness of rehydration. It was obvious that samples dried at a dry-bulb of 200° to 180° F. and a wet-bulb of 95° to 140° did not rehydrate so rapidly or so completely as did samples dried at lower temperatures. It will be noted that none of the samples dried at a minimum dry-bulb of 180° and over to less than 3 percent moisture rehydrated successfully. Reducing the size of the piece by grinding increased the rate of rehydration in 9 out of 12 samples. After 10 minutes of boiling with- out soaking these ground samples were as well rehydrated as were the whole slices after 1714 hours of soaking and 20 minutes of boiling. Even in these ground materials the ability of the sample to rehydrate was decreased as the temperature in the drier was increased and as the finished moisture was decreased. Interpretation of these results would have been difficult or impossible if single tests had been made, and the final evidence was obtained only when the ground samples were shown to react in the same manner as the whole slices, thus in- dicating that rehydration failure was a characteristic of the entire tissue in the sample and was not merely case hardening or other surface © effect. Since the purposes of rehydration tests in the plant are to check the technique of drying and to insure high quality in the finished products at all times, a clear understanding of the distinction between rehydration and cooking should be made. In the strict sense, rehy- dration refers to the replacement of water in the dry sample, and when the material is held long enough at low temperature, a coeffi- cient of rehydration of 100 will be attained and the shape of the sample will be completely restored. The only change in the sample results from imbibition cf water. Cooking, on the contrary, is heat treatment of food for the purpose of bringing about physical and chemical changes, and the only criterion for degree of cooking in vegetables is a change from crisp, hard tissue to a soft, tender texture. The natural taste and flavor of the vegetable should be retained. It has been pointed out that the rate of rehydration is greatly . accelerated at high temperatures; thus rehydration and cooking may proceed at the same time but at different rates, and the change in the chemical composition of the solids at 212° F. may result in complete disintegration of the tissue before rehydration is complete. Moreover, from the standpoint of quality testing there is a limit to the time that vegetables can be held if the finished product is to be edible. Off-flavors develop at less than boiling temperatures, and inedible products result from long boiling. The method for recon- stitution, then, must take into consideration the relative time at which the sample will be held at room temperature and boiled. Establishing these times for each vegetable will make it possible to arrive at conditions that are compatible with optimum quality. Nature and Condition of Material It is difficult to explain the fact that products of one plant may rehydrate differently from those of another, even though the equip- ment and process are similar in the two plants. It is possible that VEGETABLE AND FRUIT DEHYDRATION 167 variety, growing environment, maturity, and storage of raw ma- terlals affect the rate and effectiveness of rehydration as well as the color, taste, flavor, and texture of reconstituted products. In tests at the Western Regional Research Laboratory, rate of cooking, taste, flavor, and texture were drastically changed when carrots were permitted to become overmature, but the rate and completeness of rehydration were not affected. This is illustrated in table 15, which shows results from three varieties of carrots grown in one locality and dehydrated by one method. The code numbers, P767, P771, and P775 represent immature, high-quality carrots, the code numbers P119, P120, and P121, overmature and fair in quality of raw ma- terial. The rate and total degree of rehydration were essentially the same but the rate of cooking and the taste and flavor were definitely and consistently poor in the older samples. Definition of Terms Four expressions denoting the imbibition of water by the dehy- drated products are commonly used: reconstitution, rehydration, re- freshing, and cooking. Reconstitution implies restoration to the con- dition prior to dehydration in weight, size and shape, texture, color, flavor, composition, structure, and other observable factors. Thus the degree or completeness of reconstitution can be described only par- tially in quantitative terms, since it is difficult, and in some cases 1m- possible, to make a quantitative evaluation of these constituent fac- tors. While restoration of weight, color, form, and shape can be measured quantitatively, other factors such as taste, flavor, and texture are subject only to qualitative measures. The restoration of the weight of the product through the imbibi- tion of water is relatively easy to measure and, when the moisture content of the material is determined before and after dehydration, the percentage of restoration of weight can be calculated from the drained weight of the rehydrated sample. An example is shown in table 14. In this sample a rehydration ratio of 6.00 was equivalent to 79.4 percent of the original weight of the sample. | Methods for calculating certain related ratios are illustrated in table 14. The decision regarding calculation rests with the operator and will be influenced by the purpose of the experiment and the informa- tion that is available on the composition of the sample under study. Nevertheless, it is evident from the figures in table 16 that the rehy- dration ratio is not a true index of reconstitution. For instance, the four samples of dehydrated potato strips when rehydrated under ex- actly the same conditions gained 4.5, 4.7, 4.8, and 4.9 times the dry weight of the sample; the coefficients of weight restoration varied, however, from 78.5 to 124.3 percent. The gain in weight in rehy- dration was directly proportional to the solids in the dry sample, but to have attained the original composition the Nebraska Triumph sample required restoration to 85.0 percent water, while the Colorado Russet required only 75.5 percent. 168 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Adnl Jo [[’ 10 ysow AQ poziusode1 puvw psduNoUO Id JO pjtui=+-+- ‘Aunt ore e10UL IO J[vy AQ poziusoded pue pjtu=-+-+ ‘Aint jo Ajylsourm Aq pozTusode1 9081j=-+ [40U 10 eUOU=() ¢ "9d UB10 MOT[VA VIVIOPOT ="O “A “JA /95UB10 su01S= °O “1g ‘osUBIO OIBIOPOTT="O “IW z ‘IZ pue ‘OZL ‘6IId 18 SoTdmvs 9INjJeUIIOAO ‘plo ‘G21 pue ‘TLL ‘29! dq 218 Sojdues vINJeWIUM SuNOA ; +++ 4+ aL Se +++ 44+ 44+ t4t+ 4 4 444+ 44+ t+ 44 + ++ + + + ++ 4+ 4+ +++ 4+ oe + totdtotttotttetetet+tett+ aor toetttetttettt4+4+4+e4+4+4++4++ oe oo to + + +o ptt cwern +4+4+4+44+44+44+4+6+64+4+4+4+444+ ob to t+t4+44+ + WOS ¢ 0IN4X9,L, ¢ [eanjeu OAT A ¢ JOOMS O4se I, Beer Ke STG. Nine cena OD emma e201 02 RES ove Ne es cP ee es A OD eam ils, 0Z Rept Ie eee teas Foe OP ee seaikes OL pga Caen sg oa ap aks a dun dq L ¢ Sen Seat RRS eC Aces ee Tg O) NSF Hh 13} 0G alate Wie ise Ghee duinyd Ayire 9 0% BEIT SO ee ee Td OD igen |.8¥, ¢ SS geagerngh cre, Saeed ee PePyUlI AA | oG G 5 Uae eX Oh ae ee Eee ol Oop | $6 0G Se eS Bae ae op | (06 0G eta AE anen rage Sect Tren Raped CDiaes lacs: OL Poneto Gana EMER eS dun[d | vor G Seay i gE SS ROE: OD eae |Er9 0G Se re Se te raat dunjd Apie gq | €2 0% Gee Se ES ee Oop" | 29 ¢ RECT Soe Sipe domes pepyUulIM | 09 g REPRE ake omen ae ete op} FOL 0G ee ae ERE cee ae ee Oop ~~~" | Sat 0G ge hae aN Me cue CURR op "| 16 ¢ Rice ge eo Seen eee aoe CULT [etealiac Ol G Sie eh Cl Fre ena Veeera ne ean en OPsss == l=7S 0G Sea es eee duinjd Apire yy | 9 0% nef Le Spy RN see Seoae ODgstes | =ST G eerste Me nn a epee pepyultA | 89 g quadag | saynurnr uor} W047 -B10ISO1 polog JUSTO A Se tasitenbanie Soe OD oe oe oe oo°oceorhktkr SAINI ea itenbhesihenie ROOST SS SoOoQooOs |S Soe enh oe are NOT AOL SLNOET AYOLIe A saiyjarioa § Buipnjour sjo1u09 payouphiyap ut fzyonb pun uo1jnjrjsuovay—G] ATAV I, op op VEGETABLE AND FRUIT DEHYDRATION 169 Taste 16.—Dehydration ratios versus coefficients of restoration of weight of various samples of potato [Potato strips 52-inch thick boiled 10 minutes, with no soaking] | : Colorado 7 . Colorado Nebraska Item Russet Katahdin Pawnee | Triumph Water in original sample______________ percent __ 75. 5 We 5 80.6 85.0 Wiatern Inidny: Sample was eas ee Gonz 7.4 5.6 Weil 6.4 Weight of reconstituted sample ________ grams __ 47.0 48.0 45.0 49. 0 Ideal weight of reconstituted sample_--___ dos 37.5 42.0 47.9 62.4 Coefficient of weight restoration__--______-_____ 124.3 114. 4 94.0 78.5 TRO GH OVaL IPN) Se ee 4,7 4.8 4.5 4.9 Summary of Rehydration Tests Tt is suggested that the following conditions be met by all who are interested in making rehydration tests: Work out a time and temperature sequence suitable to the material being tested. Determine the time of soaking and boiling that is compatible with optimum quality in each sample. Always run a series of tests at various times and temperatures and evaluate the data on the rate of change in coefficient rather than on a single determination of a coefficient. Start the test with at least enough water to submerge the pieces, but do not use so much water that excess amounts are present at the end of the test, especially when quality tests are being made on the samples. Shake or stir if necessary to insure wetting of all the pieces during the test. Control ther ate of heating so as to prevent rapid and variable losses of water while boiling. Use unit heaters set up in such a way as to prevent rise in temperature from radiated or convected heat. QUALITY TESTING The human reaction to food is defined by four sensory perceptions. The appearance, the feel in the mouth, the taste and flavor, and the odor all contribute to the edibility of the food. Certain characteristics or qualities can be measured quantitatively: Examples are color, size, shape, and in some instances attributes of texture, such as resistance to shearing, breaking, and penetration. Other quality factors are measured by subjective techniques and at best are qualitative in char- acter. Either one of three procedures may be used with more or less confidence in evaluating the purely subjective qualities of a given product. These are described below with certain suggestions as to their value. One procedure is to reconstitute a series of samples by a standard method, call in 25 to 80 employees, and ask them which they like best. A refinement of this method is to assign an arbitrary number, let us say 10 for the best sample, 9 for the next best, 8 for the next and so forth, and ask the graders to rate the samples and assign numbers to them. The difficulties in this procedure are obvious. saya gh MAT Rem 1 i | = eal eau + Good rset we een ode. 8 | 0 | 4 1 All samples were boiled 5 minutes without soaking. 2See Table 15 for full code. VEGETABLE AND FRUIT DEHYDRATION 171 Procedure for Quality Tests Samples are reconstituted by methods already described. ‘These are accumulated in sets of 10 to 20 individual samples. A small amount of the material is placed in a paper cup, and the cups are numbered in a series. Care is taken to avoid numbering that permits identifica- tion of the sample. Each juryman is provided with one complete set of samples, which he grades without consultation with his neighbor juryman. The charts are collected and summaries are made of the opinions of the graders. The results are weighted qualitatively and reports are made on each quality factor (table 17). For example, the carrot samples in table 15 were graded by 10 experienced graders and the results were summarized by the use of symbols Oto +++. The colors were determined and the degree of re- turn in form was noted. Thus Sample P767 was sweet in taste, nat- ural in flavor, moderately soft and tender when it had reached 68 per- cent of its original water content and had been boiled for 5 minutes without previous soaking. The color was good, and although the cubes were still somewhat wrinkled, the sampie was reported good. Several weeks later other lots of the same varieties of carrots were harvested, dehydrated, and tested organoleptically. In this instance there was little sweet taste in the sample, the flavor was natural but weak, and it was crisp and tough after 5 minutes of boiling. This would have been reported poor but since it was obviously underdone, other sets were prepared by boiling 20 minutes without soaking and 5, 10, and 20 minutes after soaking overnight (1714 hours). When all the tests were summarized, it was found that 20 minutes of boil- ing without previous soaking yielded the maximum quality in the samples. Long soaking gave higher coefficients, and in two instances equivalent quality, but in sample P120 the quality after soaking 1714 hours and boiling 20 minutes was poorer than when boiled without soaking. It lost taste and flavor and became very turgid and hard. Jury Selection and Training All jurymen should be trained to recognize the qualities in a stand- ard sample before they are permitted to pass on unknown samples, and, when available, standard samples should be included in the daily series. The following suggestions may be helpful. All graders or jurymen should be persons who like fruits and vegetables; who have poise, intelligence, and integrity; whose living and eating habits are reasonably regular; whose health is reasonably good (certainly free from gastro-intestinal diseases). A jury of five or more is recom- mended. The testing should be carried out over a short period of time, and the room should be quiet enough to permit concentration on the part of the grader. Under no circumstance should’ a grader be interrupted or dis- tracted while rating the samples. The grading should be done in a room free from smoke, stale odors such as come from pipes and ciga- rette butts, old samples, and disagreeable chemical reactions. The grading of odor is difficult and for the most part unsatisfactory, but offensive or stale odors should’ be noted by the jurymen and given due weight in the final score for the sample. 172 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Summary of Quality Testing Methods of reconstitution must restore and preserve to a maximum the appearance, edibility, and nutritive value of the product. Since dehydrated vegetables are blanched before drying, the restored ma- terial will have the characteristics of a partially cooked vegetable. Color, taste, flavor, and odor are highly concentrated during de- hydration, but because of losses during blanching and drying, optimum quality in the rehydrated material is usually obtained before all the water has been replaced in the material. Because of variations in raw material and techniques of blanching, drying, and storage, it is recommended that each plant provide staff and facilities for daily rehydration and quality tests on the samples being produced. PROCESSING COSTS The subject of operating costs is discussed below in two sections: (1) Labor requirements and (2) segregation and analysis of process- ing costs. The general problems of labor distribution and labor re- quirements are discussed first and certain aspects of labor which are closely connected to segregation and analysis of costs are considered further under the latter heading. Labor Requirements The number of employees in a dehydration plant is by no means fixed, and, because a large number of factors affect labor use, prelim- inary estimates of labor requirements are usually rough approxima- tions. Labor requirement is affected by type of process, degree of mechanization, efficacy of equipment, effectiveness of plant lay-out, proper balance between operating steps, condition, variety, and grade of raw material, specifications for finished product, labor laws and customs, working conditions, ability and training of employees, safety measures, method of pay, morale, and operators’ individual prefer- ences and policies. Not all of these factors can be evaluated in advance. Observations made in canneries and dehydration plants and the opinions of ex- perienced plant operators are the best source of information on this subject. The discussion presented here has been developed largely from such sources. Table 18 shows the approximate labor distribution in dehydration plants of moderate size, handling about 50 tons per day (unprepared basis) of 7 vegetables important in the dehydration program. The trimming, sorting, and inspection labor varies almost in direct pro- portion to the size of the plant. Thus a 100-ton plant drying potatoes can be expected to require from 60 to 100 women on the trimming belt ; a 10-ton plant, 6 to 10. This direct relationship does not hold true for the other operations. As size of plant increases, the labor re- quirement per unit of output for these other operations decreases. Because of the need for at least one or more employees for each of many operations, regardless of the throughput at those points, the smaller plants are at a disadvantage as compared to the larger ones, which can make more efficient use of labor. Except for sorting, trim- ming, and blanching, the labor requirements are substantially the same for these 7 vegetables. Nh ar ali hs A ae I EO a a VEGETABLE AND FRUIT DEHYDRATION 173 TABLE 18.—Approzimate labor requirements * per shift for various vegetables in a dehydration plant handling 50 tons per day, unprepared basis (dehydrator Labor based on use of truck-and-tray tunnel driers) A Rutaba- Job fae Cabbage |} Carrots | Onions | Potatoes | gas and ee turnips P Feeding to preparation line___-_-_ 1-2 M 1-2 M 1-2 M 1-2 M 1-2 M 1-2 M 1-2 M O perating autoclave, sizer, and/ OF PeCCleLs Scan cae 3-5 M — 0-1 M 0-1 M 0-1 M 0-1 M 0-1 M Sorting and trimming________-_-- 20-25 F 5-10 F | 20-25 F | 25-35 F | 30-50 F | 20-25 F | 25-35 F Spreading on blancher belt____-_- -—— 0-2 F 0-2 F -—— 0-2 F 0-2 F 0-2 F Placing trays on conveyor_-____- 1M 1M 1M 1M 1M 1M 1M Spreading on trays_______--____- 1-2 F 1-2 F 1-2 F 1-2 F 1-2 F 1-2 F 1-2 F Me OaG imo CArS= et ee ee eee 2M 2M 2M 2M 2M 2M 2M Moving cars and operating drier-_ 2M 2M 2M 2M 2M 2M 2M SGrapin ey trays ask ee eee 2-4 M 2-4 M 2-4 M 2-4 M 24 M 2-4 M 2-4 M Final inspecting=_..->___-2--__- 26 F 4-8 F 2-6 F 4-8 F 4-8 F 2-6 F 2-6 F Packaging, crating and ware- { 3-4 F 3-4 F 3-4 F 3-4 F 3-4 F 3-4 F 3-4 F OUST G2 eae eee 2-3 M 2-3 M 2-3 M 2-3 M 2-3 M 2-3 M 2-3 M Other: ISONIC e ee A eS 1 1 i 1 1 1 1 HOTEWOMAN Sse aaa 1 —— 1 1 1 1 1 Helpers, cleanup, tray wash- er, carpenters, mainte- ; MAT COs = Sewer Grows eS 4-6 M 4-6 M 4-6 M 4-6 M 46 M 4-6 M 4-6 M Total number per shift: (2) OY eS a Se ae 17-25 14-20 14-21 14-21 14-21 14-21 14-21 WOT CTS ie = ete a 26-37 13-26 26-39 33-49 38-66 26-39 31-49 ioremmlamnise 22 erc ae sa Ss 1 1 1 1 1 1 1 HOKE WOMAN ee eee 1 - 1 1 1 1 1 1M=male; F=female. 2 Labor requirements for packaging depend upon type of container used. Labor figures shown here are based upon the use of 5-gallon cans, automatic sealing machines, and prefabricated cartons, boxes, or crates. The use of metal foil containers or other types of packages will involve a different labor set-up. The method of peeling materially affects the number of trimmers needed. Abrasion peeling of potatoes may require as many as 50 women per shift in a plant handling 50 tons per day. Lye peeling may reduce that number to between 30 and 40. Flame or radiant- heat peeling, brine peeling, or other peeling methods may also result in a lower labor requirement for trimming. The type of dehydrator affects labor requirements. A 50-ton tunnel requires from 10 to 15 employees per shift for loading and stacking trays, moving cars, operating the drier, scraping trays, and washing trays. If a conveyor-type dehydrator is used, and a suitable me- chanical arrangement is available for spreading the product evenly over the conveyor belt, from 2 to 4 employees may be necessary to handle the drying operations in a plant of this size. Segregation and Analysis of Processing Costs Direct operating charges are much greater than capital charges in a vegetable-dehydration plant, since in many plants the monthly labor, raw material, and packaging costs are more than the original capital outlay for buildings and equipment. The present section deals with the problem of proper segregation and analysis of the various operat- ing charges. Its purpose is to assist operators in developing a system of cost analysis that will indicate accurately the relative importance of various cost factors and the effects of changes in operating procedures. A proper segregation and analysis of operating costs is of value to the plant operator as a means of (1) comparing the operating costs for different methods of preparation, drying, and packaging (par- ticularly valuable to prospective dehydrators) ; (2) comparing dif-. 174 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE ferent raw materials; (8) obtaining accurate control over all phases of operation; (4) keeping the operator informed on current pro- duction costs; and (5) providing a proper and detailed record of operations for future reference or audit. In order to simplify their consideration, operating costs are divided into the following main groups: Raw material cost, preparation cost, drying cost, packaging and warehousing costs, indirect and overhead costs (accounts must be set up in which these charges are accumulated before distribution to the first four groups). This is a convenient grouping for discussion, since it covers the major steps most commonly followed in plants that handle vegetables or fruits. Each major step can be considered separately, and factors peculiar to that step need not be confused with any other. From an operating standpoint also, such a grouping has merit. In keeping these costs separate, the operator can compare various methods of operation. If tunnel drying on trays is to be compared with drying on a conveyor, costs associated with the actual drying steps are readily available for comparison. For some purposes, a further break-down is desirable. For example, the problems involved in peeling and trimming present a broad field for study. The relative advantages of lye peeling, brine peeling, heat peeling of various kinds, abrasive peeling, and hand peeling, together with related trimming costs, warrant much consideration. For the purpose of making such studies, a detailed break-down of preparation costs 1s valuable. For example, peeling, trimming, and blanching may each be given separate consideration. RAW-MATERIAL COST As a rule, raw material is the largest single item of cost in a vege- table- or fruit-dehydration plant. In some instances, it may amount to as much as 50 percent or more of the total cost per dry pound. Any attempt to reduce operating costs should, therefore, include careful study and control of the raw material. Variety, growing conditions, time of harvest, handling, and storage, all of which affect the quality of raw product, are extremely important in determining costs, yield, and quality of dry product. An operator should contract for his raw material well in advance of actual need, so that the production, trans- portation, and storage will be under his direct supervision and the product delivered to him will be the best obtainable. The cost of raw product is magnified in the cost of the dry product to the extent of the overall shrinkage ratio.7 On cabbage, a cost in- crease of only a dollar a ton will result in an increased cost of almost 1 cent per dry pound. Ona given vegetable, the quality and condition of the raw material will determine largely the overall shrinkage ratio. It is advisable in many cases to pay relatively high prices in order to obtain high-quality foods, since lower preparation losses and fre- quently lower drying losses will increase the yields and more than offset the higher purchase price. A careful analysis of the interrelated variables (raw-material grade, preparation losses, trimming costs, and drying ratio) is essential if the “The “overall shrinkage ratio” is the ratio of the weight of unprepared raw material to the resultant weight of the finished product. The ‘drying ratio” is the ratio of the weight of prepared material entering the drier to the weight of the same material leaving the drier. The proper use of these terms helps to avoid confusion. VEGETABLE AND FRUIT DEHYDRATION 175 most economical combination of raw materials and preparation meth- ods is to be attained. Graphs such as those shown in figures 73 and 74 S a o Coa Drying Ratios 3 tol OF RAW MATERIAL POUNDS OF DRY PRODUCT PER TON fe) 10 20 30 40 50 PREPARATION LOSSES (PERCENT ) Figure 75.—Yields of dry product at various preparation losses and drying ratios. can be conveniently used to study the combined effect of these variables. For example, we find from figure 73 that a raw material which has a 80 ] | | ane | | a | 4 f 7 a E | | | ow | Ses | | wo 60: i - 1 fio = = Raw Material and Trimming} Sw Cost Per Unprepared Ton | | =O | | = = — -— - + ——— = a | Ss | | | 22 40 pea SS | Ie aoa | fea = eat t wo | qo | zee Se re <6 1 : ec I | | ] ! 0 ial Read aera | (e) 100 200 300 400 500 600 DRY MATERIAL PER RAW TON (POUNDS) Figure 74.—Raw material and trimming costs per dry pound at various yields of dry product. 20-percent preparation loss and a 5 to 1 drying ratio yields 320 pounds of dry product per raw ton. If the combined raw-material 176 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE and trimming cost is $60 per unprepared ton, we find (fig. 74) that the cost per dry pound is 19 cents.1S Ifa cheaper raw material, costing only $50 per ton including trimming, is found to have a 30-percent prepara- tion loss and a 51% to 1 drying ratio, the charts indicate that this ma- terial also costs 19 cents per dry pound for raw material and trimming. Quality of final product may be the deciding factor in this case, if waste disposal is not an important problem. The operator will find that similar graphs are applicable to other operating problems. Certain assumptions must be made and some caution exercised in the use of graphs such as those shown in figures 73 and 74. For the pur- pose of these graphs, it is assumed that all changes in weight during preparation, either through loss of material or changes in water con- tent, are included in the preparation loss. When wood trays are being used in the determination of prepara- tion loss, errors may be caused by variation in tare weight of the trays, caused by variations in wetness of the wood. This difficulty can be largely overcome in test runs by wetting the trays before loading and using the weight of the wet trays as the tare weight. The problem is obviated by the use of metal trays. The changes in weight from the raw material, as purchased, to the prepared material, ready for drying, include such items as dirt re- moved in preliminary washing, culls graded out, the actual preparation losses (peeling, coring, trimming, etc.), and leaching losses or water pick-up during the peeling, cutting, washing, and blanching. Ex- cessive washing after cutting and especially after blanching may re- sult in losses of 10 percent or more on potatoes. In test runs, the drying ratio should be determined on the plant- prepared material, ready to go to the dehydrator, rather than on small samples. (Determination of dry ratios on samples of different raw materials is, nevertheless, a valuable means of making preliminary comparisons. ) Inspection losses on the final preduct are frequently so small that they can be neglected. If inspection losses are large enough to war- rant a correction on the yield, this correction can be made by a deduc- tion from the yields shown by the graphs for given values of prepara- tion loss and drying ratio. It may be impossible to determine this in advance of actual plant operations. If fines are removed from the dry product before packaging, they may be either a total loss or may have a market value at a lower price. This will require a further cor- rection to the yield. The simplest procedure is to determine yield after removal of the fines and credit the operations separately with any return obtained from the sale of the fines. Failure to take account of the many factors tending to decrease the final net packed weight may result in ruinously erroneous conclusions regarding the probable over-all weight shrinkage from raw material to final product. Solution of the problem outlined above should, how- ever, be relatively easy in an operating plant, since it can be based on actual operating tests. A greater danger lies in the assumptions that are likely to be made in planning for a new plant where previous experience is not avail- able on the raw material or process. The only data will be hypotheti- 18 The preparation loss used for this purpose must be an overall preparation loss, deter- mined in actual test runs, and must take account of the possible sources of error discussed in the following paragraphs. VEGETABLE AND FRUIT DEHYDRATION LEZ. cal figures—moisture content of the raw material and estimated peel- ing and trimming losses, with perhaps allowances for inspection and screening losses. Suppose that a mixture of No. 1 and No. 2 potatoes is being used. The moisture content is found to be 78 percent, cor- responding to a drying ratio of 4.3 to 1. The total loss from raw to finished product is assumed to be 25 percent, giving an over-all shrink- age of 5.7 to 1. Few people would assume a loss of 89 percent, which would raise the ratio to the 7 to 1 value frequently found in actual operation on raw material of this character. The difference may le in dirt and cullage losses and leaching losses during preparation or failure to use representative material in determining the moisture content. The cost of raw material includes the following items: Purchase price, brokers’ fees and other costs incidental to purchase, hauling charges, costs of handling and storing prior to delivery to preparation line, salaries and expenses of personnel engaged in raw material pur- chase, and other expenses, including indirect and overhead costs. PREPARATION COST The preparation of a vegetable or fruit for drying is usually under- stood to include all of the steps that take it from storage through the last operation prior to loading on trays or belts, or into a kiln, for drying. If the product is blanched or sulfured on trays, tray loading is a drying cost and blanching and sulfuring expense is still a prepara- tion cost. The major steps for most fruits and vegetables are pre- sented below. They may be in proper order for some but not for others; certain of the steps will not be necessary in all cases, and additional operations may be needed on some products: (1) Feeding to preparation line, (2) sorting, (8) washing, (4) sizing, (5) peeling, (6) trimming, (7) slicing, dicing, cutting, stripping, ricing, etc., (8) washing, (9) blanching, sulfuring, or checking, and (10) disposing of waste. Two factors in preparation largely determine the economy of op- eration: The judicious use of labor and the yield of prepared product from unprepared. Equipment costs are also important but are usually small in the long run, compared to the cost of labor displaced. Total labor may run as high as a third of all processing costs, and prepara- tion labor is by far the iargest single cost item or combination of items in preparation alone. Capital and operating charges on prepa- ration equipment amount to only a small fraction of preparation labor in most vegetable- and fruit-dehydration plants. Studies on labor efficiency and labor replacement offer a most promising means for effecting reductions in operating costs. Increased output per employee may be achieved by proper training and intelligent supervision. A piece-work or bonus system, coupled with rigid supervision and inspection, usually results in lower labor costs, especially in the trimming operation. A reduction in the num- ber of workers can often be made by more uniform operation, the elim- ination of process steps, or the installation of labor-saving equipment. The importance of the last point is sometimes underestimated even in large plants. 569074—44 12 178 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE Figure 75 shows roughly the value of equipment that can be pur- chased and operated with specified amounts saved in labor costs. The following assumptions have been made: Interest and taxes, 10 per- cent per year ; repairs and maintenance, 10 percent per year for 1-year and 2-year write-offs, 15 percent per year for 5-year, and 20 percent per year for a 10-year write-off; hourly operating cost per $1,000 of investment, 10 cents; number of hours operated per year, 24 hours per day, 200 days per year. A decrease in labor costs of only $1 per hour will pay the operating and capital charges on equipment conserva- tively estimated as costing $4,000 to $5,000 with a 5-year write-off. A uniform flow of product along the preparation line is essential for most economical operation. An even flow requires fewer employees and smaller equipment than does an irregular or spasmodic flow. Losses caused by temporary shut-downs are often large. Most items of cost continue during shut-downs, with the exception of those for INVESTMENT EQUIPMENT LABOR COST (DOLLARS PER HOUR) Figure 75.—Equipment investment justified by savings in labor costs. raw material and packaging supplies not used and a small amount for utilities. It may not be practical to save labor costs by dismissing the help, because it is usually impossible to anticipate the length of a shut-down. The amount of loss due to a shut-down may be roughly estimated by assuming that the sale value of the lost production less the cost of materials saved (raw material and containers) is an actual loss. For example, a shut-down of the preparation line, due to a shortage of raw material, results in a loss of 20 cents for each pound of lost production if the selling price of the product is 35 cents per pound and the combined raw-material and container costs are 15 cents per pound. This amounts to $100 per hour in a plant which normally turns out 500 pounds of finished product per hour. Careful treatment of the raw product through all stages of prepara- tion reduces the processing cost per dry pound by increasing yields. The two steps in preparation that offer the best opportunity for decreasing material losses are peeling and trimming. VEGETABLE AND FRUIT DEHYDRATION 179 The care and skill exercised by trimmers have an important effect on material losses. Proper training and supervision are especially 1m- portant for trimming personnel. The plant operator should know at what point a further increase in trimming rates results in excessive raw-material losses. A saving of only 5 percent of the raw product by slower and more careful trimming will pay for the employment of two extra employees if the hourly labor cost per employee is $1, includ- ing overhead, and’ the plant is handling 1 ton per hour of raw product costing $40 per ton. If the plant processes 5 tons per hour, 10 addi- tional trimmers could be justified by a 5-percent material saving. The efficient operation of each item of equipment is most essential. The manner in which a machine is operated may cause variations in processing costs greater than the capital charges on the machine. For example, some cutters produce a considerable amount of chaff unless fed at optimum rate. The problem of waste disposal is properly a part of preparation. The quantity and kind of waste are largely the result of the prepara- tion method. For instance, flame or radiant-heat peeling causes a less serious sewage problem than abrasive peeling because the quantity of waste is much smaller. Lye peeling results in a waste product that may require special treatment, deepnding upon the disposition made On it: Preparation costs include the following main items: Labor, equip- ment charges, utilities (mainly water, power, and fuel for providing steam), chemicals such as salt, lye, sulfur, etc., waste disposal (cost of disposal or credit for sale of peels, trimmings, etc.) and other expenses, including indirect and overhead costs. DRYING COST Drying covers the processing operation after delivery from the blancher up to and including delivery from the finishing bins. If no bins are used, drying ends with emptying of the trays or belts. Equip- ment charges are a larger proportion of the drying cost than is the case with preparation. For most vegetables, a few.employees operate a piece of drying equipment that may cost as much as all of the prepa- ration equipment put together, or even more. For some fruits, the ratio of drying cost to preparation equipment cost is even greater, especially for those requiring no peeling or cutting. A minimum of labor costs can be achieved by the use of a continuous- belt drier, or, with the tray drier, the installation of all practical labor- saving methods and devices. The cost of heat and power for many products may not amount to over 1 cent per dry pound. Direct-fired air heating is nearly always more efficient than indirect heat where suitable fuel is available. The installation of properly designed units is an engineering problem, but the capital charges and operating costs of these units can be a subject of cost analysis even before the unit is built or operated. Allowable temperatures, maximum or minimum humidities, and desirable drying times are predetermined technologically. Within the range of these conditions, the plant operator must find the most economical point of operation. Figure 76 shows the approximate cost of heat and power for evapo- rating 100 pounds of water with different percentages of recirculation, 180 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE with an outside temperature of 60° F. and 60 percent relative humidity. The following assumptions have been made: Cost of fuel oil, 3 cents per gallon, or coal, $5.50 per ton; cost of electricity, 114 cents per kilowatt- hour: 60 percent fuel efficiency ; constant air flow; main- tenance of a constant wet-bulb depression at the cool end. Under the conditions mentioned, 75 percent recirculation is most economical. For any particular vegetable, and for different weather conditions, similar charts can be drawn. It is possible that minimum costs may not be obtainable under conditions that are optimum from the stand- point of product quality. The plant operator must often determine the best compromise among the three factors: Product quality, product cost, and plant output. Discussions of operating costs usually assume a balance between the preparation line and the dehydrator. Obviously, the balance is dis- ae OF nae & fee COMBINED fo) COST PER 100 POUNDS OF WATER EVAPORATED (CENTS) @ COST OF FueL/ 0) 10 20 30 40 50 60 70 80 90° §=6100 RECIRCULATION (PERCENT ) FicurE 76.—Cost of evaporating water in air-blast dryer at various percentages of recirculation. turbed by anything that changes the capacity of either one. For example, the output of the preparation line may be increased by an improvement in raw material, thus increasing the amount of material fed to the drier. If the dehy drator has reserve heating capacity, it may be possible to dry the increased quantity by decreasing the amount of air recirculated, thus shortening the drying time. (The alternative expedient of increasing tray loading may be undesirable from the standpoint of quality because of increased drying time, and, for the same reason, may not increase the quantity of material the dehydrator will dry in a given period of time.) The increase in fuel cost due to lower heat efficiency will usually be more than offset by the value of the increased output of the plant. In other words, if there is no short- age of fuel, fuel economy is relatively less important than labor efficiency. or as eee VEGETABLE AND FRUIT DEHYDRATION 181 The main items of drying expense, without regard to order of im- portance, are: Labor, equipment charges, fuel and power costs, and other expenses, including indirect and overhead costs. Packaging and Warehousing Costs For purposes of discussion, several operations are included in this group, as follows: (1) Final inspection; (2) air desiccation; (3) grinding; (4) packaging, filling, replacing air with inert gas, sealing, labeling, boxing or crating, and (5) warehousing or car loading. Final inspection might logically be included in a separate group. It is, however, a relatively minor operation and for that reason is included here insead of in an independent classification. The number of inspectors varies according to the quality of the raw material, care used in preparation, and quality of drying, as well as the require- ments of the purchase specifications or grade standards. The main items of packaging expense, not necessarily in order of importance are: Containers, labor, utilities (including air desicca- tion), inert gas, equipment charges, and other expenses, including indirect and overhead costs. Indirect and Overhead Costs Indirect and overhead costs are included together, since there is ap- parently little to be gained in attempting to separate them. Costs that cannot be directly and wholly charged to any of the groups men- tioned previously are included in this group as follows: Administrative salaries and expenses, interest on investment and taxes, depreciation of plant and equipment, rental of building or equipment not chargeable directly, insurance on plant (labor insurance is a cost of labor), factory clerical salaries and expenses, laboratory salaries and expenses, equipment and building repairs and replace- ments not chargeable directly, fire extinguishing apparatus and other safety precautions, laundry, clean-up labor and janitorial services, vacation labor, first-aid supplies and expenses, transportation of help, miscellaneous supplies and expenses. In distributing these various expenses to the different steps of oper- ation each item of overhead or indirect cost theoretically should be analyzed and prorated separately on a basis equitable for that partic- ular expense. In actual practice, it is not always practicable to do this. It is advisable, however, to handle as many items as possible in this manner. For those items not handled separately, a grouping is satisfactory. Laundry, vacation labor, transportation of help, and other similar charges can be equitably distributed on a labor basis. Depreciation, insurance and taxes, maintenance and repairs, etc., are more logically spread on an equipment investment or other similar basis. Individual treatment of each item is also preferable in distributing indirect and overhead expenses among the various products handled. This method of distribution should be used wherever practical. Charges not so handled can be grouped and distributed on a basis equitable for the group, such as one or more of the following: Value of finished product, time dehydrator is used on each product, quantity of material produced, direct labor cost, and raw-material cost. 182 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE The size of the dehydration plant is a factor determining the de- gree of detail used in prorating each item. In asmall plant, too much attention to such details is unwise. In a large plant, considerable de- tail is warranted. Estimated Processing Costs in Vegetable Dehydration Table 19 presents a partial summary of estimated processing costs in vegetable dehydration, and table 20 shows labor cost per dry pound. The cost elements included are raw material, direct and indirect labor, packaging, and utilities. TABLE 19.—Estimated costs per dry pound of dehydrated vegetables, exclusive of overhead costs and profits, in a dehydration plant handling 50 tons per day, unprepared basis Ruta- Table : Sweet- Item Cabbage | Carrots | Onions | Potatoes | bagas or beets turnips potatoes Form in which prepared-_--_-_-_-- Slices Shreds Cubes Slices Strips Slices Slices Pounds per 5-gallon container _- 8 Ul 17 12 10 12 12 Assumed overall shrinkageratio_| 13 to 1 20 to 1 11 tol lltol 7to1/|10.5to1 5 to 1 Processing costs per dry pound: Cents Cents Cents Cents Cents Cents Cents Labor, direct and indirect! 2 | 10. 0-14. 5} 10.5-16.5| 8.0-11.5) 9.0-13.0) 6.0-10.0| 7. 5-11.0 4, 0-6.0 @ontainers|skenge es eae OHO lees 5.5 2.5 ts) 4.0 3H HO) WWiGilitiesise ceca cae eas A 1-2 12, 1-2 1=2 1-2 12 12 FING Gaylene eRe ey ccriakine Aa 16. 0-21. 5| 17. 0-24.0} 11.5-16.0) 13. 5-18. 5) 11.0-16.0} 12.0-16.5} 8.5-11.5 Raw material costs per dry pound: Cost at $20 per ton________- 13.0 20.0 11.0 11.0 7.0 10.5 5.0 Cost at $25 per ton________- 16.5 25. 0 14.0 14.0 9.0 13.0 6.5 Cost at $30 per ton________- 19.5 30.0 16.5 16.5 10.5 16.0 etd, Cost ay $35 per ton_______. 23. 0 35.0 19.5 19.5 12.5 18.5 9.0 Cost at $40 per ton________- 26.0 40.0 22.0 22.0 14.0 121.0 10.0 Cost at $45 per ton______-_- 29.5 45.0 25. 0 25. 0 16.0 23.5 11.5 Total costs per dry pound not ae mms hae 7 including overhead costs or profit: ! Raw material at $20 per ton_| 29. 0-34. 5) 38. 0-44. 0| 22. 5-27.0} 24. 5-29. 5} 18. 0-28.0) 22.5-27.0) 13.5-16.5 Raw material at $25 per ton_| 32. 5-38. 0| 43. 0-49. 0} 25. 5-30.0} 27. 5-32. 5) 20. 0-25. 0) 25.0-29.5) 15.0-18.0 Raw material at $30 per ton_| 35. 5-41.0| 48. 0-54.0] 28.0-32. 5} 30. 0-35. 0} 21. 5-26. 5) 28.0-32. 5) 16. 0-19.0 Raw material at $35 per ton_| 39. 0-44. 5} 53.0-59,0| 31. 0-35. 5} 33. 0-38. 0} 23. 5-28. 5) 30. 5-35.0) 17. 5-20. 5 Raw material at $40 per ton_| 42. 0-47. 5] 58. 0-64.0| 33. 5-38.0| 35. 5-40. 5| 25. 0-30. 0) 33. 0-37. 5) 18.5-21.5 Raw material at $45 per ton_| 45. 5-51.0| 63. 0-69. 0} 36. 5-41. 0| 38. 5-48. 5| 27.0-32.0| 35. 5-40. 0! 20. 0-23. 0 1 The low limit of labor cost is a summation of the low estimates for each individual operation, as shown in table 18; it is very unlikely that any plant will operate with an absolute minimum of labor in all operations. 2 See table 20 for cost details. 3 The cost of packaging includes 25 cents for a single 5-gallon can and,30 cents jfor] the wire-bound wood box holding 2 cans; the total per can is 40 cents. 4 In natural gas areas, many plants are operating at a cost of less than 1 cent per dry pound for utilities: Gas, electricity and water. Utilities may run considerably more than that amount in high-cost regions. Such costs have been roughly estimated to range between 1 and 2 cents per dry pound. A more accurate estimate is not warranted since the plant location, type of fuel, and operating procedures are not specified. TABLE 20.—Labor cost per dry pound in vegetable dehydration plants handling 50 tons per day, unprepared basis Labor cost per hour Out- Bul Direct and supervisory ? Labor Vegetable per cost OUTS hee geal oh iit 7 aa | EEN LTE GO , er dry dry Indirect} Total | P : basis ! Fore-|£°re- pound Men Women wom-| Total man an Db. Dol. Dol. Dol. | Dol. Dol. Dol. Dol. Cts. Table beets__.____-- 320/12. 75-18. 75)15. 60-22. 20} 1.25) 0.85/30. 45-43. 05/2. 05-2. 55/32. 50-45. 60)10. 0-14. 5 Cabbarem as are 210/10. 50-15. 00) 7. 80-15. 60} 1.25)_____- 19. 55-31. 85/2. 05-2. 55|21. 60-34. 40/10. 5-16. 5 Carrots Se eee ct 380/10. 50-15. 75)15. 60-23. 40) 1.25 . 85/28. 20-41. 25)2. 05-2. 55/30. 25-48. 80) 8. 0-11. 5 Qnionstasseeeeso se 380/10. 50--15. 75)19. 80-29. 40) 1.25 . 85/32. 40-47. 25/2. 05-2. 55/34. 45-49. 80! 9. 0-13. 0 IROLAtOeS 2 see a= — 600)10. 50-15. 75 22. 80-39. 60} 1.25 . 85/35. 40-57. 45 2. 05-2. 55/37. 45-60. 00) 6. 0-10. 0 Rutabagas or turnips. 395 10. 50-15. 75/15. 00-23. 40) 1. 25 . 85 27. 60-41. 25 2. 05-2. 55|29. 65-43. 80} 7. 5-11. 0 Sweetpotatoes_____- 830,10. 50-15. 75'18. 60-29. 40 1.25/ _. 85/31. 20-47. 25,2. 05-2. 55/33. 25-49. 80! 4. 0- 6.0 See footnotes on next page, VEGETABLE AND FRUIT DEHYDRATION 183 TABLE 20.—Labor cost per dry pound in vegetable dehydration plants handling 50 tons per day, unprepared basts—Continued ‘ Average hourly output per 24-hour day. 2 Assumed hourly labor rates: Men, 75 cents; women, 60 cents; foreman, $1.25; forewoman, 85 cents. Number of employees taken from table 18. 3 Indirect labor is estimated as follows: Since this indirect labor charge will be applicable to 3 shifts the approximate cost to each hour’s out- put will be 34, or $2.05 to $2.55. Nuuee of 5 sys employees ost per Position 2 (1 shift per hour day) IBOOKKEC PETS stuart tater wwe wen Pee S ET Sas ey, eS SNe oe Ree lor 2 | $0. 75-$1. 50 SCEMO PT AD NE Tate eh inne re ses oN es A Sie A es es epee a ee a ed 1 . 65 IDAVCOH CLOT kts ex mete ope eet EO ON ee ce i Bee Mee Lene WE lor 2 . 75- 1. 50 SPELT TING Ci frees ta eat a ee SaaS 2 SS Mk hed ets re by het Sad on 1 1. 50 BESET Ree T YY 9h Ft eee gece LUNs cea A UR MBN eek ens Peel tem ARS Be 1 1.25 IBIS HERCHEITISE ae awed etl een err ATENS Feet) Bilao) Panel SST e 1 1.25 AICO fel eee ices ihe oy es Lake WR RTER Soe en oe east a ty re Ee 6to8 | 6.15- 7.65 Other direct and overhead costs have not been included in this cal- culation. Some operators believe that total overhead costs should not average more than 50 percent of direct labor, while others say that these costs may be equal to or even greater than the cost of direct labor. Still others believe that overhead costs have no relation to labor and cannot be accurately estimated on a labor basis. Wide variations occur from plant to plant because overhead costs in vegetable dehydration depend on such factors as the length of the operating season, cost of buildings and equipment, local conditions, and managerial policies. The complexity of these interrelated factors is such that no general estimates of overhead costs have been attempted. The cost figures, although not complete, are useful guides within the indicated limits. A prospective operator can combine these figures with data specifically related to his proposed operation and thus more accurately estimate what his costs are likely to be. The figures are based upon continuous operation which is rarely experienced in commercial plants. Where operations are interrupted or discontinuous, suitable corrections must be applied. It is apparent, | also, that the cost estimates must be adjusted in any particular situa- | tion according to labor rates, shrinkage ratios, and operating procedures. | HANDLING SPECIFIC VEGETABLE AND FRUIT CROPS Vegetables In the following pages information is presented on the dehydration of specific vegetables. The process and to some extent the equipment required in dehydration vary with the product, and in addition the products vary in requirement for rehydration. The information that follows is more detailed for those crops that are dried in large quan- tities than for others. For convenience, information bearing on certain steps has been assembled in tables 21 to 26. Table 21 shows the type of piece, blancher loadings, and time of blanching for 18 vegetables. Since it is important that the plant operator know the moisture content of the material to be used, the approximate ranges in moisture content of the vegetables are shown 184 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE in table 22, together with averages. From these percentages the ratios, in weight, of water in the vegetables to “bone-dry” matter have been calculated and are shown also. Bone-dry matter must not be confused with the finshed product, which contains a low percentage of moisture, as shown by the maximum percentages permitted under Government procurement specifications. The ratio of water to dry matter in the raw product is useful to the operator, because it shows him how much - water is contained in the raw commodity and makes readily calculable the weight of water that must be removed. TABLE 21.—Blancher loading and blanching time for specific vegetables appre Suggested = : blancher blanching Vegetable Type of piece loading time in live per square ese F.) foot : Pounds Minutes IBGanS: iM aes eer seen en ee eee Whole; :shelled-292= see ig 15 IBGANSS STAD) Sieeeen eee ete ea Cups Sa a eae gee | 2.0 5-10 BGC iS Ree a Ce ee ee ee A 1s' 800) Ce ear eens ne NORD Rens Surette Leen LS RE ie ore 2,5 25-40 1 BY 214 ROVE pe ee eit Dee tiegty ree tea noe ET SEK RY @ubessslicess sinips=——= eee 3. 0-4.0 2,5 2-4 Cabbave sss Gane ae eee ee SHEGS 25 = sis Sea eee 1. 0-1. 25 3 2-4 CAELOES ee a eee Cubes; ‘slices; ;SiripS ===> =e 3. 0-4. 0 24-10 Celeryss S22 aoe: aetna eet Bees SHGGS Fs 26 ae ee ee ee 1. 0-1. 25 31-2 (@HarGAS wAss setae en eee Gees Bec a [Brimmedileaies =e ek ee 8 32 Gor Sweetie ee. se Se ere eee iWholejkernelsOnuco Was | ee 48 Kidlewtice. (See enn Sia a ok ene “Drimmedjéaf. eee ee .8 32 INTUISGAT EET ECM Sas = ean eee ere ee ore CO ess ee a rien aN een .8 32 ONIONS Meee Se es Se ee Shreds Slices 22s ase eee ese eee ena Tos 41-15 RAarsnilpS See ies eis ee eee oes ara eee @ubes; sliceSistEips a= 3. 0-4. 0 2 6-10 IPCAS SET CON ke eee Se ee Whole, shelled ss =." Sek eee 15 5 PObatOESE ERS ea eA EPR ee @ubes;\slices,stripss= = eee 4.0 22-10 Potatoes, rice ee eee Quartereda ee Sees ee ee ee ae Petter aoe 20-25 UD) Qe es a ee ee eae Sliced ssa eee i ere 4.0 6-10 Rutabagassec soe yaa ee ek eee ee @ubes; slices stripSs ee 3. 0-4. 0 2,5 5-10 S DIAC Se ee ne ee hrimmed)leqisessss2 sane eae eae .8 32 Sweelpotatoess === ase te wan eee CubessshcesnSitlpss- === eee 3. 0-4. 0 2,6 6-10 ROMATOCS =e ee a SHeGS oop Sea ee ee eee 1 2-3 1 Large-seeded lima beans may require 1 to 2 minutes longer for proper blanching. 2 Government specifications require blanching until the peroxidase system is inactivated. 3 Government specifications require blanching until the catalase system is inactivated. 4 Government specifications do not require blanching of onions. 5 As the peroxidase test is interpreted at present, it is usually impossible to destroy the peroxidase system in rutabagas and beets in less than 30 minutes, but if blanched as suggested above, the dried products will probably keep well in storage. 5 Sweetpotatoes should not be permitted to come in contact with iron during blanching. TABLE 22.— Moisture contents of fresh and dehydrated vegetables Approximate moisture | Ratio of moisture content, Maximum eS content of fresh vegetable! to bone-dry matter! | moisture Vegetable | content of | Range | Average Range Average | er | Percent Percent | Percent IBGans ina eee Sn ee ete | 958. 9-71.8 | 66. 5 1. 4— 2.5.) 22.0 5 Beans snap sese= ash oe ee a ee 78. 8-94. 0 | 88.9 3.7-15.6 | 8.0 5 IBCCTS= Same e Cree ee dE Dee eer ae| 82. 3-94. 1 87.6 4.6-15.9 | 7.1 25 Ca Da oe eee ae OS ee ee 88. 4-94. 8 92.4 7.6-18.2 | 12.1 24 CArrOtS a ee ees 83. 1-91. 1 88. 2 4. 9-10. 2 7.5 25 @elenyets atte eae ee Do ee ee 89. 9-95. 2 93. 7 8.9-19.8 | 14.9 24 CD Ard aS WiSS ae a a ee eas ph heen 89. 9-92. 9 91.0 8.9-13.1 | 10.1 3(10.8) 4 COormeSweetee ie en ie Se 61. 3-86. 1 73.9 1. 6- 6.2 2.8 5 SY CASS ee el Eo eee tit SL St ae See eee 81. 4-91. 2 86. 6 4.4- 9.3 6.5 4 Mustardiereens-" sane ers eeu sine 86. 7-95. 7 S2E2 6. 5-22.2 | 11.8 3(12. 2) 4 Onions) 32525 eee ae te eR Res 70. 2-95. 2 87.5 2. 5-19. 8 7.0. . 2(8:8) 24 IPAKSHIPS 52a. ee ee em Oe ee Pe 72. 6-89. 2 78.6 2. 6- 8.3 BBY) ae peas jereene ss ete a a ee ees 56. 7-84. 1 74.3 |. 1.3- 5.3 2.9 5 Potatoes 45? 82 se Fe ee ee ee 66. 0-85. 2 UisSile da d— 008 3.5 3(4.0) sine oO ee We Se eee ee ee ween My Se eas, er ae ene Se Rs a ey Ar la i at 6 io 2 IRiUtabacasee oe aay es aun eee 86. 1-91.8 89.1 652-112 a 8e2 25 Spinachi eas) = cme eset eye eee 89. 0-95. 0 92.7 8.1-19.0 | 12.7 4 Sweetpotatocsases a ee ee 58. 5-82. 7 68. 5 1.4- 4.8 2.2 2(3.0) 27 MomMatOeS es S32 ooo Se ee es 90. 6-96. 7 94.1; 9.6-29.3 | 15.9 5 1 Calculated from data of Chatfield and Adams (10). 2 According to Government procurement specifications. ; Pelt in parentheses obtained at the Western Regional Research Laboratory, Albany, Calif. ut. 5 Riced. VEGETABLE AND FRUIT DEHYDRATION 185 The drying ratio and its converse, the drying yield, are shown for 18 vegetables in table 23. These ratios and yields have been calculated from the changes in the moisture content during the drying step. The drying ratio is the ratio of the weight of the material entering the de- hydrator to the weight as it leaves the dehydrator commercially dry. The drying yield is the converse of the drying ratio and is expressed 1n percentage. These ratios are useful to designers of dehydrators, to operators, and also to prospective operators who wish to compare yields of product from various types of raw materials, since it can usually be assumed that the moisture content of the blanched, prepared material is approximately the same as that of the raw material. TABLE 23.—Drying ratios and drying yields for specific vegetables * | | | Drying ratio 2 | Drying yield 3 Vegetable aa a a ee aes | Range | Average | Range Average | | | Percent | Percent IB CAS sol ae es Ar hs et Die ed |= PRBS eka PEC | 29. 6-43. 2 35.2 IBEATISH SNAP sete wegen ean: a ae De ee | 4,415.8] 8.5 | 6.3-22.3-] 11.6 SCE Seen oe ety Pe wee te Cats el eee ee ey ee R= IGGL 7.6 | 6.2-18.6 13.0 @abbaae es ste tie ees he A Bele BOHR 4 121986 5.4-12.0| 7.9 Warr stem ne eee amb let cote ere! eee | 5.6-10.6 | 8.0 | .9.8-17.7 | 12.4 (OYE ES ap a a FR ae Eke | 9.5-20.0 | 15.2 | 5.0 9.8 6.5 Ghrardha Si wisSmaee i bane ete ae ren ae | 9.5-13.5 | 10.6 #4 (11.3) 7. 3-10. 5 9.3 4 (8.8) COGN ASWCC bo re ee en eet Wear enstes Oa. | 2.4- 6.8 3.6 14. 640. 7 27.4 TCG DS aig ae ea kee nn SA | 5. 1- 9.7 Teal 10. 2-19.6 | 13.9 IVEISFATONETLC ONS Bees eee eek ees ate ee net (2 JON Ovone 44 (2.6) 4.4-13.8 | 8.1 4 (7.9) TiO TNS ee ae re Se SE | 3.2-20.0] 7.6 4(9.4)} 5.0-31.0 | 13.0 4 (10.6) TEATS TS oem ee eee ees Coe yrs ee SEA Bil rac. 11. 3-28.8 | 22.5 IRCAS AGT CC ieee aes Noe a re aie ee ee CA BS | 2.1- 5.9 3. 6 | 16.7-45.5 | 27.0 POCA OCS Stee Set aa ee Wi Poker Con Sen 0. es ees 2.7--6.3-| 4.2 4(4.7)| 15.7-36.0 | 23.6 4 (21.3) TUTTE AD AG AS meee se ere ae eg cee ie ee 6. 8-11. 5 8.7 8.6-14.6 | 11.4 RSH ODT NGI Sate Seep ree EE Nea ai enas or et Ae ae ee 827=19: 2) | 1351 5. 2-11. 4 7.6 SiW.eCLDOLALORSS toe ee et ee 2.2—- 5.3 | 2.9 4(8.7)| 18.6-44.6 | 33.8 4 (26.7) SOMMAUOCS Eee sane ae ee ae es ee hE Le ae 10. 1-28.7 | 16.1 3.4- 9.4 | 6. 2 | 1 Calculated from data in table 22. E : : 2 Drying ratio= weight of prepared, blanched material entering drier divided by weight of dried material leaving drier. ; 3 Drying yield=weight of dried material leaving drier divided by weight of prepared, blanched material entering drier x 100. ; 4 Data in parentheses obtained at the Western Regional Research Laboratory, Albany, Calif. The operator is more directly interested in the overall shrinkage ratio—that is, the weight of unprepared raw product required to yield one weight unit of finished product which meets specifications. This may also be expressed as the reversed ratio, usually as a percentage, and is then known as the overall yield. The overall shrinkage ratio is always substantially higher than the drying ratio, and the overall yield lower than the drying yield, because all weight losses incurred at various steps of the process, such as culling, washing, peeling, trim- ming, and inspecting, must be discounted. Estimates of such losses are presented in the discussion of certain vegetables but it must be remembered that these losses vary widely. Suggested tray loadings are shown in table 24 for various systems of air flow. These loadings cannot be regarded as specific recom- mendations; instead they are to be regarded as suitable for trial. Ex- perience with various materials will indicate necessary modifications. Loadings for cross circulation and through circulation of air are listed separately. 186 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE TABLE 24.—Dehydrator-tray loadings suggested for specific vegetables and differ- ent systems of air flow Cross cireula-| Through eir- - es : tion of air | culation of air Vegetable Type of piece per square per square foot foot r Pounds Pounds Beans: imac. tyne ho kien ay ee ee eee Wiholes-sis 22 sp = ee eee ees 14) 3-4 IBeanss snapest 22 Sis Bo Paw aa Oj Ui rete Sct ar dn ple se irs Weep R re Se 15 6-12 CUT ES ES Rates Re ees ee Re 1. 3-1.8 6-12 IBGCtS att se ens ed ea Slices#ieneg<. ee ei ties tee eee 1. 3-1. 5 2-3 STRIP Soa take ot ee eeeee Us ee Ae eee ae | 1.5 6-12 Cabbage: te ae Ck nals 07 ae abe me pesaervas SHrEGS ae Lele Vek SSS ee HE ee aye 75-1.3 6-8 {Cubes PES ec et OR eT ie MRE eS 1. 3-1.8 6-12 Carrots eee he See ease pe ee ee SLICES Bee eA ee ap ne oe eee eee 1. 0=1:3 2-3 \Strips I i a ee DE Pi ec ea do Ga SP UL 12-175 6-12 Celernyaek cs ORR he eg eee Slicese= = Foe pes hot PP Ae ye cee ae een fe 1.0-1.5 6-12 CharGeiS wisseyesesec a eee eae imrimmed leah 6.2 = Ses ee ee 75-1. 3 3-6 Gorm ls wee tasssen ee oe a eee tia Wiholeskernelset cae steamer pee © 1.5-1.8 3-4 KSA eens eles Nw ON eee is Se Same ASMOn SO waSsy Chan Ses ese pe ae |e epee | ee ee IM GOR ARG UEG RING Leas oe oe ipods] Oe B Se SEN SEN Dy PRE Sa tO | Eg tee | OO = ONION SSE ace fe an, ope eS Slicesishreds tase] = che Pele ee eee 1.0-1.3 6-8 PRATT Sheree oo eR ee ea eee nce Sameras fOr DOSS Hs ee ee a Sai eee | er eee Re asHeTe ene amas tener ae ee gah pees ee Same as for lima beams_____~=________- fe eae ia ee i | ee eee (CUT ESO OEY exces Okay ot Rare eee 1.3-1.8 6-12 STIGES Hee rs Seg ae ee een ioe ee ee 1.0-1.3 2-3 Potatoes. --_---_-------------.-----. Strips a SiMe saa BU So A 1.2-1.5 6-12 RIGS es NW We Re Sed Re 1s ON ee ee eee RII PAD AGAS Sts ten atten ee ee oe ee Sameastfonr Carrots ese see 2a Sees RE | eee ae eee | SPinachweace ter ieee Me ae eee Shion GS oye Shae oniol ss | ice ee nae Oe CWP CSE Cine SEE Aer eens 1! 3-1.8 6-12 STGeS Se He Gre a ae Sole ee Sees 1.0-1.3 2-3 SWeetpolalOeSa= nasa eas eee fee Ast: GCG ise 2) UR Loy Aco rman eae teh 6-12 TBST STAG kagtepee Rees RE NN RO Eee amet Si bls 1.0 2-3 TOM AtOCS ese ta ee eee alee Beamer ee 2 STICGS 21s eR Ep pat ae 1. 3=1.8 2-3 When a bin drier is used for finishing the dehydration of vegetables, the most suitable depth of loading will vary from 2 to 6 feet and must be determined by trial. Loading depths of not over 3 feet are sug- gested for cabbage, sweet corn; and tomatoes. The air entering the bin finishing drier should have a dry-bulb temperature of 120°-130° I’. and a relative humidity of 10 percent or less. (See p. 107.) Variations between varieties and within a single variety due to maturity, cultural conditions, or storage conditions make it necessary to determine safe operating temperatures by trial. The general prin- ciple to be followed is that the finishing temperature shall be carried as high as possible without damage to the product. To serve as a guide the temperature conditions for different systems of dehydration are shown in table 25 for various commonly used types of drying equipment. Types of equipment and their operation are discussed in previous sections of this manual. For some of the vegetables, in- formation is included on storage of the dried product after it has been packaged. Table 26 contains the weights of certain dehydrated vegetables that can be packaged in standard containers. 187 “AJOLIVA YTIM SolivA ¢ “AH oGS JSBOL BV 0G P[NOYS Pus [009 OY} 7e UOISSoIdop q[Nq-JeM OY ‘“Soansy JO QT UWIN[OD 99s Soin} eVAdUI9}-q[Ng-J9M LOW z “A OG JSVIL 1V 0q plNOYSs pUd [009 9} 7B WOTssoi1dsp Q[Nq-J9M IZ r 06 OST OGL 006 06 OST COL GOT OSI 002 OT OI c9T OO GT Bicerl eoh Sene ns Sires aoenee $00} CULL, Z C6 O9T OZI 00G C6 O9L COL OLT OST 00Z-O8T O9T OZT OLLI OCGRO0GES Es Sana arenes $00}B}0dJIIMG i) OOT OLT OL 00z OOT OLT OLL OST OT 00% OLT OT O8T QO GL. ve A ESS eee cece, Rameyintenat yoeuids ia 06 OST OZI 00% OOT-G6 GGT-OGT COL OLT OL 006 CG I-OST OZT OLT OO Gey te a ea aeay ems SVsVq BIN yy U8 OL OO GOT 06-8 OFT OE! C6 GPL OOT GOT OFI-OET ¢ OOT crt GOT iy a ae ee OVUG) an OOT OLT OGI 00G OOT OLT OLT OST OGL 002 OLT OGI O8T QO Gisheeeeatal esses SUd0As PABISNYL ea OOT OLT O31 00G OOT OLT OLT OST OZI 002 OLT O POTATOES Of all the vegetables that are dehydrated, potatoes are the most im- portant. The demand for dehydrated potatoes exceeds that for any other vegetable; in fact, the total present demand for potatoes is almost equal to demands for all other dehydrated vegetables combined. The national commercial production of potatoes in 1942 amounted to 11 million tons. Potatoes are grown in all States. Maine, with a production of 1.8 million tons, or 12 percent of the national total, out- ranked all other States. Idaho with 918,000 tons and New York with 822,000 tons were next in importance.. Five cther States—California, Minnesota, North Dakota, Pennsylvania, and Colorado—each produced over 500,000 tons. The entire production of these eight States was more than half the total national crop. Potatoes are grown dur ing all seasons in one part of the country or another. In some of the Sarin n States, the harvest starts early i in the spring, while in southern Florida and in the lower Rio Grande Vallev potatoes are harvested toward the end of the year. Early pota- toes are a relatively unimportant part of the total production. The season of availability for the intermediate and late crops extends from the beginning of harvest in late.summer to at least the late spring of the following year. Yields per acre are influenced to a large extent by climate and the use of irrigation. They vary widely from year to year and among the States. The 1930-39 average yield per acre for all States was 3.4 tons, compared with 3.9 tons in 1941. For oleae ation many operators prefer the type that becomes white and mealy with cooking. Experimental testing of potatoes grown in any given area, prior to the establishment of a new dehy dration plant, is addy isable as a means of determinating (1) the operating conditions that make the best dried product, (2) the acceptability of this product for use, and (3) the yield of dried product obtainable from the raw material. The variety of potato most widely grown in the United States is the Irish Cobbler. Because of its remarkable ad laptability, it is grown to some extent nearly everywhere. The Triumph is second in im- portance and acreage, with large concentrations in the Middle West, VEGETABLE AND FRUIT DEHYDRATION 203 and Katahdin is the third most important variety. The Green Moun- tain group is important. in Maine and the neighboring States. Pro- duction of the Russet Burbank group of varieties is concentrated en- tirely in the Northwest. In California, the White Rose is the leading variety. Rural Russet and Rural New Yorker No. 2 are grown ex- tensively in certain States in the regicn of the Great Lakes. The con- ditions under which these varieties are grown affect their suitability for dehydration. A variety of potato, to be suitable for dehydrating, must be grown in a district and under conditions to which it is suited, and where it will give optimum yields. Russet Burbank when grown in Idaho, for example, is excellent, but the same variety grown in Maryland, where it is not adapted, makes an inferior product. Likewise, potatoes satis- factory for dehydrating can be grown in a certain district only if varieties adapted to that district are used. Limited tests have shown the following varieties to be among the best when grown in the States indicated: Michigan: Rural Russet, best; Irish Cobbler, Green Mountain, Chippewa, Katahdin, and Sebago, very good. New York: Green Mountain and Pioneer Rural best; Katahdin and Chippewa, very good. Washington: Russet Burbank and Sebago, best. Idaho: Russet Burbank, best; Sebago, Triumph, and Katahdin, very good. Colorado: Triumph and Katahdin, best. Maine: All the varieties named above produce excellent or very good stock for dehydrating. Tmmaturity or other factors that result in high water content defi- nitely impair quality for dehydration. Immature “new” potatoes are not suitable for this purpose. Only mature tubers, fr ee from disease and bruises, should be stored. A storage temperature of 40° F. is low enough to keep mature potatoes dormant 8 to 5 months. At this temperature, however, they may become mildly sweet. If stor ed at 40° or lower, the potatoes may show marked yellowing and browning at the center of the piece after being dehydrated. For short-time storage, a temperature of 50° to 60° results in good texture, color, and flavor in the cooked product. Potatoes stored at the lower temperatures should be held at 60° to 65° for 3 to 4 weeks just prior to dehydration. Under these conditions the sweet taste will be lost and a satisfactory dried product obtained. The relative humidity recommended for potato storage is 85 to 90 percent. Potatoes are commonly stored in pits or large bins of 150 to 1,000 bushels in the Northern States, but in milder climates they should be stored in small units. Heat insulation and ample ventilation are needed to provide the best conditions for storage. Potatoes should always be stored in the dark because in the lig! ht they become green and unfit for food as a result of the development of solanine, a bitter poisonous substance. In most districts potatoes are handled and stored in bags. After delivery to the plant, measures must be taken to make sure that the crop will remain in good condition until used. An even, moderately cool temperature should be provided, in insulated rooms if necessary. The bags should be stacked so as to provide enough aeration to prevent heating. When the weather becomes cold, heat should be provided to prevent freezing. Since potato handling is 204 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE almost an all-year job, maintenance of cool storage to reduce spoilage 1s Important. Thorough washing is the first step in the preparation of potatoes. This operation can be carried out by running them through a revolv- ing corrugated-drum or squirrel-cage washer equipped with sprays. Following washing, grading to size should be performed if abrasion or radiant-heat peeling is used, since grading speeds up the peeling operation and reduces waste. With lye or brine peeling, grading to size 1s less important. Trimming is necessary after the peeling operation to remove the eyes and black spots or unsound or damaged portions that remain after the product has passed through the peeling machines. The to- tal waste from peeling and trimming varies with the grade and size of the tubers, the peeling method used, and the care exercised in peel- ing and trimming. In commercial plants this waste is rarely lower than 15 percent, and may rise to 30 or even 40 percent if the raw material is inferior or the operation is conducted carelessly. With abrasive peeling the average over-all loss is about 23 percent and the range 18 to 27 percent. With lye, brine, or heat peeling, the losses may be considerably less—as low as 12 percent. The peeled and trimmed potatoes pass to mechanical cutters where they are cut into slices, cubes, or strips (julienne style) in accord- ance with the form desired. (Seep. 34.) After being cut the potatoes proceed to a washer where the loose starch 1s removed. This operation is then followed by blanching. If delay between cutting and blanching is unavoidable, the cut ma- terial must be held under potable cold water or a clean 2 percent salt solution. This procedure will protect the cut product from discol- oration as the result of enzymatic oxidation. The prepared material should not be held for more than 1 hour before blanching. If the potatoes are to be riced, the peeled and trimmed potatoes can be quartered or sliced and then blanched. (See table 21.) After being blanched, the material should be sprayed again with clean water in order to remove loose starch that might cause the pieces to stick together during dehydration. Delays between blanching and dehydration should be avoided. In any case, the material should not be held longer than 1 hour. Quartered potatoes that are to be riced are exposed to live steam until cooked, usually for 20 to 25 minutes. After this operation. and while still hot, they are passed through a mechanical ricer directly to the drying trays. ; Information on the moisture content of potatoes and on the maxt- mum moisture content permitted by Government specifications for de- hydrated potatoes is given in table 22. The drying ratio and drying yield are shown in table 23. Tables 24 and 25 contain information on dehydration. (See pp. 184-187.) Tin cans have been replaced to some extent by 5-gallon square cartons as containers for dehydrated potatoes. Dehydrators will re- quire between 250 and 380 five-gallon cans or cartons for every 10 tons of raw untrimmed potatoes that are to be dehydrated (table 26). De- hydrated potatoes are moderately sensitive to heat, being less suscep- tible than carrots and more so than sweetpotatoes. In 1921 Gore and Rutledge (78) reported that dried potatoes containing 6.3 to 6.6 7 ee VEGETABLE AND FRUIT DEHYDRATION 205 percent moisture content did not discolor in 700 days at 35° F., dis- colored slowly at 75° and browned rapidly at 105°. The color and texture of rehydrated potatoes are influenced by the variety, growing environment, maturity, predrying storage, predrying preparation procedures, time and method of blanching, drying condi- tions, and methods of rehydration. Under-blanched dehydrated potatoes will rehydrate slowly and incompletely. Hf over-blanched. the pieces may disintegrate in part or entirely when rehydrated at boiling temperatures. The most unsatisfactory samples are those that partially disintegrate, forming a mush in the water while the centers of the pieces remain dry, hard, and chewy. It is assumed that cubes, strips, and slices are produced because the forms are desired for specific table preparations; the rehydration procedure should therefore pre- serve these forms. Samples of high quality will be rehydrated suc- cessfully by boiling without previous soaking, but since table prepara- tions, such as fried, escalloped, and hashed browned potatoes, are best when unboiled potatoes are used, the rate and completeness of rehydra- tion at room temperature, as well as at the boiling point, should be determined. Because of the great number of factors that affect quality, some of which can be overcome by changing the conditions of rehydration, it is recommended that each producer conduct a series of tests for rehy- dration and quality on each lot of potatoes. An acceptable product should be rehydrated to satisfactorily plump pieces without becoming mushy or watery. ‘The rehydration ratio (the rehydrated weight di- vided by the dry weight) will vary with the size and shape of the piece, the time held in water, and the temperature of the water. Rehydra- tion ratios obtained with high-quality samples are given in table 27. TABLE 27.—Rehydration ratios obtained with high-quality samples of dehydrated potatoes Boiled without soaking Boiled after soaking overnight Soaked PERE WG Ticebafae ote bib an eee eerie tee ee Rel | ha OV EL= Form Size night, 5 min- 10 min- | 20 min- 5 min- 10 min- | 20 min- not utes utes utes utes utes utes boiled Inch Cubess— og eae %e 2-3 3-4 3-4 3-4 3-4 4-5 2-3 Slicesz 2s as eee BANG | heats ae wp Ha a Ge Gia eee ciy bn | eee Ste cad ea Dee OR eS Cea Strips