Division of Agricultural Sciences UNIVERSITY OF CALIFORNIA ''''•>ti£} COMMERCIAL COOLING OF FRUITS AND VEGETABLE CALIFORNIA AGRICULTURAL Experiment Station LTR'RAK^ Extension Service UNIVERSITY OF CALIFORNIA DAVIS MANUAL 43 Price, $1.00 CAGMAM 43 1-44 (1972) To obtain additional copies of this manual or a catalog listing other manuals and free publications available, see your University of California Farm Advisor (offices located in most California counties) , or write to: Agricultural Publications University of California Berkeley, California 94720 Orders of 10 or more copies of any one manual receive a 20 per cent discount off list price. All manuals are shipped prepaid. When ordering manuals, send orders and payments to the above address. Make checks or money orders payable to The Regents of the University of California. This manual presents information on cooling of fruits and vegetables for persons concerned with designing and operating coolers and for those who will be cooling produce. Part I dis- cusses the postharvest physiology of fresh produce in relation to cooling procedures. Part II describes the leading methods of cooling and the products and conditions to which they are adapted. Part III presents suggestions and data relative to design of coolers, including recently measured effects of venting and stacking patterns on forced-air cooling. Part IV presents cool- ing data, including product requirements, tables of cooling times, and formulas and conversion factors useful in cooler design. Emphasis on various topics in this publication is necessarily arbitrary. Included is informa- tion on design of forced-air coolers, hydrocoolers, ice bunkers, and air humidification that may be useful to shippers as well as to designers. No attempt is made to cover the design of vacuum coolers or mechanical refrigeration systems — this information, which would require considerably more technical detail to present, is available from other sources listed herein. THIS MANUAL is one of a series published by the University of California College of Agricul- ture and sold for a charge which is based on returning only a portion of the production cost. By this means it is possible to make available publications which, due to relatively high cost of pro- duction or limited audiences, would otherwise be beyond the scope of the College publishing program. December, 1972 The authors: F. G. Mitchell is Extension Pomologist, Marketing, Agricultural Extension, Davis; Rene Guillou is Emeritus Associate Specialist in Agricultural Engineering in the Experiment Station, Davis; R. A. Parsons is Extension Agricultural Engineer, Agricultural Extension, Davis. CONTENTS Part I— THE COMMODITY Causes of postharvest deterioration 3 Effects of temperature on produce 3 Ethylene physiology of fruits and vegetables 4 Wilting and shriveling caused by water loss 4 Relationship of temperature to produce injuries 6 Chilling and freezing of produce 6 Protecting produce temperature during marketing 7 Part II— COOLING METHODS Room cooling 7 Ceiling jets for room cooling 8 Bays for cooling and storage 8 Containers used in room cooling 8 Cooling in railcars or trucks 10 Cooling fruit for controlled-atmosphere storage 10 Cooling with package icing 11 Vacuum cooling of vegetables 11 Hydrocooling of produce 12 Checking hydrocooling operations 13 Forced-air (pressure) cooling 14 Shelf-type forced-air coolers 16 Width of container stack for forced-air cooling 17 Exposure of stacked containers for forced- air coolers 18 Containers for forced-air cooling 18 Conduction cooling of produce 19 Controlling humidity 19 Humidifying with fog spray 20 Packed towers for cooling and humidity control 21 Ice accumulators used with packed towers . 22 Using ice bunkers as a source of coolant . . 22 Liquid nitrogen and liquid and solid carbon dioxide as coolants 23 Choosing the cooling program 23 When to cool 24 Measuring temperatures 26 Part III— DESIGN CALCULATIONS Hydrocooler dimensions and water flow . 28 Air channel dimensions in forced-air coolers 29 Cooling calculations 30 Part IV— COOLING DATA Cooling and storage requirements for California fruits and vegetables 33 Removing heat by respiration 34 Produce cooling times by method of cooling 34 Useful constants 34 Acknowledgments 42 References 42 COMMERCIAL COOLING OF FRUITS AND VEGETABLES Part I— THE COMMODITY Causes of postharvest deterioration Fruits and vegetables are living organisms which undergo all physiological and pathological proc- esses associated with life. To sustain essential chemical and physiological activities, they draw energy from the food reserves stored within them prior to harvest. They are in a continual state of deterioration from harvest on, and suc- cessful marketing depends upon reducing the rate of deterioration by slowing the processes which cause damage. Deterioration of fresh produce (the commod- ity) results from many things, including physio- logical breakdown, physical injury to the tissue, moisture loss, or invasion by microorganisms. Decay-producing fungi which attack fresh fruits and vegetables are also living organisms, and may be major contributors to deterioration and loss. Some decay organisms can directly pene- trate healthy tissue; others enter only after prod- uce has been weakened or injured. Each fruit and vegetable is, therefore, a complex living system of tissue and microorganisms, and satis- factory produce management requires protection of the produce while discouraging growth and spread of microorganisms. All of the above factors can be interrelated, and all are influenced by temperature. Thus, understanding the relationship of cooling to causes and effects of deterioration is essential if produce is to be properly protected. Like all other living organisms, fresh fruits and vegetables respire through a complex series of chemical reactions. Simply stated, the starches and sugars stored in their tissues are converted to carbon dioxide and water. The process (fig. 1) utilizes energy from the stored food reserves, and oxygen from the surrounding air. If oxygen absorption is blocked, normal respiration cannot proceed and fermentation will quickly destroy the produce. Heat, released as a result of respiratory proc- esses, represents a portion of the energy orig- inally stored in the plant during photosynthesis. The amount of heat produced varies widely with the commodity, and even among varieties (fig. 2). Fruits noted for their long storage life, such as certain types of grapes and apples, typically have low respiratory rates and release relatively little heat. Some vegetables, such as broccoli and asparagus, have high respiratory rates and re- lease large amounts of heat. Highly perishable fruits, such as strawberries, release intermediate amounts of heat. Effects of temperature on produce The life processes of a perishable commodity are regulated by the catalytic action of large protein molecules called enzymes. Enzyme activity is temperature sensitive and increases two to four times for each 18°F temperature rise. Tempera- ture is thus the greatest determinant of fresh produce deterioration rate and, consequently, of potential market life (fig. 3). Produce temperatures are critical from the instant of harvest when postharvest deteriora- STORED FOOD (STARCHES AND SUGARS) CARBON DIOXIDE WATER HEAT NORMAL FLAVORS AND TEXTURE FERMENTATION CARBON DIOXIDE ALCOHOL OFF FLAVORS TISSUE COLLAPSE HEAT EVOLUTION 1000 BTU/T0N/DAY BRUSSELS SPROUT 50 59 TEMP-°F Fig. 1. Simplified schematic diagram of fresh fruit and vegetable respiration showing the role of oxygen in avoiding fermentation and rapid destruction of produce. Energy which is released by the stored food fuels these processes. Fig 2. Examples of heat evolution representing low, intermediate and high respiration rates. Levels of heat evolved from respiration may vary from these values as a result of differences in variety, maturity, time after harvest, and other factors. PER CENT MARKETABLE FRUIT lOOr 50 60 TEMP- °F Fig. 3. General effect of temperature on the market life of fresh fruits and vegetables. Some commodities (for example, those showing chilling sensitivity) exhibit special characteristics which may cause important variations from this curve. tion begins. With highly perishable commodi- ties, a few hours' delay before starting cooling can cause damage which cannot be overcome by subsequent good handling practices. Cooling of strawberries, for example, should commence within 2 hours of harvest. The longer the delay beyond this time, the greater the deterioration of the fruit (fig. 4). Temperatures favorable for maximum growth and development of rot organisms often coincide with field temperatures encountered during fruit and vegetable harvest. Without rapid handling and cooling, therefore, rot organisms may quickly consume the produce. The inhibiting effect of low temperatures on these organisms varies with the organism involved. Certain rot organisms will continue to grow, although slowly, below 32 °F while others become com- pletely inactive at somewhat higher tempera- tures. One major cause of loss in many fresh fruits, the Rhizopus rot organism, will not grow at temperatures below 40°F. Thus, the presence of Rhizopus rot indicates a history of tempera- tures above 40°F. Ethylene physiology of fruits and vegetables Ethylene gas (C2H4) is produced by most if not all plant materials, and may have important beneficial or detrimental effects on fruits and vegetables during postharvest handling. The compound induces ripening in fruits and senes- cence (loss of green color, shedding of leaves, etc.) in other plant tissues. For the gas to exert an effect a certain threshold concentration must accumulate in the internal atmosphere of the 'issues, and produce temperature must be above a minimum level. Neither threshold concentra- tion nor minimum temperature requirements for ethylene activity are well defined. The rate of production and action of ethylene is temper- ature dependent, so rapid cooling and good tem- 0 I 2345678 HOURS AT 86 °F Fig. 4. Effect of delays before cooling on the qual- ity of Shasta strawberry during distribution. After initial delay, all fruit was cooled and handled identically. Market value was based on bright firm fruit, free of rot. perature management are desirable to limit ef- fects of the gas on ripening and senescence. For most commodities, the maximum effect occurs when produce temperatures range from 62 to 70°F. Ethylene gas is widely used to initiate commer- cial ripening of bananas and melons, and for the degreening of citrus. In the future it may be adapted for use with other commodities. Wilting and shriveling caused by water loss Wilting and shriveling caused by water loss seri- ously damages the appearance of produce and thus affects consumer appeal. Many fruits and vegetables will appear shriveled or wilted after water loss of only a small percentage of their original weight (nectarines, for example, will show shrivel after 4 to 5 per cent weight loss). Stem browning can occur in grapes and cherries which have had only minor weight loss. Severe desiccation can result in considerable produce loss; wilted leafy produce may require excessive trimming to make it marketable; grapes may shatter loose from clusters if their stems are severely dried; and seriously shriveled fruits must be discarded before sale or consumption. The loss in produce weight as a result of water loss becomes another direct loss in marketing. A 5 per cent weight loss (which is not uncommon) means 1 pound less in each 20-pound package, or 100 pounds less for each ton of produce handled. Water is lost from produce in the form of moisture vapor. Most fruits and vegetables are composed of cells loosely bound together, with considerable intercellular space which is inter- connected and leads to natural openings called lenticles or stomates (fig. 5). Water from the cells vaporizes into the intecellular spaces, and main- tains an essentially saturated atmosphere within STOMATE OR LENTICLE WOUN AR AIR SPACE SATURATED WITH WATER VAPOR Fig. 5. Schematic of common route of water loss in fresh produce. Cuticle is a natural waxy covering found on some fruits and vegetables. the product. Water vapor may then move to the outside atmosphere through the lenticles, through stems or stem scars, through any injured area, or directly through the cuticle (fig. 6). Water vapor will leave produce in direct pro- portion to the difference between its internal concentration and that of the surrounding at- mosphere. Proper relative humidity (RH) is important in preventing water loss from fresh produce, but temperature of the produce and its surrounding atmosphere, as well as air velocity also affect the amount of loss. Water loss from warm produce to warm air is particularly serious under windy conditions or during transport on an open ve- hicle. The effect of air velocity is to sweep re- leased moisture from air surrounding the prod- uce. Air velocity is not a major factor in total water loss during cooling, as it affects the rates of both drying and cooling about equally. How- ever, any air movement that continues after cooling is completed will continue to remove moisture. Air movement can cause serious loss during storage. Unless air is very humid, it is extremely important to limit its movement in the storage area to the least that suffices to carry away heat produced by respiration of produce and heat leaking into the area. Air movement of 12 feet per minute is often sufficient to maintain desired temperature during storage, if the produce has been thoroughly cooled. Water loss at this air velocity is about one-half of that at a velocity of 100 feet per minute (1.1 miles per hour). Figure 7 depicts the influence of management variables on water loss of grapes. Although more severe handling practices (upper line in figure) resulted in greater weight loss, the treatments shown are less extreme than those often en- countered in commercial practice. The following points about RH, air tempera- ture and air velocity should be noted: • Each hour of exposure to warm, dry air re- sults in over twice as much water loss as holding in humidified storage for 1 week. • Use of colder air during cooling reduces water loss only slightly, but leaves produce at a much better temperature for storage or shipment. • Humidifying air in a cooler reduces water loss by 0.2 per cent of produce weight dur- ing cooling (32°F, 95 per cent RH, com- pared with 32°F, 75 per cent RH). Fig. 6. Air injected into neck of the pear causes immediate and simultaneous appearance of air bubbles over surface of the fruit, thus demonstrating the open structure of the tissue. (Photo courtesy E. C. Maxie.) PER CENT WATER LOSS HELD AT 80»F, 20%RH FOR 6 HOURS . COOLED AT 40°F, 75% R STORE 32 °F, 7 2 MPH 5 %RH AIR PRODUCT LOADED AT 40° F, TRANSPORTED AT 40° F DELAY BEFORE COOLING 6 HOUR COOLING 7 DAY STORAGE 7 DAY TRANSIT Fig. 7. Water loss resulting from two different cool- ing and handling regimes for table grapes. Lower line illustrates nearly ideal conditions; upper line illustrates the additive effect of a series of typical commercial practices. • Humidifying air during storage reduces water loss by 0.2 per cent of produce weight in 3 or 4 days. • Reducing air velocity during storage reduces water loss by 0.2 per cent of weight of prod- uce in 3 or 4 days — but this should only be done after the produce has been thoroughly cooled. Loading cold produce into a cooled transport vehicle can further reduce water loss. Many transport vehicles maintain produce tempera- tures at or near 40°F and consequently it is common practice to cool produce to this temper- ature before loading. Loading produce at 32°F with the interior of the transport vehicle at 40°F can reduce moisture loss. If RH in the transport vehicle is 75 per cent, loading produce at 32°F should reduce water loss by approximately one- half. For produce that is not subject to chilling injury, a further reduction in water loss can be achieved by setting the thermostat in the trans- port vehicle as low as possible without danger of freezing. Relationship of temperature to produce injuries Bruises and other wounds can cause serious produce deterioration during marketing and will also usually speed deterioration from other causes. Bruising accelerates respiratory activity (and, often, the ethylene production) of tissue; this causes fasiei heat release and ripening and so shortens potential shell life. Because bruising damages the natural moisture barrier on fruits and vegetables, the rate of water loss is acceler- ated. Bruised areas also serve as entry points for U^ c%**** A**% |t>*^ J 1 0 k^T" »** ! £1**** I (*•*♦ 1 1 < *%* Clk **> * ti*-*%* 1 £)*%«»> t|M** Ott I j[|***# t 1 * H%. |JM*m 1 Fig. 20. Pallet loads of strawberries are placed at the two sides of the wall blower so as to allow a plenum (air-chamber or passageway) for air being drawn through produce to the blower. sons, et. al, 1970, 1972). This difference forces air through the stacks and carries produce heat away primarily by flow around the individual fruits or vegetables in the containers (figs. 18- 22) , rather than by flow around the outside of the containers (as in room cooling) . In forced-air cooling, cooling speed can be regulated by adjusting the volume of air. Rapid cooling can be accomplished with adequate refrigeration and a large volume of air flow per unit of produce. Using a large air volume to speed forced-air cooling reduces area of floor Fig. 21. Forced-air cooling unit in operation with pallet loads of strawberries. The fabric cover in front seals the air-return plenum, and air is thus forced to pass through side slots of containers and around the fruit in order to return to blower. The blowers used in this procedure are controlled by time clocks. ^ 15 Fig. 22. Forced-air cooling in bins prior to packing. The central plenum here is blocked with plywood pan- els which cover the top and back openings of the stack. Air is thus forced to pass through container side vents and around produce to reach exhaust blowers. Blowers should be stopped when cooling is completed. Each station in this installation is capable of cooling 60 tons of produce. space needed by produce being cooled for a given rate of produce handled, but increases the cost of air circulation and may require a larger refrigeration system. Forced-air cooling usually cools in one-fourth to one-tenth the time needed for room cooling, but still takes two to three times longer than hydrocooling or vacuum cooling. Room coolers can sometimes be converted for forced-air cool- ing, but such conversions will usually only speed cooling by a factor of 2 to 3 (compared to the time required with conventional air circula- tion). A forced-air cooler should be designed and operated to substantially reduce or stop air flow through the produce as soon as it is cooled — con- tinued flow may cause serious water loss from produce unless humidity is near the saturation point. For forced-air cooling design details, see page 29. Shelf-type forced-air coolers I 'his special application allows small lots and incomplete pallet loads of produce to be cffi- ciently cooled. Single pallets are placed side by tide against a plenum (air-chamber or passage- way) usually in one row on the floor and in one or two rows above on elevated brackets or shelves (fig. 23) . The interior of the plenum is under suction or, in some coolers, under pressure. Dam- pers at each pallet position are opened by con- tact with the pallets, allowing air to be either drawn inward or forced outward through the stacked containers on the pallet (fig. 24) . Dam- pers can be arranged to open only as high as the stack of containers on the pallet, allowing stacks of various heights to be cooled without adjust- ments or loss of air. The same idea may be used to cool produce in pallet bins. Bins with slatted or screen floors are used. When each bin is placed it opens a damper connecting the plenum with the space beneath the floor of the bin and between its sills. Air from the room is drawn down through the pro- duce in the bin and into the plenum. Produce placed in a shelf-type cooler starts to cool immediately, merely by being put into position. No attendance is needed except to re- move the produce when cooling is completed to avoid excessive desiccation. Every pallet or bin is accessible for removal at any time and may be immediately replaced by another, allowing vir- tually continuous use of all the cooling positions. These advantages must be weighed against the costs of shelves, dampers and floor area in rela- tion to the volume of material being cooled. 16 Fig. 23. Shelf-type of forced-air cooler for cooling strawberries. Floor space utilization, which is not efficient with this system, is increased by use of pallet shelves. Pallet loads of varying heights can be accommodated. Width of container stacks for forced-air cooling In forced-air cooling, downstream produce (produce furthest from start of air flow) is warm- est because it is contacted by air that has been warmed as it passes through the stacks. However, for any given air flow per unit weight of prod- uce, the downstream cooling rate is not signifi- cantly affected by length of the air path through Fig. 24. Damper arrangement for shelf-type forced-air cooler allows each pallet to be cooled independently. Dampers are opened as lever at bottom of each damper panel is depressed when pallet is placed on shelf. 17 the produce. Wider stacks, which simplify han- dling and reduce the floor area occupied by air channels, are therefore possible without delaying the cooling of downstream produce, although static pressure required to move the air increases rapidly as stack-width is increased. Shipping containers are commonly stacked three-wide on pallets and placed so that air flows through the stacks across the pallet width. Ar- rangements that provide longer air paths through the containers save space but also re- quire higher air pressure and result in more uneven cooling. Shorter air paths allow faster cooling and less air pressure but require more floor space (fig. 25). One plant in Arizona stacks grape boxes two-wide on pallets which are then conveyed through an air-cooled tunnel; the air- flow is large enough to cool the fruit from 90 °F to 40°F in 1 hour. In one Mexican plant, tomato boxes are moved by hand trucks and placed in single stacks against air slots in the wall of air- return plenums for forced-air cooling. Stacks 1-foot wide permit flow of a large vol- ume of air for fast cooling with low static pres- sure (fig. 25). However, 60 per cent of the floor area is occupied by air channels, which means that this system is practical only if that much area can be economically given to the channels. Such a short air path has been utilized for cer- tain conveyor-type coolers, coolers using vertical air flow, or coolers where containers are placed against air vents in a wall. Most forced-air coolers utilize stacks 3 or 4 feet wide; reasonably good cooling rates are attained with moderate static pressures, and these widths make economic use of floor space. Floor space may be more efficiently used with stacks 6 feet or more in width, but static pressure require- ments are then usually unreasonably high for efficient cooling of downstream produce. Wide stacks should be used only for some special rea- son, such as package and produce combinations needing only unusually low static pressure in relation to air flow. Exposure of stacked containers for forced-air coolers Containers stacked for forced-air cooling should be arranged so that the tops of the stacks are exposed directly to the source of cooling air. Air that leaks into a stack is then added to the Bow through the downstream boxes. This tends to offset the slower cooling of the downstream boxes that results from their receiving the warmest air. If tops of stacks are exposed directly to the air return, souk ail then le;iks out of the top of the stack, thus decreasing flow through downstream boxes; downstream boxes then receive both the least and the wannest air, and this tends to make cooling very uneven (fig. 20). Vertical air flow through produce in bulk, or in top and bottom vented containers, lias advan Width of stocks I ft Least head on stocks 04 in Floor area occupied by air channels 60% ^ Widthofstacks 3 ft Least head on stacks 90 ir Floor area occupied by air channels 27 % ^ Width of stacks 6 ft Least head on stocks 6 0 ir Floor area occupied by oir channels II % In each of these arrangements, air flow I cf m per lb of produce, average 7/8 cooling of downstream produce 3 hours , static head loss in air channels 20% Fig. 25. Cutaway view of forced-air cooling of stacks showing static pressure (head) requirement and floor area utilization to achieve similar cooling with differ- ent stack widths. tages, though it has not been generally used. Air channels for vertical air flow are above and below the produce and may be of ample size without adding to the floor area. The high cost of grating or of vented floors with adequate air passages beneath them has been an objection. Relations of air flow to cooling rate, static pres- sure, and length of air path are the same for vertical as for horizontal air flow. Containers for forced-air cooling In forced air cooling the packing method and the containers must permit a satisfactory volume of air flow with a reasonable pressure difference across the stack (Mitchell, ei "/., 1971; Wang and 18 ♦ ♦ * -> -* — — — GOOD / "■- 1 1 \ \ -| \ — A ""■"■**» — > > — > — — — -* BAD Fig. 26. In forced-air cooling the top of the stack should be exposed directly to the source of cooling air so that boxes receiving the least air also receive the coldest air. Tiers should be spaced Vi- or %-inch apart to allow cold air that passes between the boxes to mix with the warmer air that passes through them. Dotted line shows position of dampers when not in use for forced-air cooling. Tunpun, 1968). Packs in which spaces between fruits or vegetables are occupied by packing ma- terial (such as paper wraps on pears) are not suitable to forced-air cooling; film bags or liners prevent air from passing through a pack and also are not suitable for forced-air cooling. Fruit packed in plastic trays will cool satisfactorily if the containers are designed to let air pass both over and under each tray. Grapes packed in vented lidded containers cool well by forced-air despite their compact mass. Air passing between unlidded trays of strawberries makes good con- tact with the berries as it eddies over and among them, making them one of the most satisfactory crops for forced-air cooling. Adequately vented containers are essential in this method of cooling. For moderately fast cool- ing of multiple-tier arrangements of corrugated paper containers, containers should have at least 4 per cent of their side areas vented. Cooling with a given air flow is somewhat slower with small vents because more of the air is forced between containers and less flows through them. Small vents are more difficult to line up during stacking, but the chief objection to them is the very high static pressure required to force an air flow adequate for rapid cooling through them. For example, solid-sided wooden grape boxes require about 10 per cent longer to cool with the same air flow than do slatted-side boxes, and the static pressure required for cooling solid-side boxes is about double that needed for slatted-side boxes. Small differences between similar containers sometimes have considerable effect on their cool- ing behavior in forced-air cooling. New con- tainer designs and produce packs must be tested under measured air flow and static pressure con- ditions before any close prediction of their cool- ing times can be made. (For further information on design calculations see section beginning on page 29.) Conduction cooling of produce Water loss from produce during cooling and storage may be prevented by using virtually vapor-tight containers. Although cooling by con- duction of heat through such containers is slow, their cooling rate is satisfactory for slowly-respir- ing produce if a reasonable portion of the con- tainer surface (all sides of the container, for example) is exposed to cold air. Apples and pears packed in perforated-film liners (which allow some gas exchange but little water-vapor loss), and produce packed in nonvented curtain- coated containers, are examples of conduction cooling practices. Grapes can be packed in wooden boxes having moisture-resistant, unvented liners, or in wax- or plastic-coated corrugated containers, with pads inside the containers that slowly release sulfur dioxide. Fruit deterioration is retarded by the released sulfur dioxide, by the naturally slow respiration of grapes, and by the effect of the vapor barrier in the containers in restricting moisture loss from grapes and stems. Because of these packing modifications, cooling time need not be as short as in the usual grape packing methods (Gentry and Nelson, 1964). Humidity in a storage area cooled by dry coils would be quite low if all the produce in it were packed to prevent escape of moisture. If this is done low humidity is not objectionable, and moisture-induced weakening of corrugated con- tainers is slowed. Controlling humidity Humidity in a produce cooler or storage area depends on the balance between losses and gains of water. Moisture in the air is usually not more than may be gained or lost in a few minutes 19 (O'Brien and Gentry, 1967). If gains and losses do not balance, air humidity soon changes until they do. Most water is lost from condensation on cooling surfaces, although absorption of mois- ture by containers, packing materials, walls, floors, and ceilings may sometimes be important. Water may be gained by evaporation from prod- uce, by evaporation from water used to cool air in a packed cooling tower (see page 21), or by fog sprays (see below). Air circulation should be sufficient to limit the temperature rise of humidified air. Relative hu- midity (RH) in cooling or storage is reduced by about 4 per cent for each 1°F rise in air temper- ature. Condensation may sometimes be reduced by use of large, effective cooling surfaces oper- ated at a close-to-air temperature. The figures immediately below give calculated values for water condensation in a cooling plant with a return air temperature of 35°F. WATER CONDENSATION ON COOLING SURFACES RH of return air at 35°F 95 90 80 70 60 Condensation (gallons per hour per ton refrigeration) AT(°F)* 20 0.5 0.4 0.4 0.3 0.2 10 0.5 0.4 0.3 0.1 0 5 0.5 0.4 0 0 0 4 0.4 0.3 0 0 0 3 0.4 0.2 0 0 0 2 0.3 0 0 0 0 1 0 0 0 0 0 * AT here is the temperature difference be- tween the air returning from the cooling room and the cooling surfaces over which that air will pass. In California, cooling surfaces are usually de- signed to operate (when under full load) at about 10°F below air temperature in storages and 2()°F below air temperature in coolers. Rela- tive humidities of about 70 to 80 per cent may be expected under these conditions, and cooling surfaces will have heavy condensations of water evaporating from the produce. Larger cooling surfaces (three to six times larger and more ef- ficient than usual) operated at 3°F below air temperature could reduce evaporation and con- densation by half with 90 per cent RH. Cooling surfaces operating at 1°F below air temperature under lull load, and at 95 per cent RH, would have to be 10 to 20 times as large and effective as those commonly used. I \ually, dry containers and pallets absorb con- siderable moisture from produce after it is cooled to within a degree or two of ;iir temperature; ;u'r humidity may fall to low levels at this time unless watei is added to the system. Humidifying with fog spray Humidity in coolers and storages may be kept al propei levels by using fog sprays to offset con- densation of water on cooling surfaces. The quantity of water to be added, and the resulting condensation on the cooling surfaces, can be estimated from the text table on this page. Add- ing 1 gallon of water per hour per ton of refrig- eration will offset condensation under severe con- ditions, and will allow a margin for absorption of water by dry interior surfaces at the start of an operating season (correspondingly more fre- quent defrosting of cooling surfaces will be re- quired). Fog sprays raise RH and reduce water loss from produce; condensation accompanying their use also increases effectiveness of the cool- ing surfaces, and evaporation of the fog cools the circulating air. Nozzles using compressed air to atomize the water and discharging between 1 and 2 gallons of water per hour, and between 1 and 2 cubic feet of air per minute at air pressures of 40 to 60 pounds per square inch, can be used in most situations. Larger nozzles may be used where there is room for their long spray plumes to dissipate. Fog spray does not settle out of mov- ing air, but it may create a nuisance by collect- ing on surfaces against which it impinges. It is cheaper and less troublesome to change pulleys on an air compressor, so that it maintains satis- factory pressure on the nozzles while operating continuously, than to use a start-stop system with an air tank and a pressure regulator in the line to the nozzles. Only a small air-chamber is required to absorb pulsations in the air supply. A humidistat which automatically starts and stops the water supply is sometimes used, al- though manual regulation of a continuous water supply is often more satisfactory. The humidistat must be very carefully adjusted and maintained to sense humidities above 90 per cent, and it will behave erratically if temperature varies. Also, starting and stopping the full spray vol- ume makes a room alternately too wet and too dry. With manual regulation, a heavy spray may be used when the refrigeration load is heavy, or to dampen the interior surfaces of a cooler or storage area at the start of the season; when the load is light, a fine spray will maintain a steady, high humidity. Minor changes in the character of the spray are commonly needed only twice a day in coolers or active storage areas, and much less frequently in storage areas to which new produce is not being added in great quantity. Air expansion commonly cools nozzles by 2 or 3 degrees and so they need protection against freezing even if the surrounding air is near but not below 32° F. Nozzles and water lines can be protected by wrapping them with electrically- heated tape. One type of pneumatic atomizing nozzle mixes air and water outside of the nozzle (see schematic of external and internal mixing nozzles in figure 27). Air is discharged beside the water nozzle, creating a suction that allows water to be sup- plied at a pressure of 1 or 2 pounds per square 20 inch above or below atmospheric pressure, de- pending on whether a heavy or a light spray is needed. If they receive water from a common supply, nozzles of this type must all be on the same level to avoid significant differences in water pressure. Water-supply lines must be large enough to avoid significant friction, and must be laid out to avoid formation of air pockets (which will not be blown out by low water pres- sure). External mixing nozzles are often ar- ranged to syphon water from small tanks in which the water level is controlled by float valves. This makes no provision for changing the water supply to regulate the weight of the spray, so a better arrangement is to admit water from any convenient supply to the nozzles through a needle valve. The valve may be ad- justed to provide heavy or light spray by regu- lating the flow of water rather than its pressure. External mixing nozzles are best adapted to in- stallations in which the simple control system is an important advantage, and in which the water supply may conveniently be laid out to minimize friction and to avoid the formation of air pockets. I Another type of pneumatic nozzle mixes the air and water inside the nozzle, before they issue from a common orifice (see diagram of internal mixing nozzle, figure 27). Water is supplied at a pressure close to that of the air, rendering -fric- tion in water lines and differences in elevation of t Fig. 27. Preferred fog-spray nozzle arrangements for produce coolers and for storage areas. Positive control of air and water volumes allows stable ad- justments for heavy or light sprays. EXTERNAL MIXING NOZZLES 21 Water pipe laid out to avoid air pockets, which low water pres- sure will not blow out. Hand control of water volume for heavy or light spray. Strainer Needle valve Spray on or off by hand or by humidistat. Shut off valve General water supply Strainer Air chamber Continuously running com- pressor sized for required air volume to nozzles. INTERNAL MIXING NOZZLES Variable speed to supply the volume required for heavy or light spray. Stop or start by humi- distat. Strainer Air chamber I — Water pump Float fed water tank Strainer Air chamber Continuously running com- pressor sized for required air volume to nozzles. nozzles much less important than with external mixing nozzles. Also, any air pockets are blown out of the water lines by the high water pressure, thus allowing formation of air pockets to be dis- regarded in the pipe layout. There are compensating disadvantages to this arrangement, however. Interconnection of air and water supplies in the nozzles can allow con- tamination from the air lines to be blown back into the water lines in case of failure of the water pressure. Public health regulations require fail- safe protection of back-flow from such a system into the general water supply. Moreover, the pressure in many water supply systems may not always be adequate to force water into nozzles against the 50 or 60 psi pressure needed for good atomization. A satisfactory method is to supply water to internal mixing nozzles with a small piston pump drawing water from a tank filled from a general supply by a float valve. A variable speed drive on the pump allows volume to be adjusted for a heavy or light spray. Spray adjust- ments are more positive and stable if water and air are supplied at fixed volumes rather than at fixed pressures, as is sometimes recommended. Internal mixing nozzles are generally most satis- factory in larger installations, where using a pumped water supply is not a serious objection and where it is advantageous to avoid the large, carefully laid out water piping or small tanks at each nozzle which are necessary for external mixing nozzles. A third type of fog nozzle discharges water through a minute orifice at a pressure of 400 to 600 pounds per square inch, thus providing satis- factory atomization without use of air. Elimina- tion of the air system is offset to some extent by the cost of a high-pressure water pump and extra-heavy water pipe and fittings. No adjust- ment of spray volume is possible here, and the tiny nozzles are more subject to stoppage than are larger ones. A spinning disc atomizer is sometimes used in produce coolers and storages. Water is atomized by the velocity with which it is thrown from the edge of a rapidly revolving disc, and a small air blast is provided to discharge the spray. Each unit (capable of humidifying a small room or zone within a larger room) costs several hundred dollars but is complete in itself and needs only inexpensive electric and water connections. This type of atomizer merits consideration if only one or two units are needed. Packed towers for cooling and humidity control Spray-system regulation and maintenance is eliminated if air in a cooler or storage area is cooled by contact with showering cold water in a cooling tower containing a packing material. Air moving upward in the tower can be brought to within a degree or less of the temperature of the water supply, and close to 100 per cent RH. 21 Demist er pads COOLED SPACE Closely spaced filaments or other packing Falling spray forms or melts ice on refrigerated coils Fan Pump Low -level float switch stops refrigeration when available water is frozen on coils. Ice load is controlled by water overflow or manual refill. Freezing inhibitor (if used) controls temperature. Fig. 28. A packed cooling tower in which cold water is sprayed over packing and air counter-flows through tower for cooling. Various types of packing material can be used to produce good contact betyeen air and water in a limited space. In California, an arrange- ment of horizontally-stretched plastic filaments is widely used. Wood-slat or plastic demisters remove water droplets from the air, which en- ters the cooler or storage in ideal condition (fig. 28). Prefabricated towers can be purchased com- plete with fans, packing, and demisters, or they can be constructed from purchased materials and components. Ice accumulators used with packed towers Packed cooling-tower systems can be supplied with cold water from a tank containing mechan- ically refrigerated coils on which ice forms when the refrigeration load is light and melts when the load is heavy. Since the refrigeration system can operate continuously, it needs only a frac- tion ol the capacity that would be needed if it were required to cool an entire day's production during a few hours on a hot day. (Provision is often made for cooling the water with purchased ice in case of failure of the mechanical system.) Water must be circulated in the tank by a pro- peller, or agitated with air, to melt ice evenly. As an alter native arrangement, a refrigerated (oil may he used over which water flows from above, and which accumulates ice. This system Occupies more space than a coil in the: tank but good heat transfer between water and ice is assured without special agitation. A device that senses the thickness of the ice and stops or starts the refrigeration system ac- cordingly is needed. To operate efficiently, freez- ing of ice on the accumulator coils must be stopped while ample open channels for water circulation remain be'tween coils. Air tempera- tures below 32°F may be obtained by adding a freezing inhibitor to the water; the inhibitor must be nontoxic because of possible carryover of droplets to the produce (food and environ- mental protection aspects should always be checked). Resistance of hardware to corrosive effects of the inhibitor should also be consid- ered. The solution must be neutralized if sulfur dioxide reaches it from grape fumigation. Such a solution is calculated to lower the RH of the air by about 1 percentage point for each 2 degrees that temperature is lowered. Using ice bunkers as a source of coolant Initial costs and fixed charges for an ice bunker are usually a small fraction of those for a com- parable mechanical refrigeration system, and the ice bunker is simple to operate and maintain (Guillou, I960). On the other hand, the daily cost of ice is usually several times the cost of power needed to operate a mechanical system, lor a short season the total annual cost for an ice bunker can be less than for a mechanical s\stem. Some- operations nray best be done by combining a smaller mechanical system to oper- 22 ate throughout the season with an ice bunker to carry a short peak load. An ice bunker should cool return air at least half-way to ice temperature. Failure to do this may be due to any of these conditions: • The ice may be fused, channeled or in too- large blocks, or not deep enough; a 5- or 6-foot depth is about minimum for good cooling. • Warm air can leak into the bunker or the ducts; look for leaking ice hatches. • The bunker can be poorly insulated, es- pecially if exposed to the sun; ordinary concrete has little insulating value. • The bunker may be too small; its horizontal area should be about 1 square foot for each 100 cubic feet per minute of circulating air. A room may remain warm in spite of adequate air cooling in the bunker because of: • Inadequate fan capacity for the amount of warm produce being cooled or for size of plant. • Air circulation restricted by high water in the bunker or some other obstruction; pump strainers and pumps should be readily accessible for cleaning and servicing while there is ice in the bunker. • Excessive warm air entering through doors; using an air curtain (consisting of an air flow directed across an open door) allows as much heat leakage as leaving 10 to 25 per cent of the door area open, and may be useless if badly adjusted or if exposed to winds or drafts. Recirculating melt water by showering it over ice reduces channeling and ice-cone formation, raises RH, and greatly improves cooling. How- ever, the additional cost of an adequate shower system partially offsets the low-first-cost advan- tages of an ice bunker. A reasonable operation would provide a shower of 0.25 gallons per min- ute per square foot of bunker — with an air flow in the ice bunker of 100 cfm per square foot of bunker, thus cooling the air through the same temperature range that the water is warmed. Ice refrigeration maintains high RH only when air temperature is close to ice temperature. Ice tends to maintain the dew point of the air near 32°F, resulting in 88 per cent RH if air is 35°F but only 78 per cent RH if air is 38°F. Using salt to lower ice temperature will consid- erably lower the RH. Fog-spray humidification in ice-refrigerated coolers greatly reduces water loss from produce being cooled, thus improving its appearance. Liquid nitrogen and liquid and solid carbon dioxide as coolants Liquid nitrogen, liquid carbon dioxide, and solid carbon dioxide (dry ice) are sometimes used in rail cars and trucks, both as refrigerants and to modify the atmosphere (see page 10 for dis- cussion of this modification). There is a direct relationship between cooling effectiveness of these materials and that of water- ice. Thus the quantity of the materials needed for a given cooling situation can be easily cal- culated. Vaporizing 1 pound of liquid nitrogen or liquid carbon dioxide and warming the vapor to 32°F absorbs as much heat as melting 1.2 pounds of water-ice. Vaporizing 1 pound of solid carbon dioxide and warming the vapor to 32° F absorbs as much heat as melting 1.8 pounds of water ice. These materials usually cost much more than water ice and thus are quite expensive for ordinary refrigeration. However, a spray of any of them quickly cools air in a vehicle to far below freezing, and thus makes them uniquely valuable for cooling interiors of frozen-food trucks after truck doors have been opened. The same materials can also be used to cool produce rapidly. This fast cooling, plus the atmospheric modification provided by nitrogen or carbon dioxide, may justify the higher cost. When cool- ing fresh produce, the cold gases must be diluted with warmer air to raise their temperature to a safe level before they reach the produce and cause freezing. As with any other cooling sys- tem, resulting cold air will cool the produce only if it is effectively circulated through the load. Sufficient circulation is not assured by either the generated volume of gas or natural convection within the vehicle. The cold gas that is intro- duced is dry but will become saturated by only a fraction of the moisture that would be con- densed on the surfaces of heat exchangers sup- plying the same refrigeration by other methods. Thus the use of these materials as coolants will not necessarily aggravate the problem of main- taining a satisfactory RH. Liquid carbon dioxide boils below the freez- ing point of the solid unless under pressure. Consequently, part of the vapor given off by the liquid often solidifies as it expands, so pipes and nozzles through which the vapor flows must be arranged to avoid clogging. Choosing the cooling program The choice of a cooling method involves several factors, including produce perishability, am- bient environment, packing method, speed of marketing, and the effect of other protective ser- vices. Available cooling methods include various forms of air cooling and, for certain commod- ities, hydrocooling, vacuum cooling, package icing, and top icing. The time of cooling may be: immediately after harvest; before, during or after packing; during storage or transit; or even in a split cooling regime (partial cooling before packing and final cooling after packing). Deci- sions must be made as to: whether rapid cooling is necessary, either to protect produce or aid in shipping; what final temperature produce should be cooled; how much temperature varia- tion can be tolerated during holding or trans- port of produce; and what special conditions may be necessary during cooling (as, for exam- 23 pie, fumigation, humidification, or atmosphere modification). Interrelationships between cool- ing, handling of produce, growth and spread of microorganisms and the package, the packing, and the marketing procedures must be consid- ered— each decision may influence or be influ- enced by other distribution practices. Effects of cooling method on produce and its handling. Air-cooling is widely used, and such systems can be relatively trouble-free if properly designed and operated. Rapid or slow air-cool- ing methods are available, and produce can be cooled at any point in the handling program. However, blockage of air circulation by contain- ers and packing materials can be difficult to overcome unless care is taken, and can result in slow cooling and uneven produce temperatures. Fortunately, it is possible to arrange packing methods, package design and packaging mate- rials, and stacking patterns and air-distribution systems to achieve efficient air cooling. Vacuum cooling is rapid but requires a fairly large produce-surface to mass ratio for efficiency, so its use is restricted to a few commodities such as leafy vegetables. Because cooling is by water evaporation this method causes some shriveling and wilting, although this may be partially over- come by thoroughly wetting produce prior to cooling. Evaporation from damaged tissue can quickly desiccate small injuries, thus making them more prominent. Several cooling methods, including hydro- cooling, package icing, and top icing, bring produce into contact with water, and this may be an advantage in eliminating water loss, or even in returning some water lost by wilted produce. Hydrocooling is especially attractive where handling speed is important. However, wetting of some commodities can create prob- lems which overshadow these advantages. Con- tamination of water by microorganisms can be difficult to control; wherever water enters wounds or open cavities it can cause greater losses due to fungal decay, and fungicidal treat- ments may be needed to lessen this problem. With some commodities wetting may result in other types of injury, including surface brown- ing and intensification of russeting. Wet com- modities are also more difficult to sort and grade, and some packaging materials may be damaged from wetting. Hydrocooling is sometimes credited with an- other advantage that may be more potential than real. Continuous-flow cooling can be made an integral part of the packing line — if produce can be moved from the field directly through a hydrocooler and packing line, rchandling costs normally associated with cooling are eliminated. I his saving is achieved only if the harvest and packing operations arc precisely synchronized. As such a situation never lasts for any sustained time, these savings are not achieved for an economically feasible period, Typically, produce flow from the field is in- dependent of the packing operation. Thus some provision must be made for produce accumula- tion between harvest and packing. If, under these conditions, the hydrocooler is an integral part of the packing line then the warm produce must be unloaded and held at ambient tempera- ture while awaiting cooling and packing — and no savings in handling are achieved. If the hydrocooler is separated from the pack- ing line, produce can be cooled as it arrives from the field and held in cold storage for later pack- ing. In such storage, various types and amounts of produce can be accumulated for more orderly operation of the packing facility. If produce is hydrocooled before storage it will have better temperature protection, which helps compen- sate for any extra handling needed. If field deliveries for continuous -flow cooling systems exceed receiving capacity, the produce must either be speeded through the system (re- sulting in incomplete cooling) or must remain warm while awaiting cooling. By contrast, most stationary air-cooling installations have a much greater receiving capacity than continuous-flow systems, so cooling can be started immediately although it will proceed slowly during the time that refrigeration demand exceeds capacity. Determining the speed of cooling produce. Rapid cooling is essential for highly perishable fruits and vegetables, and for less perishable produce when harvest temperatures are ex- tremely high (grapes grown in desert regions, for example). Slower cooling methods usually in- volve simpler facilities (less refrigeration and air flow capacity) and less handling of the produce and thus cooling costs can be less. Rapid cooling enables a shipper to thoroughly cool and load his produce on the day of harvest. During periods of short supply, or periods of declining prices, this enables a shipper to pro- tect the quality and condition of his produce while responding to these market conditions. Although rapid cooling has been said to "shock" a commodity, this idea has never been substantiated in tests. When injury has been found, it was related to something other than speed of cooling — for example, water damage from hydrocooling, or freezing injury from use of too low a temperature. Not all fast cooling methods will cool the produce at the same speed. Vacuum cooling (for leafy produce) and hydrocooling are the fastest methods that can be used. Forced-air cooling can be almost as fast if a large volume of air (several cfm per pound of produce) is used, but somewhat slower cooling (using reduced air -flow rates) may satisfy the needs of the produce and the shipper and be substantially less expensive (fig. 29). When to cool Typically, harvested fruits and vegetables in Cal- ifornia are cooled after packing, but recent handling and packaging changes have created 24 100 8 10 HOURS Fig. 29. Comparison of the speed of cooling peaches by different methods; average fruit-pulp temperatures. (Guillou, 1960.) interest in the possibility of cooling before packing. One reason for this interest is industry concern with excessive produce deterioration prior to packing. Delays between harvesting and packing sometimes exceed a day, and during such delays produce can deteriorate through ripening, moisture loss, and growth and spread of microorganisms. Unless delays can be elimin- ated, cooling before packing may be the only practical method to reduce deterioration. Care- ful management may minimize such delays so that only a fraction of the produce would re- quire cooling prior to packing. Cooling of produce in containers after pack- ing is often difficult because of the insulating effect of wraps, shims, pads, and liners around the produce. Additionally, the problem is some- times aggravated by tightly stacking nonventi- lated containers on pallets, by generally poor air circulation, or by insufficient refrigeration capac- ity. By cooling in relatively-open harvest con- tainers before packing, better contact between produce and cooling air or water is obtained. Cooling before packing involves the total pro- duct, including culls and material diverted to alternate outlets (canning, freezing or drying, for example). Unless cooling of diverted ma- terial helps prevent spoilage or deterioration or facilitates processing, additional cooling cost must be charged to the packed produce. If 20 per cent of the produce is diverted, cooling cost of the packed produce increases by 25 per cent; a 50 per cent diversion would double cooling cost. Cooler capacity must be large enough to accommodate produce that is to be shipped and produce that is diverted. Some rewarming of cooled produce will occur during subsequent packing operations, and this can result in increased operating costs and re- duced protection for produce (fig. 30). For ex- EFFECT OF PRODUCE DIVERSION ON COOLING COST OF PACKED PRODUCE IN PREPACK COOLING Gross produce weight (in pounds) Per cent of needed to provide gross weight 1 ton packed diverted from produce after Cooling cost packing diversion factor* 0 2000 1.00 10 2222 1.11 20 2500 1.25 30 2857 1.43 40 3333 1.67 50 4000 2.00 * To obtain actual cooling cost, multiply this factor by the basic cooling cost per ton of pro- duce. For example, if a cooling operation costs $10.00 per ton and 30 per cent of the produce is diverted to culls or elsewhere, the total cost per packed ton of produce would be SI 0.00 x 1.43 = 514.30. ample, if produce is exposed to air 40°F warmer than it is, and to a 3 mph breeze, it can warm more than 25°F in 30 minutes. If produce tem- perature had originally been reduced by 40°F, this warming would nullify over 60 per cent of the initial cooling. Only careful sorting during picking can con- trol the amount of produce diverted during packing. However, partial cooling of the pro- duce before packing followed by complete cool- ing after packing can substantially slow deterior- ation and reduce costs. With a reduced produce- air temperature difference, produce rewarming will be less. Refrigeration losses of cooled produce can be reduced by enclosing the packing facility to re- duce air movement. Air conditioning of the 25 PER CENT OF COOLING LOST 100 SAMPLE TEMPERATURE °F •*t80 60 90 120 TIME- MINUTES Fig. 30. Effect of delays in packing and air move- ment on rewarming of exposed apricots, nectarines, peaches, pears and plums. (Mitchell, 1969.) packing facility can substantially reduce pro- duce-air temperature differential. As an alterna- tive, dumping and palletizing operations can be done in refrigerated areas to shorten the time that produce is exposed to ambient temperatures. Some new mechanized packing methods greatly speed the packing operation, thus reducing pro- duce exposure time. Cooling after packing will continue to be used because of lower cooling costs that can be achieved, but these lower costs will result in greater economy of operation only if produce quality is adequately protected. Therefore, ship- pers must carefully manage harvest and pack- ing operations to minimize delays before cool- ing. Cooling speed and uniformity in the packed containers can also be increased by using forced- air cooling, by proper venting of containers and use of packing materials that do not restrict air flow through the container, and by proper pal- letizing of containers (see pages 14-19.) Measuring temperatures Just as temperature of coolant air or water can- not be determined by measuring coil temper- ature, so produce temperature cannot be de- termined by measuring temperature of sur- rounding air. Therefore, good temperature management depends upon accurate temper- ature measurements taken in the produce and in the COOling facility. Equipment lor measuring coolant temperature. The choice of heat-measuring instruments will depend on the needs lor versatility, the com- pleteness oi measurements required, and the de- gree of automation desired. Wall-mounted mercury or alcohol-filled glass- stem thermometers provide good temperature measurement at low cost; long-stem types can be read more accurately. Thermometers having remote-sensing devices allow measurement of coolant temperatures in inaccessible locations. The simplest of these cost little more than a good standard glass-stem thermometer. Record- ing thermometers which use bulbs for remote sensing of temperatures up to 200 feet away from the indicator can be obtained with as many as four sensing bulbs for multipoint recording. They are fairly precise, and relatively inexpen- sive (a few hundred dollars, based on 1972 prices). For installations where more sensing positions are required, thermistor probes or thermocouple wires connected to a recording thermograph are used, with the latter being generally more economical and requiring less maintenance. Such a system combined with a 12- to 24-station recording thermograph costs between $1,000 and $2,000 (based on prices in 1972). Equipment for produce temperature measure- ment. Inexpensive pocket thermometers having alcohol- or mercury-filled glass stems are made with pointed tips for easy insertion into produce. A cold fruit or vegetable may absorb sufficient heat from a warm thermometer to significantly change the temperature in the area of the mea- surement; to prevent such errors the thermom- eter's temperature should first be lowered by a preliminary insertion into the fruit or vegetable, and actual temperature measurements should start with subsequent insertions. Bimetallic dial thermometers (units that mea- sure temperature by differential expansion of two different metals) for manual measurement of produce temperatures are easily read under difficult light conditions, but are not necessarily more accurate than glass-stem thermometers. Bi- metallic thermometers with dials large enough for accurate reading generally cost between $10.00 and $15.00 (1972 prices). Thermistor thermometers are easier to use than pocket bulb thermometers. Thermistor probes enclosing a hypodermic needle tip can be purchased for use with small produce or hard- to-reach locations. Because this probe is small, temperature equilibration with the produce is generally not necessary. (A thermistor probe and dial indicator cost between $100 and $200 in 1972). Thermistor probes are easily damaged and must be carefully protected. Any of the multi-station recorders used for coolant temperature measurement may also be used for measuring produce temperatures. With such equipment, probes may be installed to record both coolant and produce temperatures simultaneously. Calibration of temperature-measuring equip- ment. All temperature-measurement equipment should be calibrated at least once a year to assure accuracy. Accuracy can be easily checked by submerging the sensing unit in an ice water 26 bath. The bath should contain a mixture of ice and water, should be well agitated continuously, and should be free of contaminants (distilled ice and water). Any deviation of water temper- ature from 32°F in such a mixture will be due to contamination or incomplete mixing. The sens- ing unit should be held in the water out of contact with ice or container until it equalizes with the water temperature. Many instruments have adjustments to allow for simple correction of calibration — if adjustment is not possible, a label which specifies the temperature correction that must be subsequently applied to all mea- surements should be attached to the instrument. Measuring coolant temperatures. In a room cooler, temperature measurement is needed at several locations to locate fluctuations or varia- tions occurring in the room. Temperature-sens- ing units should not be placed on outside walls because heat from outside will cause erroneous readings. In a forced-air cooler, sensors should be placed to measure incoming air temperature. To evaluate the effect of doors or hallways on the cooler, additional sensors should be placed near them. In a hydrocooler, the temperature of water contacting the produce should be measured. Variations may exist at entry and exit points of the water, so measurements should be made at both. Measuring temperatures in vacuum coolers poses a special problem, and is discussed in the section on vacuum cooling. Measuring produce temperatures. The most im- portant decision to be made in measuring pro- duce temperatures is whether to judge cooling sufficiency by the average temperature of the produce or by the warmest temperature in the batch or mass of produce. If temperature of produce in a batch can be expected to equalize before the warmest pro- duce in that batch deteriorates materially, then the average temperature should be used. Such a condition could be expected to exist when bulk produce is being mixed during packing, or when small containers are to be mixed during sub- sequent loading. If temperature variations in a batch can be expected to continue for a prolonged period, with consequent danger of deterioration of warm produce, then the temperature of the warmest produce should be used. This condi- tion could be expected if produce is cooled and stored in large bulk bins for some time before packing, or if packed containers are stacked on pallets for cooling and subsequent shipment as palletized units. Adequate estimates of the average temper- ature of a batch of produce may be obtained by collecting temperature measurements in a grid pattern within the batch whether it be a large bulk bin, a pallet load of containers, or a larger collection of either. Several measurements should be taken top to bottom, side to side, and end to end within the batch. In determining temperature of the warmest produce in a batch, it is helpful to understand the cooling pattern of the facility. The position of the warmest produce may vary from system to system and can be affected by the type of con- tainer used. Ambient temperatures of the air around a batch of produce will give little indication of the temperature of the produce at its core or pit where cooling is slowest (cooling rate at pro- duce surface may be many times greater than cooling rate at a depth of only \/2 inch into pro- duce tissue). Thus, temperatures should be taken at the core or pit position. In room cooling, temperature measurements should be taken of produce near the center of the container, as that is the slowest cooling posi- tion. Similarly, it is important to select contain- ers from the center of a pallet load of produce. The warmest temperature to be found, then, should be the core temperature of produce in the center of a container located in the center of a pallet load of produce. In forced-air cooling, downstream produce at the closed end of the return channel will cool most slowly if the system is supplied with uni- formly cold air in one direction. Thus, tem- perature measurements at this point are most important in determining proper cooling. Addi- tional measurements at other positions are use- ful in determining the cooling pattern. Produce at the bottom of the shower is slowest to cool in a shower-type hydrocooler, and should be checked to determine completeness of cooling. In submersion-type hydrocoolers, random tem- perature checks of produce should be sufficient. Temperature measurement is time consuming, but it is a valuable management tool for evalua- ting the efficiency of the operation and must be routinely done to assure that good cooling has been accomplished. Measuring temperatures in transit. Typically, temperatures in transit are recorded by a small portable thermograph mounted in the air space above the load (such units are generally rented on a per-trip basis). Although records so ob- tained can be helpful in determining external temperature to which some of the produce has been exposed during transit, they do not indi- cate temperature changes occurring in the load of produce. They can be especially misleading if air circulation through the load is restricted, as for example by a blanket of refrozen ice (under such conditions the air over the load may be quite cold while the mass of produce in the cen- ter of the load is warm). If desired, several ther- mographs can be located in containers occupy- ing different positions in the load. During pro- longed transit these thermographs would pro- vide a close estimate of the temperature of the produce surrounding them. Such records are normally most useful when studies are being 27 conducted on containers, cooling, or loading. These portable thermographs can best be cali- brated in a constant temperature chamber by comparison with an instrument of known ac- curacy. A good temperature-measurement program for fresh produce calls for the following: • Routinely measure and record coolant tem- peratures. • Routinely spot check and record temper- atures of produce as it leaves the cooler, re- gardless of your confidence in the facility. • Check accuracy of temperature-measuring equipment and note corrections, at least once a year — equalize the temperature of the bulb of glass-stem thermometers by in- serting point of thermometer into an indi- vidual piece of produce before starting ac- tual measurements. • Take temperatures from the slowest-cooling positions (produce in the center of contain- ers at the center of pallets in room cooling, or the downstream position in forced-air cooling). • Measure core or pit temperatures of pro- duce. • Remember that air temperatures tell little about inner temperatures of produce. Part III— DESIGN CALCULATIONS Hydrocooler dimensions and water flow The following calculations are useful for pre- liminary design of a hydrocooler or for check- ing the capacity of an existing unit. Substitute the expected temperature of mechanically re- frigerated surfaces, where used, for 32°F in these calculations. Given or assumed: P = maximum weight of produce to be cooled per hour (tons) fx = highest expected initial produce temper- ature (°F) t2 = highest allowable final produce temper- ature (°F) t, = highest expected temperature of water through the load (°F) S - seven-eights cooling time (minutes) from table 3 (page 39) for other tests E - efficiency in use of refrigeration = heat removed from the product heat absorbed by ice or cooling surface use 0.50 for ordinary open installation or 0.75 for enclosed and well insulated Calculate: F = remaining fraction of initial product-to- coolant temperature difference F te-t0= 1-8 P (t1~t2) wE where t0 is the tempera- T- cooling time (minutes) from S, F and figure 35 or 36. C = conveyor capacity (tons) PT C = 00 L, W and I) = length, width and depth of load on conveyor (all in feet) to give capacity C to suit containers, nature of produce and available space for the cooler w = water circulated (gallons per minute — 10 LW to 15 LW, depending on nature of produc e) ture of water leaving the ice or cooling sur- faces (to - 32) = a measure of the effectiveness of con- (t.-t.) tact of the water with the ice of cooling surfaces; compare with figure 31. If t0 - 32) comes out more than 4, the expected (U - 10) cooling could be accomplished even with poor contact of water with ice or cooling surfaces. If good water contact with ice or cooling surfaces can be assured, choose a lower te and repeat the calculations, which will give a shorter cooling time and allow a smaller conveyor capacity. If (to - 32) comes out less than 1, the expected (te - t0) cooling could be accomplished only with exceptionally good water contact with ice or cooling surfaces. To assure attainment of the expected cooling, choose a higher te or a larger w and repeat the calculations. (to - 32) Intermediate values of may be used, (t.-t.) depending on estimated effectiveness of water contact with ice or cooling surfaces V - conveyor speed (feet per minute) L Refrigeration load (Btu/per hr) = 1800P(tl-t2) (tons of refrigeration) Ice melted per hour (tons) P(h~t2) 6.7£ 160E 28 44 42 ^40 LU c 8 c o a .6 >»£ .5 Q. -O $* .4 tf> — O o I .3 .2 5 «•- O o *- c .2 S 5 GO w .15 10 10 .15 .2 .3 .4 .5 .6 .8 1.0 1.5 2 3 4 5 6 8 10 Ratio of supply channel length to return channel length. Both measured in directions of air flow. Fig. 34. Ratio of air-channel widths for least static-pressure loss, in relation to total channel width, for parallel or cross flow in channels between produce stacks cooled by forced-air (not counter flow). Fig. 35. Chart for estimating cooling times and temperatures. Each diagonal line represents seven-eighths cooling in the time at which it crosses the seven-eighths cool line. Example: Produce cools in a test to 0.08 of initial produce-to-coolant temperature difference in 30 hours, seven-eighths cooling time is 25 hours, so would cool to one-fourth of initial temperature difference in 16 hours. z < _i o o £*• i UJ LU 0C a u. 2° 0- uj o « COOLING TIME (MINUTES OR HOURS, USE ANY CONVENIENT SCALE 3 6 9 12 15 I 12 3 4 5 21 24 27 30 33 36 39 42 45 48 51 7 8 9 10 II 12 13 14 15 16 17 54 57 18 19 CHART FOR ESTIMATING COOLING TIMES AND TEMPERATURES 31 FRACTIONS OF INITIAL TEMPERATURE DIFFERENCE LLOO scale .85 .80 .75 .70 .65.60.55.504540.35.30.25.20 .15 .10 .05 .01 I i 1 1 1 1 1 1 1 1 1 1 1 M in M liiiiliiiiliinl I 1 I I 1 I I L 1 1 1 I i i i L B scale | i m I |MM|iiii|mi| i | i j I | I | i | I | i i I I | I I i i | I n 1 1 n I . |ini|iiii| i | i | i | 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 COOLING TIME, MINUTES OR HOURS Fig. 36. To use log-log slide rule for cooling calculations set any cooling time on the B scale beneath the corresponding fraction of initial temperature difference on LLOO scale. All other cooling times and fractions of initial temperature difference will then correspond. Setting shown is for seven-eighths cooling in 3 hours or one-half cooling in 1 hour. <•- I — 75 T3 u_ t_ 0 o uj 60 o or *• z> a; »- o i/2-<-55 tr o UJ i- a. CL S50 1/4—45 .2 i/e — 40 o i/ie-37.5 £ 0 — 35 y — Initial produce temperature - \ 1/2 Cool - ^v y — Average produce temperature ^V^ y^-3/4 Cool I ^^'N^ ! .. ^^N"^^ 7/8 Cool > [ Air temperature I ^^^^ / /- — \ 1 -^^^r^^^ 15/16 Cool 1 -l -i =- i n i i i i i i 3 6 9 12 HOURS OF COOLING Fig. 37. Cooling curve needed to achieve seven-eighths cooling in 9 hours. 15 Fig. 38. Time and temperature relationship in cooling produce. Spread in temperature 2°F Spread in cooling time 2 hrs 6 9 HOURS OF COOLING 32 a coolant temperature is constant (fig. 37). It is a general measure of exposure of produce to coolant, however, and is useful in calculations whether coolant temperature is constant or vary- ing, as is explained below. Calculating the momentary cooling rate. In de- signing cooling systems, it may be necessary to know the rate at which a batch of produce will be cooling at some specific time. The maximum rate when the cooling of a warm batch is started is particularly important, since this determines a peak refrigeration load. Using: S = 7/8 cooling time (hours) (t - to) - product to coolant temperature difference (°F) R = momentary cooling rate (°F/hours) 2.08 = natural logarithm of l/8 2.0S(t-to) R = S This applies whether coolant temperature is con- stant or varying, and may also be used to esti- mate 5 from test measurements of R and (t - t0). Cooling coefficients. Cooling rates are sometimes designated by a cooling coefficient — this is the temperature reduction in degrees per hour di- vided by the average temperature difference be- tween produce and coolant. For example, prod- uce cooled 40° F in 10 hours would have cooled at an average rate of 4°F per hour. If the aver- age temperature difference between produce and coolant over the 10-hour period had been 16°F, the cooling coefficient would have been 4°F per hour = 0.25 °F per hours °F. Neglecting departures from idealized logarithmic cooling, the cooling coefficient is the same for any period of a cooling operation or at any moment. There- fore, using C for cooling coefficient (°F per hour °F): R = C(t-t0) In practice, cooling coefficients may vary consid- erably as cooling progresses, and should be meas- ured over about the same cooling ranges as those to which they are to be applied. Test results with variable coolant tempera- tures may be applied to operations with constant coolant temperatures, or the reverse, by the rela- tion between cooling coefficient and seven- eighths cooling time: C = 2.08 or ,S = 2.08 Part IV— COOLING DATA Cooling and storage requirements for California fruits and vegetables The tables which start on page 35 summarize en- vironmental information for most fresh fruits and vegetables grown in California, but because of climatic, varietal and cultural differences there may be variations from the data. (Where exceptions are known, they are noted in paren- theses on the tables.) Because handling require- ments for these commodities are often complex, the reader is referred to ASHRAE Guide and Data Book (1971) and to Lutz and Hardenburg (1968) and Whiteman (1957) for more complete information; see "References" for literature in- dicated. Ideal holding temperatures. The temperatures shown in table 1 (page 35) are those generally considered as optimum for the commodity. Un- less produce is subject to chilling injury it is recommended that it be held as closely to its freezing point as possible without danger of freezing. Many commodities respond best if held at 32°F or below. Storage near the freezing point is possible only if temperature fluctuations of the cooling system are not great enough to permit freezing. Relative humidity. An RH of 90 to 95 per cent is recommended for most fresh commodities that can be held near 32°F (table 1). However, most commodities can be held at this high RH only if uniformly cooled to near 32°F. At higher pro- duce temperatures, such high RH can speed development of rot organisms. Low temperature and high RH combined can reduce moisture loss enough to eliminate shrivel and drying prob- lems for many commodities. Chilling temperature. Chilling injury can take several forms, including surface pitting, tissue discoloration, increased susceptibility to micro- organisms, altered flavor or texture, and loss of ability to ripen. Table 1 shows temperature at which chilling injury can occur in some com- modities. Often such information may not be precisely known, and thus is in parentheses. For some commodities a wide temperature range has been reported. Chilling injury is often found only for some varieties or for produce grown under certain climatic conditions. Chilling in- jury is a time-temperature function, and thus it may be possible to hold some commodities safely for short periods at temperatures below those found to cause chilling injury. Holding temperature requirements may also vary accord- ing to ripeness of the produce. Highest freezing temperature. The freezing points shown in table 1 are the highest noted for any test of that commodity (Whiteman, 1957), with many varieties having been observed to have lower freezing points. Because of limita- tions of test data readers should be aware that some varieties may have freezing points higher than those shown here for a commodity. The freezing point of a commodity is influenced by 33 the level of soluble solids. Fruits or vegetables having low-soluble solids content may freeze at temperatures usually considered safe for them. Freezing has occurred at temperatures not gen- erally harmful in Bartlett pears containing low- soluble solids, with freezing occurring first among the least ripe pears in the lot. Maximum market life of produce. There is a maximum time that one can expect a prod- uct to remain marketable under good stor- age and distribution conditions, with consider- able variations resulting from varietal, seasonal, or handling differences. To obtain the longest market life, it is essential that the produce be properly handled before as well as during stor- age. Often special treatments noted in the "com- ments" column are essential to maintaining max- imum market life for a particular commodity. Removing heat of respiration All fresh commodities respire, with heat being produced as one of the products of that respira- tion. Many studies have been conducted to de- termine the amount of heat produced by the produce, when held at specific temperatures. In table 2 (page 38) this information has been con- verted to show the refrigeration required to re- move heat of respiration. Produce cooling times by method of cooling The results of many tests of the speed of cooling various commodities are available (tables 3 and 4, pages 39-41). As has been noted in the dis- cussion of seven-eighths cooling on page 30, it is possible to develop constant values for cooling times if the produce and its exposure conditions remain constant. Using this procedure, data from various tests have been compiled and seven- eighths cooling times estimated for a few com- modities, packing methods, containers, stacking patterns and cooling methods. (For other infor- mation on containers see figures 40 and 41, pages 42, 43.) The temperature of an exposed product or container is reduced to one-eighth of its initial elevation above a constant temperature coolant in approximately the times indicated in tables 3 and 4. Small differences in containers, packing or exposure can make considerable differences in cooling rates. These figures are offered only as a guide. Useful constants One ton of refrigeration is the heat absorbed by melting water ice, or given off by freezing water ice, at a rate of 1 ton in 24 hours. This is 288,000 BtU per day, 12,000 BtU per hour, and 200 BtU per minute. Melting 1 pound of water ice absorbs 144 BtU. Vaporizing 1 pound of water absorbs 1,073 Btu at 32°F, 1,035 BtU at I00°F. Vaporizing 1 pound of dry ice and warming the vapor to 32 °F absorbs 264 Btu (equal to melting 1.8 pounds of water ice). Vaporizing 1 pound of liquid nitrogen or 1 pound of liquid carbon dioxide and warming the vapor to 32 °F absorbs 175 Btu (equal to melting 1.2 pounds of water ice). Approximate compressor horsepower per ton of refrigeration: Evaporation Condenser temperature °F temperature °F 95 115 135 Horsepower per ton 30 1.1 1.5 2.0 20 1.3 1.8 2.4 10 1.5 2.1 2.7 For fans working against pressure (not appli- cable to free discharge): / cubic feet \ / static head, \ .. , \per minute/ Vinches of water/ Air horsepower = -^- — - v 6370 air horsepower Water horsepower = Shaft horsepower fan efficiency Fan efficiency is usually from 0.40 to 0.70 for pressure fans. In water pump calculations: (gals per. min.) (head in ft.) 3960 Water horsepower Shaft horsepower = — — Pump efficiency Pump efficiency is usually between 0.50 and 0.70. The shaft horsepower delivered to a fan or pump is converted to heat in the air or water and adds to the refrigeration load at a rate of: 1 horsepower = 2,540 Btu per hour = 0.21 tons of refrigeration If the motor is in the air stream the entire electrical input of about 1 kw per horsepower is converted to heat in the air stream at a rate of: 1 kw = 3,4 1 3 Btu per hour = 0.28 tons of refrigeration This figure applies also to heat from lights or other electrical equipment. Specific heat of fruits 0.85 to 0.95 Btu per and vegetables: pound °F, average 0.90 Btu per pound °F Specific heat of wood and paper: 0.30 Btu per pound °F Specific volume of air 12.50 cubic feet per at 35°F: pound Density of water at 62.40 pounds per cubic 35°F: foot, 8.3 lb/gallon Density of ice in 56 pounds per cubic blocks: foot Volume of ice in bunk 50 to 60 cubic feet per ers: ton If either the velocity or the velocity pressure of air is known, the other can be determined by use of the graph shown in figure 39 (page 41). 34 a .a o c 13 ° * S S o b 5 01 X> 03 to O J3 O a $ g to T3 M C O . o IS 5 a .2 PP S -8 tf I 8,1 § I Is So-* £ a ^ >" 2 » m S 2 2 S ft . ■9 g §fe 8 "S | {2 '"§ * £ "» .S - "d to fflhOrt O a> .~ 3 3 a 0 O 0 o 0 J jz crt T3 tj a a M Oj c3 >> >> (-C -^ a ft £ a o O a ft ft a a 3 DO o o O o o O > T3 ^3 m S * 0Q H "S •2 9 •3 S oj 3 3 03 O §.fe I .2 cm ~^ es 03 T3 s 156 2 ■ :B S 2 I a g co ^ > a<£ a I oo o3 £ -E to a, ^ o « ' b CM o c!, S 5 £ - ° s) v « t t I £ »/5 ■**> »/S * M" "* CO ;S CM CO CM CM N IN H H w^a r-H OS 05 lO O t- co OT O 03 O CO CM CO CO ^ O O TO CO CO CO Q« o CO CM CO o o o OS -E ft OS JS"3 m is io 2c xi « CO 13 < O r- g ft € TO > b iE u <-> C E 03 0) E M 35 H PQPQOU OOOO 35 '^3 CT fe c c ££ h 3 o o Q S ■a .9 .a -o a> o 3 ft* o US 03 *-»:s 0) 2 * O > o 'St Hi .9 >> M XI S3 ft O O o P, &J= -S J 3 M a ft +2 C e de pers die j>> >> .3 4) .§ § § 99 ft X ■+* -4* >. 'Is bo a -£ 13 1 life 53 9 o3 » w OOfflE? 03 M 03 O s ££ ^i M M G C O O ££•2" * ss £ ^ «o CM .. .. CO J< ~fe £ 13 > o o SCU 3 CD 0) O a) 2 £ 6 £ «9 * * ? N N W r-t tfl >, S3 c o >> >) * 2 o a C £ a O £ co CO CM to -r « 1-1 fe il ^r rH CM c-i w-a OS ^H CO CO id t- CM t~- t~» CM CO CO CO ---I o oo os © o o CO CM CM CO CO CO JS ft •^T3 OS OS o © m "O n lO »0 ,-H "? "? CM CM CM J, J, co co co I T3 OWhOO a> ;; jQ ?? ffi W t-1 h") O O O Pm Ph Ph 36 fi tf a £ §1 03 +3 s ° .2 V £fa oJ o > S fa >> U g T3 0)0) 03 J3 S3 * 9m 9 3 03 « O £ fl -" ft o ° - 03 .£ © ^^ I ■ 8 g o 03 O T3 of 3 ■"ES l?fa d> CO o ft +3 W 73 d art i5 3 S £ 6 ^ 03 n -Q 0) fa fl T3 3 o fa 09 SI 03 O) I o I- m fa 03 ,4 1-. o3 m g o i-s 03 03 u 3 O d 03 O ft -s $ ** o V ft o *3 03 .2 3 a .& o CO 3 CO T3 G o u d o 01 o CO a Ph U o 8 o 03 "ft ft 03 CO fa -fl 03 s d o T3 O ft CV 3 ft TJ fa -C -i 03 0 02 o -C o CD a o 03 rS fa >0 0) > ^ o .s CV T3 03 e £U „ 2 boo OfQO d A i ^3 o-o "8 -? Z 7 03 & oJ ^ 1 3|,g x a ^ >> oJ ri o c3 £ w o s R; d 03 * "8 1 H 0Q d o an o o s g CO i-H * is T S si d J Co fl .S 03 03 M M 03 03 50 g ^ ^ MNcMNINH^'HIN O CO o CO c» CO O lO "5 H O rH i-J rH -H O i i i i iO ifl "fl m 03 C7i 03 Is" I I I I o o o o oi a a m O "5 CO C3 Jj io t^ »o l« -y the product (as Btu per hour), multiply value shown by 12,000. 38 Table 3 ROOM COOLING AND HYDROCOOLING TIMES, USING DIFFERENT CONTAINERS AND DIFFERENT PACKING METHODS AND STACKING PATTERNS* Room cooling Produce Container, packing method, and stacking pattern Seven-eights cooling times*** Average Slowest Unpacked, wood boxes in pallet loadst Packed, wood boxes in pallet loadst In corrugated containers^ In wood lugs, stacked solid on pallets§ Packed in plastic tray pack containers stacked on pallets. Air velocity 400 fpm|| Wood container 20.5 per cent side vent area Corrugated containers 10-13 per cent side vent area Corrugated container 6 per cent side vent area Corrugated container 1.9-3.4 per cent side vent area Forty-eight pounds in wood box, both wrapped and unwrapped, on palletll** Forty-eight pounds in non-vented, partial telescope corrugated container, on pallet Tight-fill in 12" x 18" x 10" high telescope container, 36 lb. two 1J* holes in each side, cross-stacked §t1 54 containers per pallet } ' space between sides 48 containers per pallet 3f * space between sides Tight-fill, 36 lb, cross-stacked 3? per cent side vent, 1" spacing between containers^ Tight-fill, 36 lb, cross-stacked 5 per cent side vent, no spacel LA lug, 24 1M Wrapped in LA lug size corrugated container, 24 lb.l Tight-fill in 11" x 17?" x 8* high corrugated containers, 28 lb. cross- stacked on pallet No vent and no spacing between containers, cross-stacked^ Four per cent vent area, 1" spacing between containers, cross-stacked Four per cent vent area, no spacing, register stacked Single 4-basket crate, 2 sides exposed§ 24 hours 2.5 days 2.5 days 2 days 16 hours 24 hours 16 hours 30 hours 2-3.3 days Artichokes Grapes Nectarines 6-8 days 30 hours 12 hours 12 hours 12 hours 27 hours Plums 3-4 days 4 days 4-6 days 3.6 days 23 hours 40 hours 20 hours 39 hours 84 hours 22 hours 18 hours 27 hours Oranges Fruit in 47* square bins Twenty-four inch deep bin, no side vent** Thirty inch deep bin, no side vent No side vents 24* deeptt Six |" x 16* slots on each side Perforated metal bin i" holes on 1" centers Six f * x 16" slots on each side 33 hours 45 hours Plums 24-46 hours 23-40 hours 18-33 hours 20-32 hours Asparagus Cantaloupes Peaches Hydrocooling In bunches immersed in flowing watertt In bulk in commercial hydrocooler§§ In 47"-square bins, 2' deep. Water showered in 150 gallons per minute per bin** Free drainage from slots in bottom Bin bottom with 10 1* holes allowed bin to fill In commercial hydrocoolers|[| 150-size Anjou in open lug, water spray 4 gallons per minute per foot§ In wirebound crates, immersionll No agitation Agitation In wirebound crates, water shower 5 gallons per minute per square foot, free drainage 6 minutes 45-60 minutes 30 minutes 24 minutes 33 minutes 42 minutes 46-84 minutes 28 minutes 45 minutes Pears Sweet corn 42 minutes 51 minutes * Temperatures of exposed produce or containers were reduced to £ of their original elevation above a constant temperature coolant in approximate times shown. t Reference: Sainsbury, 1961. ft Reference: Mitchell and Mayer, 1971. X Reference: Guillou, 1962-68. it Reference: Pentzer et al., 1936. § Reference: Guillou, 1960. §§ Reference: Lipton and Stewart, 1959. || Reference: Mitchell et al., 1971. |||| Reference: Toussaint, et al., 1955. 1 Reference: Mitchell and Parsons, 1970. 1"| Reference: Perry and Perkins, 1968. ** Reference: O'Brien and Gentry, 1967. *** Times are approximate and are to be used only as guides. Small differences! n containers, packing procedures or exposure to the coolant can affect cooling rates considerably. A dash ( — ) in either column indicates that information is not available. 39 cn ^h \n o o o o o O «5 l« 03 OO CN CN .-h O ^h >0 CO cn .-h U5 N 1ft N lO M — 1 © •-! O -h o O O O O O o o o o o o o o t^ © 50 ^h CO > CM — 1 a o . "a 2* cJ ?5 .2 3 o O © © o o s 03 -^ o ^ e"a o CM o o © CD d © © o © -* d .- 60 CO % f"3 -* CM o oo co ""! r^ o o o s •- 60 O g 60 O 43 o > .2 << I 1 2 S 2 -J3 rf IS O "^ S3 .-; — q> • • go a; a*" d cp (U & a$ o> w a « |g ■a "Egg T3 ■ rt ft sa *d w on ^ 83 d S m E? 35 8 o oflft "c3 *° u 2 2 O _l UJ .010 .008 .006 .005 .004 .003 .002 ---^ -.-.-/ 200 400 600 800 1000 1500 2000 VELOCITY (FEET PER MINUTE) 4000 Fig. 39. Relationship between velocity pressure and air velocity at 35°F. 41 Fig. 40. A. Four-basket crate; partially unpacked crate of plums showing com- ponents. B. Corrugated telescope container used for many commodities. C. Wire- bound crate used for corn, celery and other commodities. D. Strawberry crate containing 12 baskets; normally lidded only on top crate: E. Cantaloupe crate. Fruit hand placed without other packing material. F. Pear box for place-packed wrapped pears; top lid budge is normal. ACKNOWLEDGMENTS The authors greatly appreciate the time and thought given by others to reviewing this material, and the many valuable suggestions that have been contributed. Particular thanks are due to Robert Her- rick, Robert Kasmire, Ed Maxie, Diven Meredith, Stuart Smith, and Noel Sommer. REFERENCES American Society of Heating, Refrigeration, and Air-conditioning Engineers. 1971. ASHRAE Guide and Data Book, Applications Volume. Amer. Society of Heating, Refriger- ation and Air-conditioning Engineers. New York. Bennett, A. H. 1963. Thermal characteristics of peaches as related to hydrocooling. U. S. Dept. of Agric. in co- operation with Univ. of Georgia, Technical Bui. No. 1292. Gentry, J. P. and K. E. Nelson. 1964. Conduction cooling of table grapes. Amer. Journal of Enology, 15(1):41— 46. Guillou, R. I960. Coolers for fruits and vegetables. University of California Agr. Exp. Sta. Bui. No. 773. 42 Fig. 41. G. Standard wood lug used for many commodities including peaches, nectarines, tomatoes, and pears; pear lug often unlidded. H. Nectarine plastic- tray pack in corrugated lug — 2 layers. I. Plastic-tray pack components for con- tainer. J. Corrugated lug with inset panels for air circulation when stacked. K. Grape lug; top liner is tucked down sides before lidding. L. Paper-wood lami- nated lug used for grapes and other fruits. 1963. Pressure cooling for fruits and vegetables. ASHRAE Journal, 5(1 1):45— 49. Amer. Society of Heating, Refrigeration and Air-conditioning Engineers. 1962-68. Unpublished tests. Haerter, Alex A. 1963. Flow distribution and pressure change along slotted or branched ducts. ASHRAE Journal 5(l):47-59. Harvey, J. M. 1963. Improved techniques for vacuum cooling vegetables. ASHRAE Journal, 5(1 1):41— 44. Amer. Society of Heating, Refrigeration and Air-conditioning Engineers. Kasmire, R. F. 1971a. Private communication. 19716. Progress in mechanicals — new top-icing method for melons solves problems. Western Grower and Shipper 42(8): 19, 27. 1972. Improved method — windrowed top-icing cools celery faster. Western Grower and Shipper 43(3): 29-30. Kasmire, R. F. and R. A. Parsons. 1971. Precooling cantaloupes, a guide to shippers. Agr. Ext. Serv. University of California, Berkeley. Lipton, W. J. and J. K. Stewart. 1959. Commercial hydrocooling of cantaloupes tested. Western Grower and Shipper 30(6). 43 Lutz, J. M. and R. E. Harjdenburg. 1968. The commercial storage of fruits, vegetables and florist and nursery stocks. U. S. Dept. of Agric. Handbook No. 66. 94 pp. Superintendent of Documents, Washington, D.C. 20402. Mitchell, F. G. 1969. Unpublished tests. Mitchell, F. G. and G. Mayer. 1971. Unpublished tests. Mitchell, F. G., G. Mayer and C. H. Campbell, Jr. 1968. Solid/Spaced — a new carloading method for tight-fill packed fruits. California Agr. 22(12): 2-5. Mitchell, F. G. and R. A. Parsons. 1970. Unpublished tests. Mitchell, F. G., R. A. Parsons and G. Mayer. 1971. Cooling trials with plastic tray pack nectarines in various containers. California Agr. 25(9): 13-15. O'Brien, M. and J. P. Gentry. 1967. Effect of cooling methods on cooling rates and accompanying desiccation of fruits. Transac- tions of the Amer. Society of Agric. Engineers 10(5): 603-06. Parsons, R. A. 1972. Forced-air cooling of table grapes. Unpublished report, 5 pp. Parsons, R. A., F. G. Mitchell and G. Mayer. 1970. Forced-air cooling of palletized fresh fruit. Paper No. 70-875. Amer. Society of Agr. Engi- neers, St. Joseph, Michigan 49085. 1972. Forced-air cooling of fruit in bulk bins. Amer. Society of Agr. Engineers, Special Publica- tion SP-01-72:38-41. Pentzer, W. T., R. L. Perry, G. C. Hanna, J. S. Wiant and C. E. Asbury. 1936. Precooling and shipping California asparagus. University of California Agr. Exp. Sta. Bui. 600. Perry, R. L. and R. M. Perkins. 1968. Hydrocooling sweet corn. Amer. Society of Agric. Engineers Paper No. 68-800. Sainsbury, G. F. 1961. Cooling apples and pears in storage rooms. USDA Marketing Research Report No. 474. Toussaint, W. D., T. T. Hatlow and G. Abshier. 1955. Hydrocooling peaches in the North Carolina sandhills, 1954. A. E. Information Series No. 3920. North Carolina Agr. Exp. Sta. Wang, J. K. and K. Tunpun. 1969. Forced-air precooling of tomatoes in cartons. Transactions of the Amer. Society of Agric. Engineers 12(6): 804-06. Whiteman, T. M. 1957. Freezing points of fruits, vegetables and florist stocks. USDA Marketing Research Report No. 196. (32 pp.) To simplify the information, it is sometimes necessary to use trade names of products or equip- ment. No endorsement of named products is intended nor is criticism implied of similiar products not mentioned. Jwi L2,'72