CALIFORNIA AGRICULTURAL EXPERIMENT STATION BULLETIN 703 JANUARY, 1948 THE COMMERCIAL FREEZING OF FRUIT PRODUCTS M. A. Joslyn and Leonora A. Hohl 500 U.S. 400 a z O 300 a. en Z O 200 ^ WEST 100 /x CALIFORNIA 0 1926- 1931-1936- 1930 1935 1940 1941 1942 1943 1944 1945 1946 THE C O LI EG E O FA GRI C U L T U R E UNIVERSITY OF CALIFORNIA « BERKELEY THIS BULLETIN is intended primarily for the commercial processor of frozen fruits. It is based on observations in the field, on available scientific and technological informa- tion, and on results of investigations carried on by the Division of Food Tech- nology over the past twenty years. Part of the bulletin contains material which has already been presented elsewhere. This has been reviewed and brought up to date in the light of more recent knowledge. Much of the material is new, presented here for the first time. GROWERS may also profit from this publication, which should be a guide in cultural and harvesting practices, and in selection of varieties suitable for freezing processes. ^ -o- -o- With processor and grower in mind, the bulletin discusses the principles in- volved in preservation freezing with considerable technical and scientific detail. A thorough understanding of these principles is important to the processor who wishes to improve present methods of freezing standard items or to develop specialty items. The processor must also know the nature of the physical, chemical, enzymatic, and bacteriological changes which occur during freezing and thawing of fruits. He must know this in order to determine what fruits and fruit products can be successfully preserved by freezing, and how those fruits should be prepared, packed, frozen, and stored. Such changes also determine how the products may best serve the needs of the manufacturer of preserves, the baker, the ice cream maker, or the housewife. Continued improvements in the freezing process have made it possible to re- tain, to a fairly high degree, the original appearance, flavor, and nutritive values of various products, such as fresh fruit, vegetables, meat, and fish. As a result, commercial freezing has been profitable, through a variety of outlets, to the grower, the manufacturer, and the consumer. Future possibilities for this method of food preservation are great, as evidenced by the continued growth of the industry. In fact, its potentialities are still largely untapped. But if the industry is to continue to show a profit, and to realize its fullest development, it must improve its methods still further, and correct past mistakes. Many of those mistakes were the result of increased war- time production, when quality was sometimes sacrificed to quantity. Now that production is returning to a more normal basis, this bulletin suggests ways in which the industry may learn from past experience, raise its standards, and in general give the manufacturer and consumer a better grade of frozen food product. Those who are interested only in practical directions for freezing may turn to the section beginning on page 55. For those seeking more detailed information on principles and practices, a list of references is included at the end of the bulletin. The bulletin is not intended as a guide to users of frozen-food lockers or home freezers. CONTENTS Page DEFINITIONS AND REGULATIONS 5 Regulations for frozen-pack fruits 6 Food and drug standards 6 Grades 6 DEVELOPMENT AND EXTENT OF THE INDUSTRY 7 Early history 7 Development 8 California production history 10 Container trends 12 PRINCIPLES OF PRESERVATION FREEZING 15 Microbial spoilage and its control 16 Enzyme activity and its control 17 Selection of varieties 19 Maturity 20 Exclusion of oxygen 20 Addition of sugar or sirup 21 Addition of acids 24 Antioxidants or reducing substances 25 Heat inactivation 28 Nonenzymatic chemical changes 30 Crystallization of sugar 31 Physical changes during freezing and thawing 31 Texture . 31 Volume 34 Weight 35 Relations of loss in weight to texture 37 HEAT-TRANSFER DETERMINANTS 37 Stages of freezing 38 Temperature changes in various products 41 Effect of type, size, and shape of container 42 Effect of initial temperature 43 Effect of freezing medium 45 FREEZING STORAGE CONDITIONS 47 CONTAINERS AND PACKAGING MATERIALS AND PRACTICES 50 THE FREEZING OF FRUITS 55 Apples 55 Varieties 55 Apples for bakers 59 Applesauce 60 Baked apples 62 Apricots 62 Varieties 62 Apricots for bakers' or processors' use 62 Apricots for dessert use 63 Berries 64 Varieties 64 Barreling of strawberries 65 Sliced strawberries 68 Other berries 68 [3] 4 CONTENTS THE FREEZING OF FRUITS (Continued) **& Cherries ^9 General discussion and varieties €9 Cherries for bakers' or processors' use 70 Cherries for dessert use 70 Citrus fruits 70 Varietal adaptability 70 Maturity, harvesting, and storage 71 Grapefruit hearts 71 Figs 72 Varieties and maturity 72 Preparation for freezing 72 Grapes 73 Melons 74 Nectarines 74 Varietal adaptability 75 Harvesting and handling 75 Packing procedures 76 Olives 76 Peaches 76 Varietal adaptability 76 Maturity, harvesting, and storage 78 Preparation for freezing 79 Special products and treatments 81 Pears 82 Persimmons 82 Plums and prunes 82 Rhubarb 83 TROPICAL AND SUBTROPICAL FRUITS 83 Avocados 83 Coconuts 84 Dates 84 Guavas 84 Mango, papaya, and passion fruit 85 Pineapple 85 CRUSHED, SIEVED, OR PUREED FRUITS 86 Velva fruit 87 JELLY AND JAM BASES 88 FROZEN FRESH FRUIT SPREADS 90 FREEZING FRUIT JUICES 91 Orange juice 92 Lemon juice 94 Grapefruit juice 94 Other fruit juices 94 FREEZING SWEETENED FRUIT JUICES, SIRUPS, NECTARS, PUNCHES, AND CONCENTRATES .96 FROZEN CITRUS JUICE CONCENTRATES 98 LITERATURE CITED 100 GENERAL REFERENCE SOURCES 106 Bibliographies jq^ Preservation freezing iQg Refrigeration i^g Fruit production 107 California Experiment Station publications on fruit production 107 Journals j07 COMMERCIAL FREEZING OF FRUIT PRODUCTS23 M. A. JOSLYN4 AND LEONORA A. HOHL5 DEFINITIONS AND REGULATIONS The frozen-fruit industry has undergone various changes during its expansion, both in the terminology employed by the industry and in the regulations governing it. This section defines some of the terms used and indicates the extent of the regulations. Originally, the process of preserving fruit by storage in bulk containers, at freezing temperatures, was termed the "cold pack process" and the product, "cold packed fruit." Diehl et al. (1930)8 have suggested "frozen pack" as a more acceptable term, and this is now widely used for fruit and fruit products pre- served by freezing in bulk containers, for use in bakery products, frozen dairy products, preserves and jams, and similar foods. "Sharp-freezing" is still used industrially for products frozen at low tempera- tures (+50 F to -200 F) in a room where there is no provision for forced air, or where only a minimum of air circulation is provided by portable fans. Since the relatively still air is a poor heat-transfer medium, fruits placed in sharp- freezer rooms freeze so slowly that many hours or sometimes days are required for complete freezing. In that time, the product may spoil by fermentation or excessive oxidation. Fruits and fruit products rapidly frozen in blast, immersion, or contact freezers are referred to as "quick-frozen" fruits. There is no entirely satisfactory definition of "quick-freezing." The one most commonly used is: Quick-freezing is the process in which, by rapid heat transfer, the temperature of the product is lowered from that at which initial ice formation occurs (usually 28 ° F) to that at which most ice formation is complete (about 150 F) within thirty min- 1 Received for publication May 16, 1947. 2 This bulletin supersedes Circular 320, Preservation of Fruits and Vegetables by Freezing Storage, by M. A. Joslyn, published in 1930, and Bulletin 551, Changes Occurring during Freezing Storage and Thawing of Fruits and Vegetables, by M. A. Joslyn and G. L. Marsh, published in 1933. 3 The preparation of fruits and vegetables for freezing in locker plants is described in Freez- ing Storage. Preparation, Freezing and Storage of Fresh Food for Home Use, by Vera Greaves Mrak. Cal. Agr. Exp. Sta. Ext. Leaflet H.D. 473: 1-6. Revised, 1947. 4 Associate Professor of Food Technology and Associate Biochemist in the Experiment Station. 5 Instructor in Food Technology and Assistant Mycologist in the Experiment Station. 6 See "Literature Cited" for complete data on citations, referred to in the text by author and date of publication. [5] 6 CALIFORNIA EXPERIMENT STATION BULLETIN 703 utes. (The desirability of rapid freezing in the zone of maximum ice formation will be discussed elsewhere.) Dry sugar is used as a color and flavor preservative in the freezing of fruit in 30-pound, or larger, containers for use in bakery and dairy products, and in preserves. The ratio of fruit to sugar is commonly expressed as weight in pounds of fruit per pound of sugar. Thus, 4 + 1 or 4 : 1 pack contains four pounds of fruit to one of sugar. Sirup is now used in place of dry sugar for consumer packages or for fruit packed in institution-sized units for dessert uses, with the exception of sliced strawberries. But the use of sirup has in- troduced problems of definition. The Food and Drug Administration has accepted the dictionary definition of sirup as "any concentrated aqueous solu- tion of sugar." It requires any sucrose solution to have a concentration of at least 65 ° Balling or Brix (per cent sugar by weight in water solution) to be termed "sirup" when applied to frozen fruit and fruit products. Exception was made for canned fruits which may be marketed as packed in light, medium, or heavy sirup. Since the use of sufficient 65 ° Bal. sirup to completely cover the fruit in small containers would result in an excessively sweet pack, and since frozen fruit packers rarely use sirup of over 500 Bal., this ruling works a hard- ship upon them. At present, if the ruling were enforced, packers would have to declare that fruit frozen with a light sirup is "frozen with sugar and water." Regulations for Frozen-Pack Fruits.— Since 1928, frozen-pack fruits have been included within the scope of the U. S. Warehouse Act. Every packer should be familiar with the regulations, which may be found under U. S. Department of Agriculture, Agr. Marketing Service, Service and Regulatory Announce- ments, No. 159, October, 1940. Food and Drug Standards.— The U. S. Food and Drug Administration has not yet promulgated standards of identity, quality, and fill of container for frozen food products, under the Food, Drug, and Cosmetic Act of June 25, 1938. Basically, however, any frozen-pack food should be "the clean, sound product obtained by packing in a suitable container, properly prepared fresh fruit, vegetable, meat, or other food, with or without the addition of sugar or salt, and maintaining it at a temperature sufficiently low to insure its preservation." Grades.— Standards of fill, identity, and quality for the various fruit products now frozen commercially are being developed by the industry. At present, the only standards of quality available are the "Tentative U. S. Standards for Grades of Frozen Fruits" proposed by the Processed Products Section, Fruits and Vegetable Branch, Production and Marketing Administration of the U. S. Department of Agriculture. These standards may be obtained through the Washington Office or from the San Francisco Office, 821 Market Street. Tenta- tive standards have been formulated for apples, apricots, berries, cherries— soin pitted and sweet— peaches, raspberries, rhubarb, and strawberries. COMMERCIAL FREEZING OF FRUIT PRODUCTS 7 These tentative grades are based on the following factors: appearance; tex- ture; flavor; odor; color; absence of defects; development; and degree of disin- tegration. Critical evaluation of grade by these standards requires considerable practice and familiarity with the product, and the tolerances allowed are based largely on subjective methods. Evers (1947) has suggested quality control pro- cedures for frozen peaches based upon these grades. There is need for develop- ment of more satisfactory grades, based on critical evaluation of consumer acceptance of the product. More exact methods for such evaluation have been developed by Dove (1946) and by the Committee on Food Research (1946). There is also need for the development of objective standards of quality. DEVELOPMENT AND EXTENT OF THE INDUSTRY Although experiments with commercial freezing of fruits were made as early as 1908, the years 7925-7936 marked the main period of experimentation. After 7936, the industry underwent a period of rapid growth, and reached a high level of development in 7947. Production and sales rose sharply during World War II, but in some instances quality was lowered. If wartime mistakes are rectified, how- ever, continued high production may be expected. If packers are to profit from the mistakes of the past, they should be familiar with some of the history of the frozen fruit industry. Its development has been rapid, especially during the war years, and because of this quick expansion, quality was sometimes sacrificed to quantity. Proper plant supervision was lacking, as was good market exploitation. As a result, some packers were left with large amounts of surplus products as soon as food supplies for civilians became more plentiful and military demands dropped. Early History.— The historical record of the frozen fruit industry has been summarized in a survey of food processing in the West (Anon., 1934). The first experimental packs were made in Denver in 1908, and the first commercial output was in Salem, Oregon (1909) and Puyallup, Washington (1911). Early development of the frozen fruit industry was most rapid in the Pacfic North- west, where chiefly strawberries, raspberries, and smaller quantities of other berries were frozen. Conditions favorable to berry production and location far from the main consuming center (eastern markets) forced the growers in that area to utilize all possible outlets. Second in production to frozen berries were red sour cherries, which in the early 1930^ were packed chiefly in the tri- state district of Delaware, Maryland, and Virginia. As shown in table 1, frozen berries predominated until the war years of 1941 to 1945. During these years, the production of strawberries and cherries decreased, and the production of frozen apples, apricots, peaches, prunes, and rhubarb sharply increased as did also that of figs, nectarines, and pears. During 1941-1945, berry production was only 34 per cent of the entire fruit pack. 8 CALIFORNIA EXPERIMENT STATION BULLETIN 703 Development.-Diehl and Havighorst (1945)* in their recent survey of the progress and prospects of the frozen food industry, have divided its history into two periods: the first, of experimentation, 1925-1936 inclusive, and the second, of rapid growth since 1936. In the first phase, preservation was limited largely to fruit intended for subsequent industrial use. This included fruits to be used in frozen dairy products, pies and other bakery goods, preserves, jams, and, to a limited extent, for fruits to be canned for salad, and as juice. During these years, the early investigators laid the foundation for future expansion of the industry by testing the behavior of different kinds of fruits to freezing and to freezing storage and subsequent thawing. They selected the more suitable Table 1 UNITED STATES FROZEN-FRUIT PACKS, FIVE-YEAR AVERAGES 1908 TO 1945* (Weights given include sugar or sirup when used) Years averaged Berries Other fruits Total 1908-1910f thousands of pounds 10 500 1,700 6,664 43,774 45,046 78,889 100,924 thousands of pounds 200 5,000 18,717 14,710 59,509 194,490 thousands of pounds 1911-1915 500 1916-1920 1,900 1921-1925 11,664 1926-1930 62,491 1931-1935 59,756 1936-1940 138,398 1941-1945 295,414 * Source of data : Statistical Review and Yearbook Number. Western Canner and Packer 38 (6) : 257-93. April 25, 1946. t Three years only; no pack reported before 1908. varieties, and developed processing methods. Packaging, freezing, storage, dis- tribution, and marketing problems were solved by the pioneer frozen food packers. Mechanization of processing, and handling methods were developed by the industry and by machinery manufacturers. Gradually, the institution channels (hotels, restaurants, hospitals, clubs) and, finally, the retail outlets were exploited. Economical methods of refrigerated storage and of distribu- tion to retailers were developed, as were refrigerated cases for distribution by retailers. Both retailers and consumers were instructed in the proper use of the frozen product. Then followed a rapid increase in production and distribu- tion, and a continued improvement in processing and packaging techniques. During the war years, sales of frozen foods increased rapidly because they were not rationed, and canned foods were scarce. The wartime conditions, however, were not without disadvantages, chief among them being the lowering of quality. The pioneers of the industry recognized that the chief justification of freezing as a means of preservation lay in its better retention of color, flavor, and nutritive value. This advantage in COMMERCIAL FREEZING OF FRUIT PRODUCTS 9 many cases amply justified the increased costs of packing, storage, and distri- bution. The provision of adequate low-temperature refrigeration to keep the product frozen at the processing plant, during transit to central warehouses, during transit and distribution to the retailer, and at the retail outlet is chiefly responsible for these increased handling charges. During and immediately following the war, the quality of much of the frozen fruit and vegetable pack, which had reached high levels of develop- ment by 1941, was poor. The factors which contributed most toward this relaxation of quality control from field to plant, within the plant, and from plant to consumer were: Shortages of experienced labor. Lack of machinery for orchard cultivation and for preparation, processing, and packaging. Shortage of packaging materials. Limitations in availability of refrigeration facilities for storage and dis- tribution. A large, unfilled consumer demand. Pressure on every packer to process as much tonnage as could be handled by his plant. The entry of new packers into the field. Not all these were inexperienced. Some were skilled food processors from other fields. But some had little or no knowledge of the fundamentals by which high-quality and low-cost pro- duction of frozen foods could be achieved. Keen competition for the limited quantity of available raw produce further influenced the trend toward lowering of quality (Joslyn, 1946a; Diehl, 1945, 1946; Cruess, 1946). According to a recent analysis (Anon., 1947), two decided changes occurred in fruit-freezing operations during the war years, 1941-1945. First, the quantity of fruit frozen increased from an average of 140 million pounds during the five-year prewar period of 1936-1940, to about 300 million pounds. Secondly, the market changed. Before the war, the biggest part of the pack (about 57 per cent) was in barrels and other containers of over 30-pound size, and was sold almost entirely to large manufacturers. Another 36 per cent was in the 11- to 30-pound class, and went mainly to smaller manufacturers, especially pie bakers. The institution trade took only about 3.5 per cent, in 1 1/2- to 10-pound containers, and the remaining 3.5 per cent was packed in 1 -pound units for retail sale. During 1941-1945, large manufacturers took only 34 per cent of the tonnage, the pack in institution sizes went up only moderately (to 4 per cent), but the production in retail sizes averaged 8 per cent, and the output in the 10 CALIFORNIA EXPERIMENT STATION BULLETIN 703 smaller manufacturing sizes reached 54 per cent. Thus, during the war period, the industry had come to rely upon small manufacturers and large institutional buyers instead of the large manufacturer for whom the business was originally established. The halting of large-scale government purchases, high production costs, poor quality of some of the packs, continued limitations upon sugar use, which kept bakers and preservers from buying the quantities they might otherwise have used, led to unbalanced inventories early in 1947 and distressed selling of both good and bad merchandise at prices below cost. Shortage of refrigerated cars for transporting the frozen food from production centers to the metropoli- tan marketing centers, saturation of the existing freezer storage space, and shortages in refrigerated sales cabinets in retail outlets have contributed to the difficulties encountered in distribution (Havighorst and Diehl, 1947). Whether the rapid development in frozen fruit production during the war years will continue or a recession occur, will depend upon whether the industry again packs only fruit of high quality and whether distributors can dispose of present stores of frozen produce. If the industry can weather the existing condition, it is likely that the forecast of Diehl and Havighorst for a continued increase in production will come to pass. In the meantime, development of new types of pack, introduction of new products, and development of new uses for existing products will do much to overcome the threatened recession. Where advertising and market exploitation are concerned, the frozen food processors have lagged far behind the producers of other food items, in bidding for public acceptance of their product. California Production History.— In California the freezing preservation of fruit products was limited at first to small quantities of barreled strawberries and, on occasion, a limited pack of apricots and peaches. The freezing of orange juice began in a limited way in 1930, but was not commercially im- portant until 1937. The production of frozen citrus juices (largely orange) for the years for which data7 are available was: YEAR POUNDS 1937 691,000 1938 3,285,000 1939 1,345,000 1940 2,297,000 1941 733,000 The small production of frozen citrus juices during the war years was due primarily to heavy demands upon the citrus processing industry for concen- trate for government use, and to restrictions on containers. 7 Source of data: Yearbook and Statistical Number. Western Canner and Packer 36(5): 239, April 25, 1944. In subsequent production statistics, production of citrus juices is included with t hat of figs, grapes, and miscellaneous fruits. COMMERCIAL FREEZING OF FRUIT PRODUCTS 11 Table 2 FROZEN-FRUIT PACKS BY DISTRICT, 1943 TO 1945* (Total pack, all fruits, including sugur or sirup when used) District 1943 1944 1945 Washington- Oregon thousands of pounds 102,041 42,223 14,986 55,016 16,053 35,968 thousands of pounds 115,441 79,249 16,135 54,750 21,526 49,723 thousands of pounds 137,991 California 166,532 Colorado-Utah-Idaho 15,843 Northeast 43,077 South . . f Midwest 42,528 31,665 Total 266,297 366,824 437,636 Source of data: Statistical Review and Yearbook Number. Western Canner and Packer 38(6) : 257-93. April 25, 1946. Table 3 CALIFORNIA FROZEN-FRUIT PACK, 1943 TO 1946* (Total weight, including sugar or sirup when used) Fruit 1943 1944 1945 1946 Apples and apple sauce . Apricots pounds 9,038,309 11,709,793 pounds 12,160,824 34,809,185 2,132,155 128,560 316,981 136,000 pounds 44,768,160 50,619,008 392,459 232,905 2,182,730 488,375 pounds 26,817,991 33,882,158 Bush berries (un- classified) 84,886 Blackberries 1,543,639 190,551 Boysenberries 4,133,965 Loganberries 656,223 Raspberries 67,906 Youngberries 1,200,247 727,761 106,775 1,005,195 693,280 580,102 15,119,361 7,851,401 1,275,963 28,350 1,518,347 1,222,667 969,741 5,832,659 1,737,476 26,955,833 23,004,394 1,522,020 Strawberries 3,916,119 Cherries (sweet) Nectarines ....'! 5,227,190 885,151 877,718 Peaches (cling) Peaches (free) Peaches (unclassified) . Pears 5,383,898 21,130,000 11,816,734 564,473 1,550,097 1,634,837 40,663,608 271,620 1,642,936 1,631,988 4,587,810 166,531,761 817,845 Plums and prunes Rhubarb 2,680,966 1,496,495 Miscellaneous f Total 1,656,212 79,248,691 683,807 111,483,816 * Sources of data : 1943 to 1946, compiled by Western Frozen Foods Processors Association in March, 1946, and March, 1947. 1946: Western Canner and Packer 38(13): 91. Dec, 1946. t Includes figs, grapes, and citrus juices. Fruit puree included in fruits. 12 CALIFORNIA EXPERIMENT STATION BULLETIN 703 Considerable quantities of Youngberries (subsequently displaced by Boysen- berries) were frozen in Los Angeles and vicinity for local use out of season, and this berry pack has increased. California, at present, is an important producer of frozen fruits. In fact, in 1945, it led even the Pacific Northwest, as shown in table 2. This was due to the large production of frozen apples, apricots, peaches, and nectarines and to the phenomenal increase in production of strawberries and other berries. Ordinarily, California cannot compete with other districts, particularly the Pacific Northwest, in strawberry production because of the higher costs (land, labor, etc.) and because of the difficulty of raising strawberries resistant to plant Table 4 CALIFORNIA FROZEN APRICOT AND PEACH PACK FOR 1946, BY SIZE OF CONTAINER* (Weights given include sugar or sirup when used) Size of container Apricots Freestone peaches Clingstone peaches 1 pound or under pounds 5,841,811 29,284 2,779,550 21,956,456 276,225 3,098,920 33,982,246 pounds 12,095,596 326,365 1,345,330 6,678,936 pounds 70,848 Others under 10 pounds 10 pounds 1,558,960 30 pounds 2,992,830 Barrels 258,400 Others over 10 pounds Total 735,335 21,181,562 4,881,038 * Source of data: Western Canner and Packer 38(13) :91, Dec, 1946. pests and diseases but still of suitable quality for freezing (Joslyn, 1930a). The large demand for frozen berries, and the limited production in the Pacific Northwest, particularly in 1945, stimulated the freezing of strawberries in California. The production of frozen fruit by variety in California during 1943-1946 is shown in table 3. A general decline in the pack of California apricots and peaches occurred in 1946 when there was also a decided shift from institutional- to consumer-size packaging (see table 4). The pack of cling peaches dropped 81.9 per cent from 1945 to 1946, while apricots dropped 32.9 per cent during the same period. The pack of freestone peaches decreased only 7.8 per cent, but in 1946 over 1 1 million pounds more were in 1 -pound, or smaller, containers than in 1945. Container Trends.— Early in the industry, most of the frozen fruits were packed in 50-gallon wooden barrels, with smaller quantities in 30-gallon barrels, and in 5- and 10-gallon kegs. The 5-gallon can was introduced and used to a limited extent in the Pacific Northwest in 1926, and commercial packs of berries in the W os 3 Eh H £ fc rH ©, OS -fa r- 1 »« IT3 pn O g | < £ o ^ > O 'So H J* «o ■& O 4} is to thousands of pounds 40,875 N CD O to rH • to • CO ! eo CO 0 ©** CM 00 t» OS CO CD CO CO CO rH CO t> rH CD ** CO to t> CO of TjT lO CO CN CN rH OS 0 rH OS OS t> CO rH o> «B 3 os »o » 3 0 os eo eo • rH t- CO r-i 0 to N 00 N O CO ^ rH CO CN 00 t- lO CN 00 rH OJ "^ O [> (N lO CO~ CN rH rH to 00 OS 8 y-i to rH rH 9 0) -Otj CO Bis lis 5e eo to 00 CD O T-T TH CO CO eo" CO t> 00 00 to CN OS rH TJH CO 00 00 CO OS CO CO y~l rH 00 CO CN CO CD to Oi CO y-i Tt< O CN O y-i 9 01 i2 w -*. 1*3 * •«* © os 00 rH CO OS CO CO CN O y-i 0 CO rH to CN CN CO to CN 00 TJ-O CN £0 CO M CO to t» O ^J1 ^ C75 of rH CO 0" y-i CN O CO to os~ OS 5 0 (D CO 0> W 09 1*3 w OS rH © © rH CO O CD CO CD 00 rH CD CO O rH OS 0 rH to CO rH CO 00 OS 00" to Ui eo rH yM CO OJ rH CO M T3-S » l"3 w OJ 00 0 0 Tf« CO CO CO lO CO CN OS CO CN 5 O t> CN OS CO CD CO y-i O CO 0> 1 CO CM 03 rH ■§■3 0 Kg N1 §0, CO 5«3 W HONt- CO tN rH rH O CO ■** k> TjT to" O rH to r-T y-{ to CN rH OS OS "a « bi 1 O W ) 1 O 1 O CO e O 1 lO § 1 O w a 0 >-• O § 0 O H M 03 X* tc X 1 0 CO 03 .Q CP £ O rH 03 X* 1 10 f= 1 03 1 rf CM 03 X2 1 £ 1-4 03 X 03 -t-» O u 03 "5 0 "5 0 14 CALIFORNIA EXPERIMENT STATION BULLETIN 703 slip-over-top 30-pound can were first made in 1927. Smaller sized cans were used first in 1928 when the first pack of fruit in 1-pound cartons for consumer use was also made. One-pound cans were used commercially to an appreciable extent in 192Q (Joslyn, 19290). Barrels and larger containers are still used (see table 5) for fruit packed for the preserve and bakery trade. But the smaller sized, fiber carton packs for retail distribution of dessert fruits have become more popular. Table 6 CALIFORNIA FROZEN-FRUIT PACK BY SIZE OF CONTAINER, 1945* (Weights given include sugar or sirup when used) Fruit Apples and apple sauce Apricots Bush berries (unclassified) . Blackberries Boysenberries Loganberries Youngberries Strawberries Cherries Nectarines Peaches (cling) Peaches (free) Pears Plums and prunes Rhubarb Miscellaneous Size of container 1 pound and under pounds 6,427,864 3,073,965 1,260 570,631 45,576 393,340 1,306,628 1,246,374 1,697,474 Other small sizes (1-10 lbs.) pounds 445,408 75,648 37,092 284,578 364,704 1,980,436 10-30 lbs. pounds 23,269,378 10,865,291 73,803 112,368 769,663 4,375 780,987 16,000 3,477,982 746,180 4,811,122 2,972,279 102,316 13,500 284,814 30 lbs. pounds 14,625,510 36,132,031 318,656 40,000 1,292,560 460,000 390,000 333,170 1,230,445 945,720 21,590,199 18,441,109 271,620 1,371,090 7,410 290,670 Barrels pounds 546,721 80,537 119,247 24,000 51,680 620,571 464,944 124,080 169,530 334,416 * Source of data : Western Frozen Food Processors Association, March 1, 1946. In California (see table 6), an appreciable quantity of apricots, berries, peaches, plums, prunes, and other fruits was barreled for the preserve and bakery trade in 1944 and 1945. Most of the fruit pack, however, was in smaller packages, of 5 pounds and over, and in 30-pound cans for the bakery and insti- tution trade. Considerable quantities of apples, apricots, and peaches were packed in 1 -pound and smaller fiberboard containers for retail distribution for dessert use in the home. The development of locker plants as distributors for the 30-pound packs of frozen fruit for home use in canning and making pre- serves out of season, has been an important recent development. While this market was stimulated by the wartime shortage of sugar, it could probably be continued as an important outlet for frozen fruit in normal times. The dis- tribution of the 1945 pack of fruit by sizes of container is given in table 6. COMMERCIAL FREEZING OF FRUIT PRODUCTS 15 PRINCIPLES OF PRESERVATION FREEZING The physical, chemical, enzymatic, and bacteriological changes occur- ring during the freezing and subsequent thawing of fruits, and the present status of our knowledge of the means of controlling or elimi- nating these changes are discussed in this section. Microbial spoilage and its control, enzyme activity and its control, nonenzymatic chemical deterioration, and changes in texture, volume, and weight are described in some detail. It is necessary to know the nature of these changes in order to determine what fruit and fruit products can be preserved satisfactorily by freezing and how they should be prepared, packed, frozen, and stored. The spoiling of foods, in general, is caused largely by the growth and activity of microorganisms, by enzyme activity, and by decomposition of certain of the constituents through reactions with each other, or with the environment (for example, oxygen of the air), container walls, etc. (see Joslyn, 1938, for detailed discussion of these factors). In the living tissue, these constituents do not react, and therefore do not decompose. But when the tissues are killed, whether in preparation or by freezing, the constituents are allowed to mix with each other and to react. Such reaction may result in decomposition. The living plant tissues also exhibit a certain resistance to the attack of microorganisms. In the living tissue the activity of the enzymes, which promote chemical action, is organized for the purpose of carrying on the ripening, respiratory, and other processes necessary for the life of the plant. The killing of the plant tissues by freezing places a definite limitation on the extent to which the natural quali- ties of the product may be retained. In the preservation of food by freezing storage, we depend upon the preser- vative effect of low temperatures and the direct and indirect effects of the for- mation of ice in the tissues of the product. To properly preserve most food products, temperatures below those at which ice formation occurs are re- quired. The undesirable effects that are produced by freezing, particularly on texture, are due largely to this separation of water from the tissues in the form of ice. Once separated in this way, most of the water cannot return to the tissue. If it were possible to reach a temperature low enough to preserve the product for the desired period without actually freezing it, preservation by cold would be greatly improved. At present we are limited to the selection of varieties of fruits which are more tolerant of freezing and to the development of process- ing methods which minimize its undesirable effects. Even with fruit juices, the separation of ice often leads to undesirable changes in appearance (clearing and sedimentation). With fruit pulps and purees, the problem is not so acute. The presence of ice crystals necessitates constant temperature storage. 16 CALIFORNIA EXPERIMENT STATION BULLETIN 703 Microbial Spoilage and Its Control Little delay should occur between the harvesting and processing of fruit, and even less between processing and initial chilling or freezing. Speed of handling is necessary to reduce the danger of fermentation and microbial spoilage during freezing. This is especially important when freezing fruit in large containers. The growth o£ all microorganisms decreases as temperature is lowered until a temperature is reached at which growth ceases. This minimum growth tem- perature varies with the environmental conditions, such as composition of me- dium, availability of oxygen, and type of organisms. It is lower for the cold- loving (psychrophilic) organisms and increases as the optimum temperature for growth increases until the relatively high minimum of the heat-loving (thermophilic) organisms is reached. The biological zero for growth was often considered to be o° C or 32 ° F, but since 1887, numerous species of microorganisms have been observed to grow at 32 ° F and even considerably below that temperature (Berry and Magoon, 1934). In general, the extreme psychrophilic organisms are fungal rather than bacterial. The lower limit for microbial growth is now believed to be some- where between 150 and 20 ° F. Ice formation, which results in increased concentration of solutes, has a marked effect on microbial growth. In frozen foods, only those psychrophilic organisms will develop which are also osmophilic (tolerant to high concentra- tions). Bacterial activity usually will occur at lower temperatures in nonacid vegetables than in acid fruits, and for this reason the latter may be stored at somewhat higher temperatures. At low temperatures,- microorganisms will not only be retarded in growth and activity, but they may also be destroyed. The destructive action of cold is particularly great during the early stages of the refrigeration process and con- tinues, though at a diminished rate, for many months. Complete destruction, however, has rarely been attained even at extremely low temperatures (Berry, 1933)- The killing of bacterial cells in frozen products results not only from tem- perature, but also from mechanical, and dehydrating effect of ice formation and from other conditions (Weiser and Osternd, 1945). Although a large per- centage of the microorganisms present— often over 90 per cent— is destroyed by cold, the surviving organisms remain alive for a number of years, and will de- velop at favorable temperatures. The destruction of the plant tissues by freez- ing usually allows a more rapid development of those microorganisms present, so that the spoilage of frozen food after defrosting is often more rapid than that of the original fresh product even though the frozen contains a much smaller initial number of microorganisms. In sound fruits and vegetables, microorganisms are present only on the COMMERCIAL FREEZING OF FRUIT PRODUCTS 17 surface tissues; the interior tissues are sterile. Thus the number of micro- organisms remaining on the fruit or vegetable products after they are prepared for freezing is an excellent indication of the sanitary conditions prevailing in the plant. Although some attention is given to this point in the freezing in- dustry, the high bacterial counts often reported in the literature indicate the possibility of further improvement in the sanitation of the harvesting and pre- paring units. Not all the types of microorganisms will develop in a given product because some of them require food elements that are either not present or not available, and because the acidity or the sugar content serve as natural checks. Thus each food product permits the growth of only certain types of microorganisms, and, in a sense, has a selective effect on them. Only those microorganisms which are capable of growing in the food need be destroyed. The others will cause no deterioration and need be killed only if they are pathogenic (cause disease). Although freezing temperatures are required actually to stop the develop- ment of microorganisms in fruits, reducing the temperature of the fruit to 400 F as quickly as possible will very markedly retard the growth and activity and reduce the danger of fermentation and spoilage during freezing. There should be little or no delay between harvesting and processing and even less delay between processing and initial chilling or freezing. The conditions of freezing must be such that all the mass of the product is quickly brought below the temperature at which rapid spoilage will occur (32°-40° F). This is par- ticularly important in the freezing of fruit in large containers where the rate of temperature change in the center of the mass is slow, occasionally permitting serious spoilage by fermentation. Precooling the fruit before packing in barrels, and frequent rolling of the barrels after they have been placed in the freezer is important. Rolling is particularly helpful in obtaining a uniform mixture of fruit and sugar and in hastening the freezing of berries packed with sugar (Ireland, 1941). The subsequent rate of freezing, and freezing storage conditions are governed by the character of the product and the length of time it is to be stored. However, because of the extremely slow rate of freezing and other disadvantages of barrels, the authors recommend 30-pound tins or large cartons for bulk freezing of fruits. Enzyme Activity and Its Control Changes in color and flavor of fruits during freezing and thawing are caused mainly by oxidative enzymes. Such enzyme activity may be controlled by selection of varieties less susceptible to these changes, at the proper stage of maturity, by removal or exclusion of oxygen, by addition of sugars or sirups, by addition of antioxidants, or by heat. Changes in color and flavor as a result of tissue damage during freezing and during slow defrosting are particularly profound in fresh fruit frozen for dessert use. Most varieties of apples, apricots, cherries, nectarines, grapes, 18 CALIFORNIA EXPERIMENT STATION BELLI I IN 709 peaches, pears, plums, and, to a lesser degree, berries, darken quickly aftei peeling or other mechanical injury. This discoloration, its resultant loss ill characteristic flavor, and production of undesirable off-flavors are caused largely by oxidative enzymes. The chemistry of the enzyme-catalyzed oxidative discoloration of fruits has been summarized by Joslyn (1941)- The browning of fruit is due largely to interaction of the enzyme, polyphenol oxidase, with molecular oxygen and a suitable phenolic substrate or color base. In the living tissue, oxygen tension is low and the phenolase is maintained in a reduced state through the activity of a dehydrogenase and suitable hydrogen donors such as ascorbic acid. On injury, the dehydrogenase is destroyed and the residual ascorbic acid is rapidly oxidized, thus permitting the formation and accumu- lation of quinones and higher products of oxidation of the mono- or poly- hydroxy-benzene derivatives. These may be either free (for example, catechol, tyrosine, etc.) or combined (catechol-tannins, caffeic acid, etc.). The phenolic substrates themselves will form brown or red pigments, or their primary products of oxidation (the quinones) may induce oxidative discoloration in tannins and similar substances. It has been well established that no browning will occur in fruit tissues until practically all ascorbic acid (vitamin C) has been converted by oxidation into dehydro-ascorbic acid. And the evidence available at present indicates that most of the loss in ascorbic-acid content in injured fruit tissue is brought about by induced oxidation by the phenolases in the presence of suitable phenolic substrates (Ponting, 1944a). In addition to the phenolases, certain respiratory enzymes may be involved in the formation of off-flavors similar to those observed in unblanched frozen vegetables. Apricots, peaches, and plums occasionally develop haylike flavors; pineapple may develop musty or fishy flavors; and certain varieties of straw- berries develop a strong manurial odor and flavor. These off-flavors probably are caused by the accumulation of intermediate products resulting from dam- age by freezing or mechanical injury to the cells. This leads to an imbalance in the chain of step-wise enzyme reactions occurring in normal respiration and metabolism, and the activation of enzymes such as catalase and peroxidase. Hydrolytic as well as oxidative enzymes may produce changes in composi- tion and flavor. The inversion of sucrose into dextrose and fructose in frozen- fruit products is well recognized, and the activity of invertase even at low temperatures has been established (Joslyn and Sherrill, 1933; Kertesz, 1942). Losses in pectin content in frozen blackberries, raspberries, and strawberries have been reported, and since, at least in the case of blackberries, such loss was prevented by heating, it is likely that pectic enzymes were involved (Joslyn and Marsh, 1933, particularly p. 27). Proteolytic enzymes may be active, also. Eckart and Cruess (1931) found bromelin to be present in undiminished activity in stored pineapple juice frozen for over three months. It has long been recognized that low temperatures and ice formation do not prevent, although they do retard, enzyme activity (see review by Joslyn, 1946&). COMMERCIAL FREEZING OF FRUIT PRODUCTS 19 Although retarded by decreasing the temperature, so active are enzymes in promoting chemical changes and so resistant are they to the destructive effect of cold, that appreciable deterioration occurs in but a few months in tissues stored at commercially available freezing storage temperatures. Enzymes have been found to be unaffected by exposure to the lowest temperatures investi- gated (4000 F below o°) and many of them are surprisingly active even in rela- tively concentrated solutions. Consequently they cannot be inactivated by cold alone. The undesirable changes brought about through the unchecked activity of the enzymes, however, may be minimized by storage at low temperatures, the lower the temperature, the longer the storage life of the product. The storage life generally is somewhat more than doubled for each decrease of 180 F in temperature, but the nature of the storage life temperature relation- ship is not definitely known. In addition to being retarded by low temperatures, the activity of the enzyme concerned in the change of flavor or color must be prevented by some suitable method of inhibition or destruction. The discoloration of fruit can be prevented or minimized by the selection of varieties of fruit low in either the polyphenol oxidase or phenolic substrate content. Oxidation may be minimized by: 1. Removal of oxygen from the tissues (either by forced respiration while immersed in dilute salt or sugar solution or by mechanical deaeration). • 2. Exclusion of oxygen from the surrounding atmosphere (as by freezing and storage in vacuumized, hermetically sealed con- tainers or immersed in sirup). 3. By the addition of substances which either inhibit the enzyme or maintain a reducing condition in the fruit tissues and surround- ing atmosphere, such as edible acids, salt, sugar, sulfurous acid or its salts, and ascorbic acid. 4. By the destruction of the enzyme by heat (Joslyn, 1934; Hohl, 1946a). Selection of Varieties.— Not all types of fruits can be frozen. The following are best adapted to freezing: Youngberries, Boysenberries, raspberries, straw- berries, peaches, nectarines, and red sour cherries. Many tropical fruits, par- ticularly pineapple, papaya, guavas, and passion fruit, can be preserved well by freezing. Some fruits, such as apples and apricots, which cannot be frozen successfully for serving without further preparation, can be prepared for use in bakery products. Most fruits can be well preserved as purees, juices, and sirups. Owing to loss in turgidity on freezing, the tomato cannot be preserved by freezing without such severe loss in crispness as to make it unacceptable. Not all fruits are equally susceptible to discoloration, flavor changes, and 20 CALIFORNIA EXPERIMENT STATION BULLETIN 703 texture changes. Even varieties or hybrids of a particular fruit vary in their responses to freezing. By selecting varieties of initial high color and flavor, changes in these characteristics may be minimized. One of the most striking examples of a variety which does not undergo oxidative browning is the Sun- beam peach. Kertesz (1933) made a study of this variety and found that it contains the necessary enzyme system, but lacks the phenolic substrate and therefore always retains its natural color. Unfortunately, there are very few of the well-known commerically grown varieties which have this nonoxidizing character. The Sunbeam peach is deficient in flavor, and very few plant ings of it have been made. Crosses of the Sunbeam with more flavorful peaches should be available in the future. Variety, however, affects the magnitude of the darkening reaction, and those fruits less susceptible to browning should be selected. Maturity.— Maturity is another important factor. The immature fruit is not only higher in tannin content but also contains a more active phenolase. The content of tannin or other color base as well as that of active phenolase de- creases with maturity. It has been reported (Caldwell, Lutz, and Moon, 1932; Lutz, Caldwell, and Moon, 1932) that the discoloration of peaches of all varieties was most rapid and pronounced in immature fruits and decreased in intensity with advancing ripeness. The best stage of maturity for freezing purposes was found to be one day before full eating ripeness. Exclusion of Oxygen.— The removal of oxygen from the fruit tissues as well as from the atmosphere surrounding the fruit is necessary for the complete in- hibition of browning. For small fruits, such as berries, packing in hermetically sealed containers closed under vacuum is sufficient. This procedure was introduced by J. E. McConkie in Oregon in 1929 and is again being used commercially. The removal of oxygen (present as such in the tissues or as organic peroxides) by forced respiration under water or brine, which is success- fully used as a pretreatment in the canning of apples and rhubarb, was not successful in early experiments with fruit for freezing (Joslyn, 1934). It was found, however, that the gases present in fruit tissues could be almost com- pletely removed by subjecting the sliced fruit in sirup (apples, apricots, and peaches) to a vacuum of about 29" until gas evolution ceased. During the course of the evacuation, the vacuum was released several times, care being taken to have the fruit covered with sirup. The fruit was found to absorb sirup which replaced the gases in the intercellular spaces, and the fruit became translucent rather than opaque. Fruit deaerated and impregnated with sirup in this manner did not brown so rapidly as ordinary sirup-packed fruit. More recent tests have confirmed the above results, and the development of the deaeration-impregnation treatment appears possible. To achieve satisfactory results, it is necessary to exclude air during defrosting as well as during freez- ing and freezing storage. COMMERCIAL FREEZING OF FRUIT PRODUCTS 21 Loss in flavor, browning, and production of off-flavors in crushed, sieved, or pureed fruits in the presence of oxygen are also catalyzed by phenolases. To avoid oxidative deterioration it is necessary to remove the oxygen from the pulp by subjecting it to a vacuum. Since the advantages of deaeration may be preserved by relieving the vacuum in the evacuation chamber with an inert gas, such as nitrogen, it is possible to fill, package, and freeze the product under a slight pressure of such gas. Such a treatment markedly improves the retention of flavor and prevents loss of vitamin C and discoloration. While deaeration is not as yet practiced commercially in the preparation of fruit purees, it is used in the production of frozen orange juice and other citrus juices. Prompt handling of fruit and fruit products during preparation for freezing and packaging is necessary to minimize exposure to air. Fruits such as peaches, which are peeled, are particularly susceptible to surface discoloration during packaging. For best color retention, all fruit should be filled promptly into the package, covered with sirup without delay, and quickly sealed and frozen. Addition of Sugar or Sirup.— Since the beginning of the industry, fruit has been frozen with added sugar. It was used to retard the development of yeast and mold and thus reduce the danger of fermentation during freezing, storage, and distribution, and to preserve color, flavor, and aroma. Sugar has long been known to retard the activity of phenolases. In 1929, P. J. Quin found that at the same concentrations, the retarding effect of sucrose was greater than that of glycerin or dextrose. The activity of peach oxidase was found to be com- pletely inhibited in solutions containing 70 per cent sucrose or 60 per cent glycerin, although use of the latter is impracticable. Sugar acts not only as an enzyme inhibitor, but also, particularly when present as a solution, as a means of excluding air. The solubility of oxygen in sugar solutions is low, and when the solutions fully cover the fruit, the rate of diffusion of oxygen from the head space in the container to the fruit is retarded. To exert its full effect, however, the sugar must be completely dissolved, and also absorbed by the fruit tissues. Sugar added dry is most effective when uniformly incorporated with crushed or pureed fruit, dissolved in fruit juices, or mixed with sliced strawberries. It is not satisfactory for use with whole berries, or halved or sliced fruit, such as apricots, apples, and peaches. The sugar solution formed on contact with the surface of the fruit usually settles to the bottom of the container, carrying with it the unabsorbed sugar and leaving the upper fruit exposed. When the mix- ture is frozen quickly the sugar remains largely unabsorbed at the point of application and exerts only a limited preservative effect. The use of sugar solutions instead of addition of dry sugar was introduced by Cruess, Overholser, and Bjarnason (1920). They suggested the use of a 6o° Bal. sirup for strawberries, 300 for raspberries and apricots, and lighter sirup for cherries. They pointed out that there was a slight difference between the concentration of sirup (sugar solution) at which the flavor was best preserved 22 CALIFORNIA EXPERIMENT STATION BULLETIN 703 and that at which the texture was best. Subsequent observations in this lab- oratory and elsewhere have confirmed these results and extended the observa- tions to lay the basis for the industrial use of sirup in freezing fruit (Joslyn, 19306). The use of sirup has the following advantages over the sugar-pack method: 1. Air discoloration is reduced to a minimum. 2. The sirup is more convenient than the sugar, especially if the latter is to be distributed uniformly throughout the mass of fruit. 3. There is less damage to the fruit during the addition of sirup than during the addition of sugar. 4. A more uniform and attractive pack is obtained as there is little or no change in fruit volume by loss of water from the fruit, and there is no settling of the fruit in the container as occurs in the sugar pack. 5. The sirup is a better aid to preservation during freezing than the sugar. It can be chilled before use and acts as a precooling agent. 6. The texture of the thawed fruit is better. 7. The sirup pack is applicable to all fruits. There is some difference of opinion among investigators as to the concentra- tion of sirup that is best. Joslyn has favored the use of medium sirups of about 400 Bal. for most fruits, except strawberries and very tart fruit, which are better with 50 ° sirup. Diehl, et al. (1939) recommended the following strengths of SirUP- PER CENT FRUIT DENSITY OF SIRUP Blackberries 40 to 50 Blueberries 40 to 45 Cranberries 50 Raspberries, black 40 to 50 Raspberries, red 50 Strawberries 45 to 50 Apples 50 Apricots 60 to 65 Cherries, sour 60 to 65 Cherries, sweet 40 to 50 Peaches 50 Prunes 50 The limit of the concentration of sirup to be used is set by the plasmolytic effects of sirups of high concentration— shrinkage and toughening of the tissue COMMERCIAL FREEZING OF FRUIT PRODUCTS 23 and extraction of much color and flavor— and by the degree of sweetness desired in the final product. Fruit also has a greater tendency to float in the heavy sirups, and requires additional precautions in packing. The volume of sirup added should be sufficient to cover the fruit in the container, and the package must be so designed as to keep the fruit submerged during freezing. In small containers, this is achieved by covering the top layer of the fruit with a parch- ment or cellophane liner; in larger containers, by the use of a perforated, paraffined paperboard cover or by insertion of a shallow, cone-shaped disk re- tained against the top rim by tabs or friction. Dry sugar is still preferred by packers of fruit for bakers' or preservers' use, to avoid the larger volumes of liquid that would have to be handled. Color retention in fruit for bakers' use, however, is obtained by pretreatments other than those that rely upon exclusion of air. With the exception of sliced straw- berries, which are frozen with added dry sugar, fruits for dessert use are now widely frozen with sirup. The absorption of sugar by the fruit during freezing and subsequent de- frosting is not large, amounting to about 5 per cent in berries frozen in sirup. In berries frozen with dry sugar, it becomes larger with the increase in ratio of sugar to fruit. Absorption varies from about 2 per cent in 1 : 6 pack to over 10 per cent in the 1:1 pack. Small quantities of sugar are absorbed by sliced apricots and peaches, but in general, the absorption is not large in fruit frozen slowly, and is even smaller in that frozen rapidly. Wiegand (1931) re- ported that Oregon (Marshall) strawberries held in sirup or with sugar at 3o°-3i° F for 24 to 72 hours before freezing were superior in color, flavor, and texture to those frozen without such treatment. Joslyn and Marsh (1933) did not find this to be true of Banner strawberries, apricots, or peaches although the loss in weight of the sugar-cured fruit was less than that of fruit frozen without such storage. Subsequently, Wiegand (1941) concluded that sugar absorption on delayed freezing is not large. While delayed freezing is not necessary for flavor and color retention in small containers of sirup-packed fruits, it is desirable for barreled berries and advisable for dry-sugar-packed fruits in retail containers. The substitution of dextrose solutions, invert sirup, or honey for cane or beet sugar solutions has not been found desirable. Fellers and Mack (1929) reported that the substitution of dextrose (cerelose) for cane sugar caused strawberries to darken, to assume an unnatural, purplish-red appearance, and to become flat, unnatural, and objectionable in flavor, and soft and mushy in texture. Observations made in this laboratory have confirmed similar effects for a number of fruits. In recent tests, apricots, peaches, and nectarines frozen in a 400 Bal. dextrose solution had an objectionable flavor and were more discolored than those frozen in a 400 sucrose solution. In addition, the dextrose crystallized out on freezing, and the fruit was covered with an unattractive mass of soft white crystals. Enzyme-converted invert sirup was found to cause a brownish discoloration accompanied by 24 CALIFORNIA EXPERIMENT STATION BULLETIN 703 an objectionable odor and flavor in both the strawberries and sirup similar to that found in the dextrose packs. Acid-converted invert sirup was found to be satisfactory, provided that the degree of inversion was not above 50 per cent. Apricots were particularly sensitive to the development of off-flavors in invert sirup and were best in color and flavor at 50 per cent inversion; peaches were less sensitive, and nectarines least. Commercial glucose sirups (low con- version corn sirups) were found to be superior to dextrose sirups in color re- tension, but fruit packed in these sirups had a slightly objectionable flavor. The high conversion corn sirups also were satisfactory in color-retentive ability. Several fruits retained more of their natural color in these sirups than in sucrose sirups of the same strength, but developed a noticeable foreign flavor. Mixtures of high conversion corn sirups with cane in the proportion of 1 part corn sirup solids to 3 of sucrose, however, were equal to cane sugar solutions. Addition of Acids.— It is well known that the pH (active acidity) of the medium influences enzyme activity. Enzymes usually exhibit an optimum activity at a pH range characteristic of the source and purity of enzyme, type of sub- strate, nature of buffer system, and temperature. Their activity decreases in regions of pH lower or higher than this optimum. Samisch (1935), for example, reported the optimum pH for the oxidation of catechol by apricot and peach phenolase, respectively, to be 4.9 while, under the same conditions, there was no marked optimum for apple phenolase in the region of pH of 3.6 to 6.6. Upon acidification, the activity of all three phenolases decreased, that of apri- cot and peach markedly so. This behavior of enzymes has been used to advantage in checking brown- ing of lye-peeled peaches during handling. After washing, the fruit is im- mersed in a 1 per cent solution of citric acid. The addition of citric acid to sweetened fruit purees or to the sirup in which fruit is frozen was found to markedly improve color and flavor retention. As much as 0.5 per cent of citric acid, for example, can be added to fruit purees, or fruits such as apricots and peaches, without making them too sour. The use of added acid is now common in the freezing of fruit products for use in ice cream and ice. More recently, the use of a combination of citric and ascorbic acid has been suggested for preventing browning of cut fruit (Luther and Cragwall, 1946). In our tests, a sirup containing 0.5 per cent of citric acid and 0.03 per cent of ascorbic has been as satisfactory for color retention in apricots, peaches, and nectarines during freezing storage as one containing 0.1 per cent of ascorbic arid alone. The fruit frozen with citric and ascorbic acids, however, does not retain its full fruit flavor so well as fruit frozen with ascorbic acid alone. Upon storage at room temperature after defrosting, the citric-acid-treated fruit, par- ticularly apricots, darkens more rapidly. Citric acid apparently exerts a cata- lytic effect on discoloration after long storage at room temperature. Added a< id was not found objectionable in apricots and peaches. COMMERCIAL FREEZING OF FRUIT PRODUCTS 25 Antioxidants or Reducing Substances.— Several antioxidants, which may act either by reducing the free oxygen present in the sirup and the fruit tissues or as phenolase inhibitors, have been used (Sater et ah, 1947)- Of these, ascorbic acid (vitamin C) is used most widely at present in the packing of frozen fruit for dessert use (Hohl, 1946a). It rapidly reduces dissolved oxygen in the sirup, maintains the phenolic substrates in the fruit tissues in reduced form, and thus prevents browning. In addition to preventing browning, ascorbic acid has a striking effect on retention of the natural fresh flavor and aroma. Although cut fruit, frozen, completely immersed in sirup, retains its color during freezing storage almost as well without added ascorbic acid, and is of acceptable color immediately after thawing, the presence of the added ascorbic acid improves color retention as the fruit reaches room temperature. The fruit must be completely covered with ascorbic-acid-containing sirup, otherwise there will not be effective pro- tection even at high concentrations of antioxidant. The usual range of concentration is from 200-250 mg. of ascorbic acid per pound of fruit. At the upper range, with a 1-pound pack of halved apricots containing 10 ounces of fruit and 6 ounces of sirup, the sirup should contain 156 mg. of ascorbic acid or 0.0916 per cent. With sliced peaches packed at the rate of 11 ounces to 5 ounces of sirup, the sirup should contain 172 mg. of ascorbic acid or 0.142 per cent. As a general practice the addition of ascorbic acid to the sirup to the extent of 0.1 per cent by weight with sufficient sirup to cover the fruit, has been found satisfactory. Theoretically, if the oxygen con- tent of the sirup and the fruit, and that diffusing into the fruit and sirup, are known, provision can be made to add the necessary quantity of ascorbic acid, but at present there is but little available data on these points. However, data on the ascorbic acid stability in frozen fruit is being accumulated (DuBois and Colvin, 1945; Bauernfeind et ah, 1946). Such information is necessary if claims are to be made for the added ascorbic acid as vitamin C. The Food and Drug Administration requires that any addi- tion of ascorbic acid be declared on the label. If the packer does not wish to make a claim for reinforcement of the nutritive value, the label may state that the ascorbic acid was added simply to preserve color and flavor. On the other hand, if he wishes to claim that supplementary vitamin C was added, the label must guarantee that the stated amount is present in the package. The labeling, to comply with the regulations, must also indicate what proportion of the adult daily requirement of vitamin C is present in a given portion of the con- tents of the package. All these regulations imply that the packer knows the loss of vitamin C during freezing, during the frozen storage period, and during defrosting, and also that he has some idea of the effect of various preparation methods upon losses of vitamin C during storage. Its lack of characteristic flavor, other than a mild acidity, the ease with which it blends with fruit flavors as well as the fact that it is an important nutrient (vitamin C) are factors in favor of ascorbic acid. On the other hand, it is still 26 CALIFORNIA EXPERIMENT STATION BULLETIN 703 relatively expensive, its antioxidant properties are weaker than those of sul- furous acid, and it is not so readily applied because it does not penetrate the fruit tissues so rapidly. Sulfurous acid and its salts (the sulfites and bisulfites) are among the strongest and best known of antioxidants. It is the most effective chemical in- hibitor of browning and can be used in much smaller concentrations than ascorbic acid or thiourea. Sulfurous acid and the sulfites, however, have the dis- advantage of bleaching the anthocyanin pigments of berries and cherries and the red pigments at the peach-pit cavity. They have an objectionable flavor Fig. 1.— Left to right, comparison of sulfited, untreated, and blanched apricots. which may be easily detected in concentrations above 25-50 p. p.m., in frozen fruits, and also have a tendency to destroy the natural fruit aroma. In our ex- perience, apricots are the only fruits for dessert use in which sulfites may be used. Apricots frozen with a sirup initially containing 50 p.p.m. of SO retain their color and flavor fairly well. This sirup can be made by dissolving 446 grams of liquid sulfur dioxide, 725 grams of sodium bisulfite (NaHS03), or 662 grams of sodium metabisulfite (Na2S205) per 100 gallons of 400 Bal. sirup. (1 gallon of 400 Bal. sirup weighs 9.809 lbs. in air at 68° F.) Sulfur dioxide is particularly useful, however, for a pretreatment of fruit, such as apples, apricots, and peaches, to be used for bakery products or jams and preserves. Halved apricots or sliced apples and peaches immersed for 4 minutes in a solution containing 3000 p.p.m. of S02, or for 3 minutes in one of 4000 p.p.m., drained well and then frozen in sealed containers, will retain their color during storage for over a year, and will retain less than 100 p.p.m. COMMERCIAL FREEZING OF FRUIT PRODUCTS 27 of S02 after storage. If the strength of the sulfite solution is carefully main- tained during treatment, the fruit tissue will be penetrated throughout but will not be excessively sulfited. It will also contain the required concentration of S02 for color retention— 50 to 75 p. p.m. for apricots and peaches, and 25 to 50 p. p.m. for apples (Joslyn and Mrak, 1933; Joslyn, 1942). With more dilute solutions, the penetration of S02 will not be complete and browning will occur in the center portions of the fruit during freezing storage and be accentuated during thawing. The Western Regional Research Laboratory (Anon., 1945a) has recommended that apple slices or rings be dipped for 1 minute in a solu- tion containing 0.2 to 0.25 per cent of S02, and then held for 8 hours before freezing to allow the S02 to penetrate. We were not able to confirm this recom- mendation for holding, in some of our tests with a different variety of apple. The treating solution containing 3000 p.p.m. of S02 (0.3 per cent) can be prepared by passing liquid S02 from a gas cylinder into a wooden or non- corrodable metal container (stainless steel or brass) filled with water, at the rate of 25 pounds per 100 gallons of water. The disagreeable fumes of S02, where liquid sulfur dioxide is used, can be minimized by keeping the solution cool, placing it in a well-ventilated place, or by adding sufficient flake lye to adjust the pH to about 4. The ratio of the weight of lye added to that of S02 should not be over 2 : 3. Sodium bisulfite or sodium metabisulfite may be used in place of liquid S02 at the rate of 41 pounds or 37 pounds, respectively, per 100 gallons of water. (100 gallons is equivalent to 13.35 cu- ft-) The immersion time is best controlled by means of a suitably arranged conveyor system. The S02 strength of the bath should be adjusted periodically by addition of sulfur dioxide or sulfite after the strength of the bath has been determined. This is done by titration of a carefully taken and measured aliquot with standard iodine solution, using starch indicator. To test the extent of inactivation of the phenolase by sulfur dioxide, and the degree of penetration of sulfur dioxide, select several pieces of fruit, especially the larger ones and those least likely to have been completely immersed, cut them in two with a stainless steel knife, and spread a freshly prepared, 1 per cent solution of catechol in water over the cut surface, with a medicine dropper. After 5 to 10 minutes, the portion of the fruit which still contains active enzymes and is likely to brown on freezing, will turn black (Ponting, 19446). Still another antioxidant which has received some attention in the literature is thiourea (thiocarbamide), known by the trade name Frulite (Denny, 1942). It is effective in inhibiting browning if the fruit is immersed in the solution and drained again, before freezing. As a result of the recently announced policy of the Food and Drug Administration its use is not approved because of possibility of injury to humans. Tests with medicinal preparations con- taining thiouracil have produced serious symptoms in experimental animals. Other disadvantages of this antioxidant are that it adversely affects flavor and is not so effective as S02. 28 CALIFORNIA EXPERIMENT STATION BULLETIN 703 Heat Inactivation.— Discoloration may also be prevented by heat inactivation of the phenolases, or blanching (scalding). When blanching is properly done, no browning can occur, but if blanching is insufficient to destroy the oxidizing enzyme, the browning may be even more severe than in untreated fruit. (This effect is caused by injury to the tissues, Fig. 2.— Center portions of apple segments after various pretreatments, freezing, thawing, and slicing. Top left, insufficiently blanched; top right, untreated; bottom left, sulfite dip, which did not penetrate properly; bottom right, adequately blanched. resulting from blanching, which allows the several reactants to be brought to- gether more quickly.) Another advantage is that blanching does not require the addition of a foreign chemical substance, and this eliminates troublesome label declarations. Disadvantages of blanching are that it tends to give the fruit a cooked flavor and softens its texture. A blanch which is sufficient to inhibit browning and still not precook the fruit is only rarely attainable. Another serious problem in connection with blanching fruit for freezing is that of cooling. To check con- COMMERCIAL FREEZING OF FRUIT PRODUCTS 29 tinued cooking effect of the blanch, the fruit must be quickly and thoroughly cooled after blanching. This has been done most frequently with excessive quantities of water either in flumes or sprays. This method has the very obvious disadvantage of causing large losses of soluble nutrients and flavor from the fruit. Air cooling, while somewhat slower, would be far better from the stand- point of saving nutrients and flavor, if practical means of achieving it com- mercially could be developed. Fig. 3.— Cooling peaches after blanching. Top: Spray cooling, best for retention of flavor and nutritive value. Bottom: Flume cooling, undesirable because of excessive leaching of soluble constituents. Blanching may be accomplished by means of live steam, hot water, hot sirup, or by electronics. Steam is the most convenient and popular procedure for commercial use. The period of blanching, whatever the method used, should be long enough to completely inactivate phenolase at all portions of the tissue, particularly in pieces of fruit that are farthest removed from direct contact with the heating medium. The temperature to which the fruit tissues must be brought varies with variety, maturity, and holding period. Heating the fruit throughout, in free-flowing steam, to a temperature of about 1850 F is usually sufficient. The 30 CALIFORNIA EXPERIMENT STATION BULLETIN 703 time of blanching for halved, medium-sized apricots exposed to steam in single layers is about 4 minutes; for sliced apples it is about 2. If heat distribution is poor, only the enzymes in the surface layers will be destroyed and internal browning will result. As a test for the adequacy of scalding, select a few pieces of fruit from the blancher, cool to room temperature, cut across, place in a saucer, and add several drops of a 1 per cent tincture of guaiacol, a 1 per cent tincture of guaiacum, or 1 per cent water solution of catechol. If the samples change in color (guaiacol to red, guaiacum to blue, catechol to dark brown or black), the blanching has not been sufficient. Considerably shorter periods of time may be used in sirup blanching, and when this is followed by sirup cooling, the flavor retention will be sufficient for bakers' use. Blanching in boiling water is not recommended. Only fruit for bakers' use should be blanched. NONENZYMATIC CHEMICAL CHANGES Nonenzymatic chemical activity producing changes in flavor and color may occur during preparation for freezing, freezing storage, and subsequent thawing. This may be purely oxidative, but may also include development of off-flavors and increase in tartness. These chemical changes are very difficult to control. Auto-oxidation, which may be responsible for part of the changes in flavor and color, can be controlled by exclusion of oxygen, or may be minimized by addition of sugar or sirup. Sugar has been shown to reduce oxidation of ascor- bic acid, even in acid solutions (Munilla and Vogelsinger, 1937; Shamrai, 1941 ; Richardson and May field, 1944). Among the nonoxidative changes is the significant increase in tartness on freezing. This has been ascribed by Pickett (1932) to an increase in titratable acidity, but the increases he found were too small to account for the marked changes observable. During freezing and subsequent thawing, some fruits acquire a peculiar flavor and an odor which is undesirable and sometimes offensive. Weak- flavored fruits, such as cling peaches, apparently lose all of their characteristic flavor upon prolonged storage, even when protected from oxidation. Grapes, apples, berries, and cherries sometimes acquire a rather undesirable flavor which seems to be more apparent in hermetically sealed containers in which the fruits have not been promptly cooled and frozen. It is thought that this may be due to anaerobic respiration which occurs in the closed containers during freezing. Peaches also acquire a pronounced benzaldehyde flavor during long storage. This is more marked in freestones than in clingstones, and in exposed peaches than in those submerged in sirup although it occurs even in the latter. Sometimes off-flavors result from the permeability of the paper containers, which allow absorption of foreign cold-storage odors and flavors, and often they can be traced to absorption of foreign flavors from the containers them- selves. These off-flavors are more pronounced after long storage. COMMERCIAL FREEZING OF FRUIT PRODUCTS 31 Crystallization of Sugar.— At times, during the storage of frozen fruits, a white, fondantlike material forms on the surface. When the fruit has been thawed, this appears as white patches of partly crystallized material. This condition is particularly noticeable in deeply colored fruits or berries. It is most prevalent under the following conditions: with fruits from which relatively little juice exudes to dilute the sirup; where high original concentrations of sucrose sirup are used; at relatively low freezing storage temperatures; and in containers that permit gradual dehydration of the product in storage. Rabak and Diehl (1944) have described this condition and have shown that it is due to the crystallization of sucrose. We have frequently observed the condition in fruits packed in improperly sealed containers, even at low sirup strengths at o° F, and have also observed formation of small rosettes of sucrose crystals throughout the body of fruit purees (3+1), particularly with persimmon pulp. Essentially this is due to the crystallization of the sugar-water eutectic mixture. (The eutectic point is the constant temperature at which a solution of water and a dissolved substance, such as sugar, will change from a liquid state to a solid state upon removal of heat. The eutectic mixture is a solid one of crystals of ice and crystals of solute, such as sucrose.) Mondain-Monval (1925) found that in the system sucrose-water, the eutectic point occurs at a sucrose concentration of 62.4 per cent and the eutectic temperature is -13. 90 C (+70 F). When a solu- tion of sucrose of this strength is cooled to +70 F a single solid phase appears. When solutions of lower concentration are cooled to below their freezing point, ice alone forms until the eutectic temperature is reached, when a mix- ture of ice and crystalline sugar forms. In addition to sucrose, the other sugars, salts, and acids present in the fruit-sugar system will form eutectics, but there is little information on the condition at which these will occur in the complex mixture. Physical Changes During Freezing and Thawing The most troublesome physical change occurring in frozen fruits is that of texture. This undoubtedly results from changes in the colloidal systems of the fruit, but our present knowledge of the reaction of plant colloids to freezing is incomplete. We do know, however, that an important factor in texture retention is uniformity in the distribution of ice crystals in the frozen product. This section suggests ways of achieving that uniformity. Changes in weight and volume also occur, and must be taken into account in choosing containers. Texture.— Cellular breakdown, resulting in softening and other changes in texture, is the most striking and troublesome physical change which occurs in fruits, particularly in those frozen for dessert use. This loss in the characteris- tic crispness and turgidity is grossly similar to that resulting from cooking. It can be minimized by selection of varieties or strains of fruit which are resistant to these changes and particularly by serving fruits immediately after partial 32 CALIFORNIA EXPERIMENT STATION BULLETIN 703 defrosting. Considerable attention has been given to the effect of rate of freez- ing, and size and distribution of ice crystals formed during freezing, upon the texture of the final product, and some attention has also been given to con- ditions of defrosting.8 It has been established that the size and distribution of ice crystals can be controlled by the rate of heat removal (or rate of freezing) during the period of maximum ice formation. When foods are frozen slowly, the ice crystals are large and nonuniformly distributed. In meat products, large ice crystals form between the meat fibers and sometimes cause rupture and dislocation of the fibers. In plant tissues, ice gradually fills the intercellular spaces and eventually forces the cells apart. Rupture of the cells as a result of internal pressure from intracellular ice crystals is not common since the natural elasticity of the cells and their relation to each other within the tissue prevent this. In both animal and plant tissues that are slow-frozen, the water removed during freezing is only partly re- absorbed on thawing; the excess leaks out of the tissues. In both tissues, how- ever, desiccation of the fibers, or cells, as well as mechanical injury, contributes to changes in texture. When animal or plant tissues are frozen so rapidly that translocation of water does not occur before freezing, many small ice crystals are formed, and these are generally uniformly distributed within the tissues. Such tissues, par- ticularly animal tissues, are much less disorganized, so that a much larger percentage of the water separated as ice is reabsorbed on thawing. In fruit tissues, however, the separation of water is a much more irreversible process, and the improvement in texture as a result of more rapid freezing is not so profound. In general, however, most investigators agree that a uniform dis- tribution of small ice crystals is desirable. The best ways to obtain this uni- formity are: 1. Increase the rate of heat removal. 2. Decrease the size of the unit frozen. 3. Select containers which have the largest surface area per unit volume, thus presenting more surface to the refrigerant. 4. Select the most efficient heat-transferring medium as a refriger- ant. 5. Reduce stagnant films and air pockets to a minimum. 6. Use a refrigerant at as low a temperature as is economically jus- tifiable. 8 For discussion of the extensive literature in this field, much of which is still contradictory see the early excellent discussion by: Glennie, A. E., Index to the literature of food investiga- tion. I)q,t. of Scientific and Ind. Research, His Majesty's Printing office. London 1Q2Q pp i-o I lcsshi ;.nd Evers(i9/J7), and Joslyn (1934). COMMERCIAL FREEZING OF FRUIT PRODUCTS 33 The early tendency to focus attention upon low temperatures alone as a means of obtaining the desired rate of freezing (at one time quick freezing was re- stricted to freezing at -400 F) has given way to a more rational evaluation of heat-transfer factors. The changes in the colloidal systems involved are undoubtedly the limiting factors in satisfactory texture retention, but unfortunately our knowledge of the response to freezing of colloids such as occur in plants is insufficient. Although the plant cell consists largely of water, it remains firm because the gels are capable of holding large amounts of water in a rigid or semirigid form. During freezing, these gels become progressively dehydrated, and the frozen system may be considered essentially as a dehydrated gel plus ice. Upon thaw- ing, water is not reabsorbed by plant gels so completely as by animal gels, and a disturbed system results. The degree of irreversibility in this process is de- termined partly by the original concentration of water in the tissues and by its distribution between free and bound forms. The average water content of our common fruits is about 85 per cent as compared with 70 per cent for fish foods and 60 per cent for meat products. The bound water content in berries, i.e., water that will not be frozen at -200 C, was reported by Daughters and Glenn (1946) to vary from 2.2 per cent to 6.7 per cent for 21 samples of strawberries and from 5.6 to 6.1 per cent for two samples of raspberries. There is some evi- dence that the higher the bound water content, the better the texture reten- tion on freezing. Our present knowledge of the freezing and thawing of colloid systems can be summarized as follows (for further details, see Clayton, 1932): When freezing is so rapid as to fix the original spatial distribution of the colloid, thawing may be either rapid or slow and yet reproduce the original structure. With slow freezing, however, resulting in large ice crystals, rapid thawing cannot restore the original condition. If the colloid does not produce a sol or gel with simple contact with water, the rate of thawing is without in- terest subsequent to slow freezing. But if the sol or gel condition follows con- tact of colloid with water, thawing of colloid systems should be slow enough to allow the colloid to take up the wat,:r provided during melting. The behavior of only a few coj'oids on freezing and thawing has been studied— eggs, gelatin, and agar agar. Some observations have been made with pectin. The use of low methoxyl pectinic acids was found beneficial by Buck, Baker, and Mottern (1944), and others, in reducing "run-off" in strawberries, and addition of these pectic substances to the sirup in which the berries were frozen produced an improvement in texture. This was not found to be true of other fruits. The denaturation of colloids by freezing is particularly important in the freezing of cloudy fruit juices where the character of the colloidal material may be so altered that it readily precipitates when the juice is thawed. This effect is particularly noticeable with apple juice, but it has also been observed with grape, berry, and citrus juices (Joslyn and Marsh, 1937). Shrader and 34 CALIFORNIA EXPERIMENT STATION BULLETIN 703 Johnson (1934) ascribe this change in orange juice to the separation of a viscous sirupy fraction, the metacryotic liquid, containing 8 per cent acid and 64 per cent carbohydrate. The quantity of this metacryotic liquid in quick-frozen orange juice is small, but in slow-frozen fruit juices, and on storage at higher temperatures (5°-i5° F), it is formed in significant amounts. In pectinous fruit juices this liquid may form a jelly which will not dissolve upon thawing. Table 7 EXPANSION OF WATER AND SUGAR SOLUTIONS DURING FREEZING* Substance Increase in volume on freezing at 0° to 5° F Water per cent 8.6 10 per cent sugar solution 8.7 20 per cent sugar solution 8.2 30 per cent sugar solution 6.2 40 per cent sugar solution 5.2 50 per cent sugar solution 3.9 60 per cent sugar solution None 70 per cent sugar solution 1 per cent decrease in volume * From: Joslyn, M. A., and G. L. Marsh. Changes occurring during freezing, storage and thawing of fruits and vegetables. Calif. Agr. Exp. Sta. Bui. 551 : 16(1933). Volume.— The increase in volume that occurs during freezing is of importance in selecting the size of container and in determining fill-in weights. Water, unlike most other liquids, expands on freezing. The increase in volume amounts to about 9 per cent. When water, fruit, fruit juices, and sirups are frozen in sealed containers, it is necessary to allow for the increase in volume on freezing to prevent bursting of the containers. Fresh fruits generally contain air in the intercellular spaces in which ice is first formed, and when frozen at relatively high temperatures, the fruits con- tract rather than expand in volume. However, when they are subjected to lower temperatures which freeze the entire fruit rapidly, an expansion in vol- ume occurs. The increase in volume of sirups of various concentrations, when prepared at room temperature and frozen at o° to 50 F, was determined (see table 7). Apple juice increases about 8.3 per cent on freezing and cooling to o° F; orange juice, about 8 per cent. An average increase in volume of 4.0 per cent was found for whole rasp- berries frozen at o° F and of 6.3 per cent for crushed raspberries. At the same temperature, whole strawberries increased 3.0 per cent, strawberries with added sugar in the ratio of 2 : 1 increased 1.2 per cent, and crushed straw- berries increased 8.2 per cent. COMMERCIAL FREEZING OF FRUIT PRODUCTS 35 Owing to the collapse of fruit, release of intercellular gases, and osmotic action of the added sugar or sirup, a decrease in volume occurs upon thawing. The volume occupied by fruit which has been subjected to freezing and thaw- ing is considerably less than that occupied by the fruit when fresh. The average decrease in volume of untreated strawberries was found to be about 6.8 per cent; of raspberries without sirup, 5.6 per cent; and of strawberries packed with sugar in the ratio of 2:1, 4.2 per cent. This decrease is responsible for the apparent "slack fill" in containers of frozen-pack berries. These results rep- resent actual changes in volume of the fruits. Owing to shrinkage in volume upon thawing, a decrease in the depth of berries in the container occurs. The reduction in depth is naturally more pro- nounced for whole, untreated berries than for berries packed in sirup. The apparent decrease in volume of blackberries, loganberries, raspberries, and strawberries in No. 10 friction-top cans, after thawing, was found to be 31.5, 27.8, 38.2, and 26.4 per cent, respectively. In berries packed in sirup, the de- crease in volume varied from none to 3 per cent. In berries packed with sugar, it varied from 4 to 6 per cent, based on the initial volume before freezing. An allowance of 10 per cent of the volume in a given container is usually sufficient when freezing occurs uniformly throughout. Nonuniform freezing, however, may burst even a slack filled and lightly closed container. Ice forma- tion occurs in regions of lowest temperature and continues into regions of higher temperature. In large containers, ice forms at the inner surfaces ex- posed to the refrigerant and moves toward the center, and in liquid products, such as juices, the soluble solids concentrate in the warmer section. It is desir- able to fill the container and freeze it so that it is completely full when freezing is completed, with no air voids present. In partially filled containers, desicca- tion may occur during freezing storage, with movement of water vapor, by sublimation, from the exposed surfaces though the air voids to the coldest surface in the container— usually the upper surfaces— where ice formation will occur. In paperboard boxes, freezing under pressure is the most efficient method of obtaining complete fill and full protection against in-the-package desiccation. Prepackaged foods, quick-frozen under pressure, will be more uniformly frozen, and occupy less volume per unit weight. They will freeze faster than those frozen without pressure, and the packages will retain their shape better even when made with lighter weight chipboard, thus facilitating casing. Weight.— A decrease in the weight of frozen fruit occurs during and after thawing. This decrease is caused by the water which separated as ice during freezing and was not reabsorbed during thawing, by leakage of fluids through tissues injured by freezing, and by the osmotic action of the sugar or sirup. It does not depend entirely upon tissue disorganization, since it is offset in part by absorption of sugar in the case of sugar- and sirup-pack fruits, but it does depend, to a large extent, upon the handling of the product during thawing 36 CALIFORNIA EXPERIMENT STATION BULLETIN 703 and draining. It is difficult to remove all of the added and exuded juice, sirup, or water from the product by draining after thawing. This loss in weight also depends on length of storage and on storage conditions, and usually increases with length of storage. It amounts to about 20 per cent to 40 per cent in samples stored at o° F for a year, thawed for about 16 hours at room tempera- ture, and drained 2 minutes over a i/^-inch mesh screen (Joslyn and Marsh, 1933)- Table 8 COMPARISON OF LOSS IN WEIGHT OF BANNER STRAWBERRIES IN CANE-SUGAR SIRUP AND IN INVERT-SUGAR SIRUP* Concentration of sirup Loss in weight Cane-sugar sirup Invert- sugar sirup 10. . degrees Balling per cent 37.4 34.1 27.5 29.1 26.5 31.7 26.2 per cent 34.3 20 30 40 50 31.3 30.2 29.3 27.8 60 28.8 65 32.3 * From: Joslyn, M. A., and G. L. Marsh. Changes occurring during freezing storage and thawing of fruits and vegetables. Calif. Agr. Exp. Sta. Bui. 551:22(1933). In general, in results reported by Joslyn (1930&) and Joslyn and Marsh (1933), the loss varied with the kind and character of fruit and was greatest in water and least in sirups of certain concentrations. The change in Balling degree of sirup was greater where the change in weight was greater. The loss in weight as a result of the freezing and thawing did not vary in a regular manner with the concentration of sirup and there was no definite relation between loss in weight and concentration of sirup, such as would be expected if osmotic action alone were responsible for the loss in weight. The loss in weight of apricots decreased with increase in the ratio of fruit to added cane sugar, but the results for Banner strawberries were rather variable. The substi- tution of cerelose for cane sugar increased the loss in weight to some extent. The substitution of invert sugar testing 76.3 ° Bal. also decreased the loss in weight. However, in these tests there was no direct relation between the ratio of fruit to sugar and the loss in weight. The data reported by Wiegand (1931) indicate that, in some instances, the loss in weight of berries frozen with sugar increased as the ratio of fruit to sugar decreased, and in others there was no continuous and regular increase. Diehl et al. (1930) report that an increase in the concentration of cane sugar resulted in an increase in the amount of water extracted as shown by increased percentages of soluble solids in the fruit. A further comparison between invert and cane sugar, made up in sirups, is COMMERCIAL FREEZING OF FRUIT PRODUCTS 37 shown in table 8. The increase in loss of weight in strawberries, in some but not all of the concentrations of sirups used, however, was not so large as would be expected from the molecular size of the two sugars. The osmotic pressure of invert-sugar sirup is roughly twice that of cane-sugar sirup. Apparently other factors besides osmotic pressure are involved in loss in weight upon thawing. Similar erratic results were obtained in a test with sliced Phillips Cling peaches. In two cases, the loss in weight in invert-sugar sirup solutions was about three times that in cane-sugar solutions of the same concentration, but in other cases, the loss in weight in invert-sugar sirup was less than in cane- or beet-sugar sirup. The variety of fruit markedly affects loss in weight. With strawberries, it was found to be less in Banner and greatest in Capitola. With freestone peaches, it is greater in Elberta and least in Rio Oso Gem. Clingstone peaches, in general, show less loss in weight than freestones. The loss in weight also increases with increase in length of storage. Relations of Loss in Weight to Texture.— The free drip, or loss in weight during thawing, has often been used as an acceptable index of the change in the colloidal state and degree of disorganization of flesh products. Woodroof (1936) reported that "the quantity of fluid lost by the cell through leakage to the outside, loss of original turgidity and degree of fragmentation of the pre- cipitated protoplasm were in direct proportion." Joslyn and Marsh (1933) found that, in general, there was a complete loss of crispness upon freezing; the juicy portions became soft and flabby, and the fibrous portions became tough. There was not an even distribution of tenderness such as has been re- ported for flesh products. There was some relation between the degree of retention of original shape and turgidity, and loss in weight. The greater the loss in weight, the more severely was the texture disorganized. Fruits packed in sirup retained their structure better than those packed with dry sugar, even though the loss in weight for the latter may have been less. HEAT-TRANSFER DETERMINANTS The temperature changes in the product at the point of slowest rate of cooling are a measure of the suitability of conditions to the preserva- tion of the product and are useful in evaluating the method of freezing used. If the product cools too slowly, it may spoil before it is completely frozen; if it warms too rapidly on thawing, it may spoil during distribu- tion unless rigorous care is taken to insure proper refrigeration. The rate at which a commodity freezes is dependent upon two sets of factors, those concerned with the nature of the product— its thermal properties, dimen- sions, and initial temperature— and those concerned with the freezing medium —its thermal properties, temperature, and velocity. Extensive investigations ol the thermal properties of fruit products were conducted in this division in 1930-1932, and the results obtained have been reported previously (Joslyn and 38 CALIFORNIA EXPERIMENT STATION BULLETIN 703 Marsh, 1930, 1932&, 1932c). The data, together with other information, are summarized in the following discussion. Stages of Freezing.— Temperature measurements show, as indicated in figure 4, that the temperature changes in the center of a container of food during freezing are naturally divided into three intervals. During the first interval, Fig. 4.— Temperature changes in No. 10 cans of water, cane sugar, and sirups during freezing. (From Bui. 551.) the chilling stage, the temperature is lowered gradually to the initial freezing point, i.e., initial stage of ice formation. Heat transfer, even in this initial stage, is mainly by conduction, so that the specific heat and over-all heat con- ductivity are the limiting factors at a constant temperature gradient. It is for this reason that, unlike heat transfer during heat processing, the rate of cooling is faster as the sugar content is increased. The specific heat of water and sugar solutions, in the range of 32c-8o° F, was found by Short (1944) to be as follows: PRODUCT SPECIFIC HEAT Water 1.00 5 per cent sugar solution 0.97 10 per cent sugar solution 0.94 15 per cent sugar solution 0.92 20 per cent sugar solution 0.86 30 per cent sugar solution 0.85 50 per cent sugar solution 0.76 The specific heat for several fruits was reported as follows: apples (83.7 per cent water), -0.89; grapes (79.3 per cent water), -0.85; oranges (80.7 per cent COMMERCIAL FREEZING OF FRUIT PRODUCTS 39 water), -0.91; peaches (89.6 per cent water), -0.91; strawberries (90.9 per cent water), -0.96; figs (90 per cent water), -0.71. As cooling proceeds, a temperature is finally reached at which ice separates, and in the second interval, the zone of maximum ice formation, the center of the mass is at a fairly constant temperature since the material has cooled to its freezing zone and heat is liberated by change of water to ice. The extent of ice formation in this zone is illustrated by the following data for sugar solutions obtained by Short. PER CENT FROZEN IN REGION OF FREEZING POINT TO 20° F 100 96.7 93.4 91.2 85.6 75.1 53.8 PER CENT CONCENTRATION OF SUGAR 0 (water) 5 FREEZING POINT 32 31.4 10 15 30.9 29.8 20 29.5 30 27.5 50 24.9 In general, as the sugar content increases, the freezing zone becomes lower in temperature, and the period of fairly constant temperature is shorter. The heat evolved during ice formation for water is 143.4 B.T.U. per pound; the latent heat of fusion for foods was found by Woolrich (1933) to be the latent heat of water multiplied by the percentage of water present in the food. Thus, for fruits of 90 per cent moisture content, it would be 143.4 x 90/100 or 129 B.T.U. per pound. In the third interval, during which ice formation continues but at a lower rate, the temperature drops and gradually approaches that of the refrigerant. This zone represents essentially the cooling of the frozen product and tempera- ture changes in it are slower than in that above freezing. In this stage, the cooling of frozen sirups is slower than that of ice and decreases as the sugar concentration increases. The slow cooling in this stage is due not only to the lowered specific heat, and heat conductivity, but also to the lower temperature gradient. The specific heat of the frozen sirup is not constant in this range, but decreases with decrease in temperature. At o° F, Short (1944) gives the specific heat of the frozen sirups, and several fruits, as follows: SIRUPS, PER CENT SPECIFIC HEAT FRUIT SPECIFIC HEAT Apples 0.69 Grapes 1.01 Peaches 0.77 Figs 0.76 Oranges 0.76 Strawberries 0.68 5 0.52 10 0.63 15 0.70 20 0.77 30 0.92 50 1.07 < X E-i Q 0)0)00lOM H0t-iHt-C0C0iH00Ot> t- J3 H H i-i t4 THrHrHiHrH tHiH-CO •3 03 .S bo 5 «s to s.s © 8 © o r fa ^^ m Oi CD OS W oqco cocooooooooo^ooi>r-4 c>od cdt>c^t>doso6o6odt>d C © CO CO CO CO COCO COCOCOCOCOCOCOCOCOCOCO '3 © © 1 1 ■8 88 CO 4 G5 -* II 1 1 1 1 1 1 1 1 1 1 co |p cot>cocococooqt>oqeoi> OJOO lOCOt>t>OC5l>i>t>t>CS CO fa CO CO COCO COCOCOCOCOCOCOCOCOCOCO -co ja « © s * c g e8 O o'TJ» *• o.S g £$ ^ oo oo t-00 t-t>t>t>©G500G5C}OOt> • ^< «MN J3 »H sg© £*£ - o 3 •a o Q, & p 1 & O, Q, Q, O, P. >? 0 E d =1 S S EJ =J i_ S-, J_ l~, M •i-l •!-< «n «rH -n E fcH *- • i3 H rlH H pq CO V M CO CO CO CQ CQ CO •g Pi .. 8 «o p a .. ■g CO CO "* lO " j) c8 cS (S c8 fl fl fl fl fl CD CD CD V CO O <„ c m o o o o o I'S a a> ft S os w w c«4Ti P,DiP,CJ,Oi2±i ai O. <^ c S '3 ^ Q 3 * »fl0^j3^^,£j goOOOO'w^ •2 3^ fe .is .-t; .13 a ^ co rti w co t- a »: r b £ o t-H K £ £ 2& 03 & & & £ £ c hi £ £ £ 6 5 spa. rt CM I! o * •§* r ea . « .3 . . ft i .§ 2 ^ fa H I COMMERCIAL FREEZING OF FRUIT PRODUCTS 41 In general, the concentration and type of soluble solids present influence the thermal properties of fruit and fruit products more than cellular structure. During defrosting in No. 10 cans, the rates of temperature change in water and sugar solutions, which are typical of other products, are shown in figure 5. The rate of temperature rise in general was affected by soluble solids, as above. Temperature Changes in Various Products.— In general, the rate and nature of temperature changes were found to be similar to those in sugar solutions. The solid and semisolid products cooled more slowly to the freezing zone, and Fig. 5.— Temperature changes in No. 10 cans of water, cane sugar, and sirups during thawing. (From Bui. 551.) below, than did water. Where the separation of much ice occurred, the prod- ucts thawed more slowly than did water. Products such as Royal Anne cherries in sirup did not differ appreciably in behavior from pasty products, such as prune pulp. Typical of the data are the rates of temperature change in berries packed with sugar or sirup, in No. 10 cans in still air, shown in table 9. The data indicate how the proportion of fruit to sugar or the concentration of sirup used affects the rate of heat transfer. No very marked differences were found in the rates of temperature change of the various berries in 400 Bal. sirup. The rates of temperature change increased somewhat with increase in the propor- tion of sugar to fruit and with the concentration of sirup used. The increase in rate of temperature change with increase of concentration of sirup in the presence of berries was not so great as for the sugar solutions that did not contain berries. 42 CALIFORNIA EXPERIMENT STATION BULLETIN 703 Table 10 RATES OF TEMPERATURE CHANGES IN 40 PER CENT SIRUP IN TIN, PAPER, AND GLASS CONTAINERS OF VARIOUS SIZES IN STILL AIR* Container 4-ouncecan 8-ouncecan 6-ounce flat can No. 1 Eastern oyster can No. 1 tall can No. 2 tall can No. 2l/2 can 1-pound flat can No. 10 Sanitary can No. 10 friction-top can 10-pound friction-top can . . . 15-pound friction-top can . . . 30-pound friction-top can . . . 5-galloncan 4-ounce Mono tub 8-ounce Mono tub 16-ounce Mono tub 32-ounce Mono tub 16-ounce Tulip Nestrite cup . 32-ounce Tulip Nestrite cup . 8-ounce Purity P. B 16-ounce Purity P. B 32-ounce Purity P. B 64-ounce Purity P. B 4-ounce glass bottle 8-ounce glass bottle 12-ounce glass bottle 32-ounce glass bottle 16-ounce Mason jar 32-ounce Mason jar 1-gallon jug 5-gallon jug 4-ounce milk bottle Freezing period at 2° F Time to reach 31° F hours H 1M IX IX IX IX IX IX SX SX 4M 7 5M X l l 2 IX IX IX IX 2 2X X 1 IX IX IX IX 3X 5% 1 Time from 31° to 25° F hours X 1 IX IX IX 2 IX X X X X X X X X X X2 IX IX Time to reach 5°F hours 6 sx m sx nx 13 15 12 31 30 40 45 65 61 QX IX ioy2 13K ioh 13 5 14 20 8 9 12X *X 12 28K 63 *X Thawing period at 68° F Time to reach 25° F hours IK 2 2 2 2X 2X 3M 2X 8 IX 9H 15 16 1 IX 2X sx 2X 2X ix 3 SX 5X IX IX 2X sx 2% sx 8 17 IX Time from 25° to 31° F hours X X X X X X 1 X 1 IX IX 2X *x 5X X X X IX X 1 1 3/T X2 X X X IX 3 X Time to reach 65° F h0UT3 6 GX ex ex sx 9X 12 sx 24 23^ 28H 30 47 46 6 7 13 9 12H 8 10H 15 19 5 7 7 10X IX 10 22 45 *X * From: Joslyn, M.A., and G.L. Marsh. Changes occurring during freezing storage and thawing of fruits and vegetables. Calif. Agr. Exp. Sta. Bui. 551:10(1933). Effect of Type, Size, and Shape of Container.— The rate of heat transfer, especially where it is limited to conduction, depends to a large extent on the size and shape of the container. In containers of equal capacity, the greater the surface exposed to the refrigerating medium, the more rapid is the cooling. The rate of heat transfer in 40 per cent sirup in tin cans of various sizes was COMMERCIAL FREEZING OF FRUIT PRODUCTS 43 found not to be directly proportional to the surface exposed per unit volume of contents, but to increase progressively with increase in the size of the con- tainer and with increase in surface exposed per unit volume. The extent of the difference in rate of temperature change between containers of various size and kind is shown in table 10. The rates of temperature change in various paper and glass containers de- creased progressively as the size of the container increased. It is impossible, however, from the above data, to show whether the size, shape, or the material from which the container was constructed was most important in determining the rate of temperature change. It was found that the rates of cooling and Table 11 RATE OF TEMPERATURE CHANGES IN BERRY PRODUCTS DURING FREEZING IN BARRELS AT 0° F* Berry product Time to reach approximate freezing zone Approximate freezing zone Time in freezing zone Total time to reach 10° F Strawberries : Without sugar days 3 3^-4 4 degrees F 27.5-28.5 20-22 23-25 25-26 23-25 27.5-28.5 days 2 4-5 5 4 4 days With sugar 1:1 7 2:1 over 9 3:1 4:1 Raspberries : With sugar 2:1 over 9 Without sugar * Source of data: Diehl, H. C, et al. The frozen-pack method of preserving berries in the Pacific North west. U. S. Dept. Agr. Tech. Bui. 148. warming in the pint cylindrical Purity Paper Bottles, the pint, tall Nestrite cup, the squat, 16-ounce Kleen Kup, and the 16-ounce flat tin can used by the industry for freezing storage of fruits did not vary materially from each other. Because the small containers were of different shapes and wall thicknesses, the effect of the materials, such as paper, glass, and tin, could not be determined accurately. However, it appeared that there was little difference in the rates of cooling and thawing of products in containers of similar size and shape. Effect of Initial Temperature.— The data presented in table 12 indicate the extent to which increasing the initial temperature increases the time necessary to reach the freezing temperature. This effect is more marked in larger con- tainers, and precooling of fruit to be frozen in 50-gallon barrels is necessary to minimize danger of fermentation during freezing. In any case, the fruit should be below 70 ° F at the time of packing. In larger containers, however, the rate of cooling in still air at o° F is very slow. Temperature changes in barreled berries are shown in table 11. In these, the temperature of the fruit at the center may remain above 400 F for over 36 hours. Loss by fermentation 44 CALIFORNIA EXPERIMENT STATION BULLETIN 703 occasionally occurs in fruits packed in this manner, especially when sugar is not added. In 5-gallon cans, 14 to 23 hours are required to cool the contents at the center from 70 ° F to the freezing zone, and the period of time in the freezing zone is from 4 to 1 1 hours depending on sugar content, and the total freezing time is over 36 hours. In 5-gallon barrels, about 20 hours are required Table 12 EFFECT OF INITIAL TEMPERATURE ON RATE OF TEMPERATURE CHANGE IN WATER, SIRUP, AND PRUNE PULP DURING FREEZING* Initial tempera- tUT6 Time Time Time Product to reach freezing Freezing zone in freezing to reach zone zone 0°F degrees F hours degrees F hours hours 179.0 sy2 32 isy2 30 139.3 8 32 19 29H Water 110.6 7K 32 19 28% 73.1 4% 32 18M 26^ 54.8 3% 32 19 2614 ' 39.2 m 32 20 25% ( 181.3 10 16.0-16.3 4H 29 139.7 9H 18.5-20.0 7 29% Sirup, 40 per cent 112.6 8 18.7-21.0 *V2 28% 73.1 ey2 18.7-21.2 12H 29 56.1 5V2 18.7-21.2 12H 28 l> 40.2 4% 18.8-21.2 10 23% t 161.9 16 15.7-16.3 4 B1H 134.3 14 15.7-16.3 6 28% Prune pulp 106.5 12 15.7-16.3 7 28 303^ v v < 73.7 11 15.7-16.3 -7V2 53.6 9 15.7-16.3 m 26% \. 34.1 7K 15.7-16.3 5V2 27K * From: Joslyn, M. A., and G. L. Marsh. Changes occurring during freezing storage and thawine of fruits and vegetables. Calif. Agr. Exp. Sta. Bui. 551:11(1933). 8 to cool to freezing, 28 hours in the freezing zone, and 108 hours to freeze from 700 F to io° F. In 10-gallon barrels, the times for these periods are 36, 32, and 1 24 hours, respectively. Agitation during freezing, such as by rolling, markedly reduces the freezing time. The presence of air spaces in the container, particularly between the inner surfaces and the product, and in the outer wrapper, markedly reduce freezing time. Care should be taken to fill the container completely; have the inner lining, or bag, and the outer wrapper as tight as possible. In plate freezers and in air freezing in. a suitably constructed frame, freezing under pressure will reduce the freezing time from one third to one half of that required for freezing without pressure. COMMERCIAL FREEZING OF FRUIT PRODUCTS 45 Diehl et al. (1930) reported that the addition of 5 pounds of ice to the center of a 50-gallon barrel of raspberries reduced the temperature at the center and increased rate of cooling. The fruit at the center of the barrel packed with ice reached 400 F in one and one-half days, while that in the barrel without ice required more than two and one-half days to reach the same temperature. Precooling the berries before packing is a more desirable practice. Fig. 6.— Multiplate quick freezer in loading position. (Courtesy of Birdseye Frosted Foods.) Effect of Freezing Medium.— Except in the case of fruit frozen in barrels for preservers' use, still air is not used commercially in freezing as it is a par- ticularly poor heat-transferring medium. Air blast freezers, either of the short tunnel type, or the more efficient, progressive cross-draft type, and contact freezers, such as plate or immersion freezers, are commonly used. In the air freezers, advantage is taken of the increase in rate of freezing with increase in air velocity. Perry (1938) has shown that doubling the air velocity can be expected to increase the heat transfer coefficient about 60 per cent and to re- duce the freezing time between 27 and 40 per cent. Doubling the air velocity, however, produces four times the velocity energy and friction loss and requires eight times the fan power. Not only is the initial and power cost of fans greater, but the fan energy which is dissipated as heat must be removed by the refrig- eration system. In commercial practice the most economical air velocity for small packages (about 1 or 2 pounds) and for tray or belt freezing is 650 lineal 46 CALIFORNIA EXPERIMENT STATION BULLETIN 703 feet per minute. For larger containers (about 20 pounds) air velocities as high as 2,000 lineal feet per minute have been suggested. Where air is used as a freezing medium, particular care should be taken to obtain uniform distri- bution of air over all the exposed surfaces of the product to be frozen, other- wise much of its effectiveness will be lost. The temperature of the air also affects rate of freezing but its greatest effect is in the zone of maximum ice formation and in the final cooling. In the initial stages, lower temperatures do not accelerate cooling so much. Thus Perry found that an air temperature of -100 F will give about 18 per cent faster initial cooling, 28 per cent faster freezing, and 48 per cent faster final cooling than an air temperature of o° F. This effect is used to advantage in the multi- stage, cross-draft freezers in which the air temperature is progressively de- creased in each succeeding stage (Finnegan, 1938). In air blast freezers of the tunnel type, air temperatures of -300 F are used to obtain freezing times of about 3 hours for 1 -pound packages, and 6 to 8 for 2- to 5-pound ones. Air as a freezing medium is poor in comparison with liquid refrigerants. Because of its relatively low specific heat (0.238), a large volume of air must be recirculated (approximately 50 cu. ft. to remove 1 B.T.U. per degree of air temperature rise) to maintain a desirable rise of the air temperature (40 F) during its passage over the food being frozen. Liquid refrigerants, such as brine or alcohol, are more efficient, but their use is limited to rigid, liquid- tight containers (Finnegan, 1935). Canned fruits and fruit products may be quickly and economically frozen either in a tubular freezer, in which the liquid refrigerant is rapidly passed over the surface of the can as it moves through, or in an agitating immersion freezer, in which the cans are rolled or carried through the refrigerant by a spiral conveyor. The freezing medium used in these immersion freezers is a denatured alcohol of the following com- position: 100 gallons of ethyl alcohol, 5 gallons of methyl alcohol, 5 gallons of ethyl acetate, and 1 gallon of aviation gasoline. This alcohol medium evaporates from the surface of the container without leaving an objectionable residue such as brine does. The freezing of food in sealed tin cans, by immersion in alcohol, makes possible a considerable reduction in freezing time as compared with an air blast freezer. The viscosity of alcohol (Wells, 1946), in contrast to that of brine, remains low at temperatures as low as — 1000 F and thus reduces the cost of pumping. Although the specific heat of alcohol is low, and large volumes must be circulated per ton of refrigeration, the rate of heat transfer is much higher for alcohol than for brine at — 300 F. Calcium and sodium chloride brines are corrosive both to the freezing equipment and the cans, and leave a residue which makes them sticky and rusty. Alcohol, on the other hand, evaporates rapidly and is noncorrosive. Ethyl alcohol is usually adulterated with methyl alcohol for use in freezing plants. Methyl alcohol has a high specific heat, but in nst be used with care because of its toxicity. The alcohol residue should be removed From the cans before casing. COMMERCIAL FREEZING OF FRUIT PRODUCTS 47 Freezing by direct immersion of fruit in a refrigerant, such as a 57 per cent sirup at o° F, can also be used, but at present such use is limited (Woodroof, 1939). FREEZING STORAGE CONDITIONS Fluctuations in temperature of frozen fruits during storage may cause changes in texture, appearance, and palatability. Storage temperature should therefore be maintained as constant as possible in the storage room, in transit by refrigerated cars, in the warehouse, and at the retail outlet. Although frozen fruit and fruit products packed with sugar or sirup may be stored at io° F for considerable periods of time, storage at — 50 F or o° F is preferred, particularly for the product packed in consumer-size containers. Fluctuations of temperatures during storage, sufficient to cause defrosting and refreezing, should be avoided, as this will cause growth of ice crystals and damage to texture. The degree of fluctuation which produces such damage, however, is not definitely known. It will depend, in general, on the percentage of water remaining unfrozen in the product and the rate at which it changes with change in temperature. In general, the lower the storage temperature, the less will be the change in percentage of water unfrozen with change of temperature. The temperature of the air surrounding the frozen product, par- ticularly if it is packed in large containers or in well-insulated fiberboard cases stacked closely, may fluctuate appreciably without producing significant fluctuation in temperature of the product (in the center of the case or interior of the stack) owing to the slow rate of heat transfer. Rapid fluctuations in temperature are less harmful than slow fluctuations occurring in long cycles. Hustrulid and Winter (1943) reported that fluctuations of 8° to io° F in air temperatures do not cause appreciable changes in appearance and palatability of frozen fruits and vegetables stored at 50 F or below. Fluctuations in storage temperature are also undesirable because they pro- duce desiccation of the product even in hermetically sealed containers. There will be an appreciable transfer of moisture vapor from the warmer surfaces of the contents to the colder surfaces of the inner walls where it will deposit as ice. The water so lost by sublimation will not be reabsorbed by the product. The storage temperature should be maintained as constant as possible, prefer- ably within ±2° F, and the temperatures at all points in the stacks and in the room should be uniform. If the storage room is refrigerated with coils (over- head or side), the temperature of the refrigerant in the coils should not be more than 50 F below that of the air. Where temperature differences exist, there will be conditions conducive to moisture transfer, by sublimation, from the ice surfaces at higher temperature to the colder surfaces. In-the-package desiccation can be further decreased by eliminating air voids in the container, and filling and freezing under conditions which assure that the container is completely .filled after expansion in volume on freezing has occurred. The 48 CALIFORNIA EXPERIMENT STATION BULLETIN 703 more uniform the temperatures at all points in the product and in the freezing storage room, and the less these temperatures are allowed to fluctuate, the less will be the water loss in the product. The desiccation problem is more important in frozen vegetables than in fruits packed in sirup. Woodroof and Shelor (1947) recently reported that strawberries, black- berries, raspberries, and peaches frozen without sugar retained their color, flavor, and texture better after 12 months in storage at -100 F than at -200 Fig. 7.— Freezing storage of cased products, showing method of stacking and use of dunnage. (Courtesy of Birdseye Frosted Foods.) to 0° F, o° to months' storage at -100 F than after six months at io° F. All of the fruits deteriorated more rapidly at a temperature fluctuating from o° to io° F than when held constant at io° F. Blackberries and raspberries stored at — io° F were superior to those stored at higher temperatures, but the differences were not so great as with strawberries and peaches. Peaches at fluctuating tempera- tures deteriorated more rapidly in flavor and color than did other fruits and also lost quality more rapidly. Frozen foods should be protected against temperature rise during transit from processing plant to central warehouses and from warehouse to distrib- COMMERCIAL FREEZING OF FRUIT PRODUCTS 49 utor. To minimize this factor, insulated refrigerated cars and trucks should be used for transit and distribution. The product should be cooled to as close to — io° F as possible before loading into the precooled, refrigerated car. Re- frigerated cars for fruit express are satisfactory for shipment of frozen food if salt is added to the ice in the bunkers. In recent investigations (Johnson, 1943) on the the most efficient refrigera- tion to be used for the rail shipment of frozen foods at different seasons of the year, it was found that in summer, for both end-bunker and top-bunker refrig- erated cars, best results were obtained by precooling the cars for 24 hours before loading, by filling the bunkers with ice (crushed ice for overhead bunker cars, and coarse ice for end-bunker cars), and by adding 30 per cent salt. Under these conditions, an average air temperature of about 150 F could be obtained before loading. Loss of refrigeration, ranging from 30 F to 160 F occurred during loading, but this could be avoided by using a portable tunnel of kapok or dry zero insulation which has a tight fit from the opening in the cold stor- age warehouse into the car. With 20 per cent added salt, the air temperature was about 50 F higher. The cars were 15 days in transit, under standard refrig- eration, and the bunkers were filled to capacity with ice and salt at all regular icing stations. These were located at approximately 24 hours of running time apart. The commodities shipped reached New Jersey with a maximum tem- perature of 150 F in the top-bunker cars with 30 per cent salt, as compared with 200 F for the same type car with 20 per cent salt. In the end-bunker cars, the maximum temperatures were about 50 F higher in these tests. During ship- ment in winter, 20 per cent salt would be sufficient. Recent investigations made by the U. S. Department of Agriculture Bureau of Plant Industries have stressed the importance of zero precooling of frozen foods before shipping them over long hauls by freight. The use of 2,000 pounds of dry ice per car, in addition to the regular ice and salt refrigeration placed in cartons on the top layer of a carload, failed to lower appreciably the over-all temperature and actually resulted in the subnormal functioning of the car. No marked benefit was obtained by covering the top and bottom of the load with heavy paper so that air would circulate around rather than between the cars. A temperature rise from — io° to + io° F in transit is permissible if the prod- uct is quick- frozen upon arrival to an average temperature of o° F by storage in a sharp-freezer room at — 20 ° F. Cased frozen foods should be stored so that they are protected by an insulating layer of air from warmer surfaces, such as the walls, doors, or floors. Dunnage or pallets should be used to provide such air spaces during transportation in refrigerated cars. If it is necessary to sharp-freeze upon arrival at central warehouses, circulation of air about the stacks should be facilitated. If this is not possible, loose stacking is best. For best color, flavor, and texture retention, the fruit frozen for consumer use should be maintained in the frozen condition until ready for serving and should be served immediately after defrosting, while it is still cold. 50 CALIFORNIA EXPERIMENT STATION BULLETIN 703 CONTAINERS AND PACKAGING MATERIALS AND PRACTICES Containers for frozen fruit products should prevent absorption of flavors and odors from the storage atmosphere; keep out oxygen; and prevent loss of volatile flavors and moisture. They should also be liquid- tight and rigid, for ease of handling. The inner surfaces must not flavor the product nor give it an objectionable odor, and must be acid- resistant. Containers must also be easy to fill, shaped to freeze quickly, and easily stacked. This section discusses various types of containers from the standpoint of these requirements. In our experience, no container fulfills these requirements so well as the hermetically sealed can, particularly one of rectangular shape. Although cans were introduced very early in the history of the industry, and the 30-pound enameled can with slip-over or wide push-in cover is still favored for the freez- ing of fruit products for subsequent processing, cartons are very generally used for retail-sized packages. The smaller sized cans for retail distribution, at first, did not meet consumer acceptance. Their early use caused some losses because consumers tended to associate cans with imperishability and stored them at room temperatures. For this reason, a wide variety of paraffined, paper- board containers (cylindrical or cup-shaped) and folding, rectangular paper- board cartons with inner bags, or liners, and overwraps were introduced and have come into wide use for a variety of products. These containers, however, are not so airtight, nor so moisture-vapor-proof, and do not lend themselves to automatic filling and mechanical handling. During the period 1931-1941, cans were used for the freezing of citrus juices, and both the consuming public and retail distributors were gradually instructed in the proper storage and use of frozen foods. During the war as a result of a shortage of tin plate, glass containers were used to a limited extent, and fiberboard cartons were intro- duced as a substitute for the 30-pound can. At present there is a growing in- terest in the use of cans for consumer-sized packages of frozen-fruit products, and one California processor of fruit filled his entire 1946 pack in rectangular cans which were frozen by immersion in refrigerated alcohol. A new, com- posite fibermetal container has a rectangular, paraffined fiberboard body with metal ends which are automatically sealed by roller-sealers. This was intro- duced during the 1946 season and proved to be an acceptable semirigid sub- stitute for the tin can. A large proportion of "frozen-pack" fruits is packed in paraffin-coated fir barrels of 30- or 50-gallon size. But 5- and 10-gallon kegs are also used. The 5-gallon can, fitted with a 6-inch friction seal in the top and coated inside and out with a protective enamel (usually berry enamel), and the 30-pound and smaller sized cans are in wide use. During the war years, a 30-pound fiberboard carton came into use. This has an inner bag of heat-sealing, double-walled, COMMERCIAL FREEZING OF FRUIT PRODUCTS 51 Fig. 8.— Closures for the 30-pound and 5-gallon can developed for freezing fruit. (From Cir. 320.) Fig. 9.— Types of containers used for retail-sized packages. CALIFORNIA EXPERIMENT STATION BULLETIN 703 Fig. 10.— Filling and sugaring 25-pound packs of apricots, in large cartons. (Courtesy of Western Canner and Packer.) Fig. 11.— Wrapping machine for cartons, with photoelectric cell for overwrap adjust- ment. (Courtesy of Battle Creek Bread-wrapping Machine Co.) moisture-vapor-proof, laminated, regenerated cellulose film. When properly sealed, it is fairly satisfactory although not so rigid and sturdy as the can. It is rectangular in shape and lends itself well to stacking. The outside dimensions are 6s/4 " x 12" x 13I/2" an<^ tne volume is approximately 1090 cu. in., or 36 cu. in. per pound of fruit. The bulk of fruit in the consumer-sized packages is frozen in folding paraf- fined paperboard cartons, usually of the top-opening or top-fill type. These are used with a heat-sealing inner bag of moisture-vapor-proof plastic film COMMERCIAL FREEZING OF FRUIT PRODUCTS 53 or with an inner liner. A wide variety of plastic films has been developed. They have good water-vapor resistance and are flexible at low temperatures (Aikeen, 1946; Rabak and Stork, 1946). The more important of these are the regenerated cellulose films coated, to make them heat-sealing and water-vapor- resistant, with cellulose acetate or polyvinylidine chloride (cellophane or Saran-coated cellophane), rubber hydrochloride film (pliofilm), and rubber films (cryovac). To improve the water-vapor-proof qualities of this container, it should be overwrapped with a suitable grade of paraffined wax paper (Mc- Coy et al., 1946). Paraffined paper is preferred for overwrap as the label can be printed on it. The quality of the seal as well as the water-vapor resistance Table 13 EFFECT OF PRESSURE FREEZING ON PACKAGE SIZE FOR STRAWBERRIES IN 1-POUND CONTAINERS Method of freezing and dimensions of container Exposed area Total carton area Volume Area per unit volume Pressure-frozen in plate freezer : 5H"x3',x2" sq. in. 63.3 76.1 76.6 80.4 75.3 sq. in. 89.8 101.6 100.9 106.8 98.0 cu. in. 30.7 37.9 38.4 40.9 36.8 sq. in/cu. in. 2.06 Air blast freezer : 5M"x4^',x4M" 2.02 Wx4K"xl3/" 1.99 5^"x4M"xlM" 1.97 6"x3U"xlM" 2.05 of the film affects moisture vapor loss, and too often in practice, the value of the liner or overwrap is lost by faulty sealing. There is a wide variation in the type and dimensions of packages used for frozen fruit. Thus in a recent survey (Anon., 1946a), 9 sizes of 1 -pound con- tainers were reported, ranging from 5I/4" x 4" x ij4" to 6" x 4" x i?4". Most of the packers freezing in the package without pressure use a 1 -pound carton having the dimensions of 5*4" x 4" x i?4", containing 37 cu. in. per pound of fruit. The 5-pound carton is 12" x 8i/£" x 3" (282 cu. in., or 56 cu. in. per pound) and the 10-pound carton is \oy2" x 12" x 4" (504 cu. in., or 50 cu. in. per pound). The effect of pressure freezing on size of container is shown in the analysis of several packers' containers shown in table 13. The data presented here indicate that pressure freezing results in a saving of 19 to 25 per cent in volume per pound and from 11 to 16 per cent in total carton area for a top-opening carton. The Eastern Frozen Food Association, after a nationwide survey among 700 processors and packers to determine the dimensions of a standard, complete functional carton for frozen foods, proposed that the following types of cartons be used (Fabian, 1946). 54 CALIFORNIA EXPERIMENT STATION BULLETIN 703 1. A 12-ounce package measuring 6" long by Wl" wide by VA" high containing 36.75 cubic inches and holding 12 ounces of frozen vegetables or 16 ounces of fruit. 2. A 40-ounce package measuring lO1/^" long by Wl" wide by 2" high containing 126 cubic inches. 3. A 10-pound package measuring \Wl" long by 12" wide by 4" high containing 504 cubic inches. All of the above sizes are multiples in size and shape so that it is possible to provide a single carton holding a given quantity of the various sizes, and to pack combinations of standardized retail packages in the same carton without lossof space. For example, a carton 12" x ioi/2" x 16" would hold four 10- pound packages, twenty-four 40-ounce, or fifty-four 12-ounce packages. Standardization of the above type in size and shape would result in a rec tangular package capable of being filled economically, and in space conser- vation during transportation, storage in low-temperature warehouses, in retail storage and display cabinets, and in home lockers and refrigerators containing o° F storage units. In addition to the rectangular cartons, paraffined tubs and cups with slip-in covers are used to a limited extent. These have a tendency to leak, particularly when the lid is improperly inserted, and are not so economical of casing re- quirements. The packaged frozen fruits are placed in fiberboard cases for shipment. Usually a 100-pound test, solid fiberboard is used as this has proved stronger than the corrugated fiberboard container. These cases should be so designed as to minimize air spaces between individual cartons and to allow complete filling. The side and end flaps on top and bottom should be cut so that they just meet in the center without gaps, and a good grade of low-temperature- proof adhesive should be used in sealing them. As an added precaution, par- ticularly for long storage, the edges and sides of the case should be sealed with a wide, moisture-vapor-proof adhesive tape; the asphalt-laminated kraft type has been used for this purpose. If the containers are overfilled or the freezing conducted so that ice formation begins at one localized surface and subse- quently extends throughout, bulging will occur. This bulging will cause diffi- culties in casing and subsequent storage and will necessitate the use of oversize packing cases. Bulging can be prevented by selecting the fill-in weights, size of container, and rigidity of container walls suited to the particular freezing system used. Where inner bags are used, air must be squeezed out of the top surface just before the bags are passed through the heat sealer. The heat-seal- ing units should be located on the conveyor belt so that a minimum of head- space is present in the sealed bag. Where freezing is carried out under pressure, lighter, less rigid paperboard and smaller containers and packing cases may be used. COMMERCIAL FREEZING OF FRUIT PRODUCTS 55 THE FREEZING OF FRUITS The remainder of this bulletin is a handbook of freezing practices and processes for individual fruits and for purees, jelly and jam bases, juices, and juice concentrates. (Tropical and subtropical fruits are listed alphabetically in the section beginning on page 83.) The more important kinds of California fruits commonly frozen, the recom mended varieties, average yields per acre, production districts and harvesting seasons are given in table 14. Table 15 shows the percentage losses in weight of fruit during preparation for processing, from which the general yields may be estimated. Fig. 12.— Unloading apples preparatory to inspection. (Courtesy of Western Canner and Packer.) APPLES Varieties.— Since apples are chiefly frozen in slices for use in pies, only varie- ties which make good pies should be chosen. Tressler and Evers (1947) state that the Greening is the best variety to freeze in the eastern states, but Baldwin, Northern Spy, Winesap, Stark, Jonathan, Rome Beauty, and Cortland are also good. In the Pacific Northwest, Yellow Newtown, Winesap, Spitzenburg, Stay- man, and Jonathan are considered superior to the other varieties for freezing. In Utah, chiefly Yellow Newtown, Rome Beauty, and Winesap are used. In Q w Q w a s o o w to s s > O W N O !zi O ■I O 02 s ° En si 3 is • coJk * £ E £ »2 3>> U g co£ ££ U. CO s*1 CO -& £ §! CO o 5 •*> "«§ •ago go I §§§ CO CO CO >» o •c S a S o i! £2 U ID ^ i P. W CO •§I'S ls.i S CO CO fl o AH CO o o 5 •O Pi P CO ^a J4 a £ Xi M a CD CD ■c CO M >> ^ o < i U3 _L >» iH *H e3 • p. in « a tH >.2 >> ee ee 1 fc> OQ >» >•* a a aSoo o^3 > S^ o flt!O.S 3£g« £o g| •as O e3 £ a WO •a £w §S £ 0-5,02 flla 111 03 ss o'E w O ■£ «> o-d a a ^ a IS 3 ■a" -s * » o g s>» fl |3|I1 t-J PU02O § 1 £ S £ o §-3 So a 2 u O ■Sow WPkW S 1.8 !§ cej Pi |» +» o « a 60 o S (N 0) c8 CCj +j >V2 00) WW a* 03 C<3 8 a© o5 * W eeTeef M.8J8 P. CD CO IP tH >» 3 §1 T3 CD I •S O o I .J 3 flog ugcf M so Oh CO CO ■a -a s » QQ i-i >; .5 ** x £ tu

H CD Pi .P S3 bo cd 2 -3 •* P bo | Pi CO "o o -o CD J-l Pi '1 2 I- +3 03 CD O CO ■a P o If (H — ■ si gco S a p ** Is v — / TO CO 9S 1 CD e3 «j ■a w Id e8 fa J-gco p 2?-^^ a 3w5 bo ■a 2 .«* CD CD CD III g ill O p > 1»o P P « ocotf *ht!H CD -P bo § i < Is a fa ° OW s w« ■ —a; h .28 otfa ° •£« S« © 2 iigl * a ► § ws-a£ a°"s £^§^ O, bo co ca Sell W 1 plus 1958 gms. sugar and mix slowly to 1020 gms. puree and mix for 20 to 30 minutes Mix to dissolve all sugar and obtain a uniform mixture Package 4156 gms. product Place in subzero storage Source of data: Johnson, G., and M. M. Boggs. New fresh fruit spreads preserved by freezing. Food. Indus. 19(ll):80-83, 201-202. FREEZING FRUIT JUICES The quantity and types of fruit juices frozen can be greatly increased when the current practices of selecting fruit for juice production, of extraction, and of freezing are improved. Large quantities of orange juice, smaller quantities of grapefruit and lemon juices, and limited quantities of other fruit juices are frozen commercially. Failure to use freshly harvested, sound fruit of high quality, and to extract the juice under conditions that would avoid contamination (by metals, un- desirable portions of the tissues surrounding the juice sacs, and air), and improper freezing and storage have resulted in the distribution of products of poor quality (Cruess, 1946; Cruess et al., 1946). The most serious defect 92 CALIFORNIA EXPERIMENT STATION BULLETIN 705 occurring in many of the frozen juices is the marked separation and clotting of the pulp particles, which occurs upon thawing, particularly in citrus juices. In severe cases, the juice, after standing, consists of a clear liquid floating above a sediment of badly curdled and clotted particles. In addition, unde- sirable changes in flavor occur. These are due partly to oxidative changes, which produce stale or terpeney flavors, and partly to nonoxidative changes such as hydrolysis of the precursors of the bitter principles (naringin in grape- fruit, and limonin in orange). The principles and practices of fruit juice production have been discussed elsewhere (Joslyn and Marsh, 1937; Tressler, Joslyn, and Marsh, 1939), and only the general directions are given in this section, together with a brief description of current practices. ORANGE JUICE Difficulty has been experienced during the last four or five years in obtaining fruit of the proper quality in California. Only fully ripe, freshly harvested, Valencia oranges should be used. In investigations carried out during 1931- 1932, it was found that juice extracted within two weeks of storage at 700 F, or four weeks of storage at 32 ° F was better in initial flavor and keeping quality than that extracted later (Joslyn and Marsh, 1934). The fruit should be at its optimum maturity as determined by color, flavor, and growing conditions. Juice extracted from immature fruit (Balling degree below 12 and acidity of over 1) will become better in flavor, and that from overmature fruit will lack orange flavor and be stale. In the Los Angeles and Riverside areas, the fruit is at its best for one month, while that from the coast counties, particularly Orange County, is better in general and is more acceptable over a longer por- tion of its growing season. Packing-house culls, if these are damaged or dis- eased, are not so suitable as orchard-run fruit. The fruit should be stored, as received, in shallow, ventilated bins, and each lot should be analyzed. Fruits should be blended as taken from the bins unless the scale of operation will warrant blending of the various lots of juices to obtain a product as standardized as possible in flavor and color. (Most con- sumers prefer a Balling-acid ratio of about 14 to 1.) The fruit from the bins is passed over a sorting belt to remove moldy or otherwise damaged fruit. The sound fruit is then washed and cleaned by passing it through a soaker bath (preferably one containing a detergent and sterilizing agent [quaternary am- monium salts or hypochlorite]). From the bath, the fruit passes under cold water sprays and over revolving brushes which scrub the surfaces as they are sprayed. The washed fruit is then thoroughly rinsed with clean water and dried by passage over revolving parallel brushes so that the surface is reason- ably dry and free from excessive numbers of microorganisms (bacteria, veast, and mold spores). The clean, dry fruit is then fed by conveyor to a size grader if automatic citrus-juice extractors are used. The latter have displaced semiautomatic COMMERCIAL FREEZING OF FRUIT PRODUCTS 93 hand-fed reamers. At present, the Brown citrus-juice extractor is used although other types of automatic extractors are being developed. Among these is the Pipkin, or "F.M.C.," extractor that extracts juice and oil separately. In the extraction, particular care should be taken to prevent contact with iron or copper surfaces and to minimize contamination with rag juice (the albedo carpellary membranes and seeds contain enzymes, bitter glucosides, and other objectionable constituents). Exposure to air should also be prevented as should contamination with peel oil (present in the outer flavedo layer). The extracted juice should be strained in nonaerating, stainless-steel strainers (usually of the helical screw type) operating in an enclosed, perforated screen housing. The perforations should not be over 0.030 inch in diameter. The strained juice should be deaerated by pumping, with a nonaerating pump, into vacuumized vessels where, by a combination of impact spraying and film (holding vacuumization at 29" vacuum or over), the occluded and dissolved oxygen and other gases are removed. The residual oxygen content of the deaerated juice should be as low as possible for best flavor retention. The deaerated juice can then be subjected to several pretreatments. If it is to be packed under vacuum in hermetically sealed cans, the juice is pumped out of the deaerator into a heat exchanger where it is precooled to 40 ° F or below. It is then filled cold into citrus enamel-lined cans, with a nonaerating bottom filler, and then closed by a vacuum-closing machine. If it is to be packed in a moisture-vapor-proof bag in a paperboard carton, the juice should be flash pasteurized to inactivate pectic enzymes and obtain maximum solution of pectic substances. This is done by passing the juice through a tubular heat exchanger in which it can be brought quickly to 2000 F, held at that tempera- ture for 15 seconds, and then quickly cooled to below 700 F (or flash pasteur- ized at 1900 F for I/2 minute) before being precooled to a filling temperature of not over 400 F. Even in vacuum-closed containers, short-time, high-tem- perature pretreatment is desirable for retention of maximum flavor and ap- pearance. Even after deaeration, the vacuum may be relieved with air without serious impairment of quality. It is preferable to use an inert gas such as water-pump nitrogen, or a mixture of 4 parts nitrogen to 1 of carbon dioxide. The nitrogen carbon dioxide-treated juice, particularly if slightly charged with nitrogen, is of fresher flavor and less subject to oxidative deterioration. It is essential that the juice be frozen as quickly and as completely as pos- sible to prevent separation and accumulation of unfrozen liquid of high solids content. Canned juice is frozen commercially by passing it through freezing tubes in which refrigerated alcohol at -400 F (Finnegan, 1941) is passed in turbulent stream over the surfaces of the cylindrical cans as they rotate slowly. Or the cases may be put into an agitating freezer in which they are rolled over in a moving stream of refrigerated alcohol in a revolving inner drum. This drum rotates in a larger, concentric, hollow steel drum. By rapid rotation during freezing, citrus juices have been successfully frozen in glass (Finnegan, 1944). Air-blast freezing in cellophane bags or in other types of paraffined 94 CALIFORNIA EXPERIMENT STATION BULLETIN 703 paperboard containers has not been satisfactory with precooled orange juice. If the orange juice is previously frozen into a thick slush in nonaerating con- tinuous freezers (such as the continuous tubular ice cream freezer or the plate freezers), or in batch vertical-flooded ammonia freezers, and packed as a heavy orange ice into paperboard containers, it may be frozen in air-blast freezers. The container should be filled so as to allow space for expansion-usually to 90 per cent of its capacity. The inner walls should be inert to the juice, and the container should be capable of being hermetically sealed. At present, only the citrus enamel-lined can and the glass jar fulfill these requirements. The largest sized container that can be frozen sufficiently rapidly when filled with liquid juice is the gallon can. If orange juice is to be frozen in larger con- tainers for subsequent processing, it should first be prefrozen to the consistency of a thick ice. To retain flavor and avoid separation, the frozen juice should be thawed rapidly, preferably with agitation during defrosting, and kept below 500 F. Small containers are best defrosted by storage over night in a household re- frigerator at 450 F. LEMON JUICE Eureka or Lisbon varieties of lemons are both satisfactory for freezing, but particular care should be taken to prevent contamination by peel oil and to reduce exposure to air during extraction and processing. Lemon juice is less stable than orange juice, but suffers less separation. GRAPEFRUIT JUICE The Marsh seedless grapefruit is the only available variety in California. Since it contains appreciable quantities of naringin, an intensely bitter gluco- side, in the early stages, and is flat in flavor when overmature, particular care should be taken to use only the fully ripened fruit, and to avoid cold-stored fruit which yields juice of poor keeping quality. Otherwise, the process is similar to that for orange juice. OTHER FRUIT JUICES The juices of apples, berries, cherries, passion fruit, and pineapple can be well preserved by freezing and, from time to time, limited quantities of these juices have been introduced commercially with some degree of consumer acceptance. Apple juice has proved most acceptable, as evidenced by the con- tinual freezing of small quantities in the Sebastopol region. Passion fruit juice has such a pronounced flavor, and is so acid, that it is usually not acceptable unless sweetened or blended with other juices. Quick-frozen canned Hawaiian pineapple juice, when first introduced commercially over ten years ago, proved popular, but was too high in price to sell readily. It is being produced again. Each type of juice presents special problems, but they all have in common the tendency for fruit pulp separation if slow-frozen or stored above o° F, and COMMERCIAL FREEZING OF FRUIT PRODUCTS 95 a tendency to oxidative deterioration. This is less likely to occur in frozen berry and cherry juices and most likely to be found in pineapple juice. Apple juice reacts somewhere between the two extremes. Apple juice is best when it is extracted from blends of several varieties of apples which are balanced with regard to acid, tannin, and sugar content. When prepared from a single variety, Gravenstein was found to be best, Winesap second, Jonathan third, and Yellow Newtown last in recent tests carried out by Cruess and Glazewski (1945). Mountain apples from Watsonville or Santa Cruz districts were better than those grown in the coastal valleys, in our ex- perience. The apple juice extracted by the conventional process (crushing the whole fruit in a hammer mill, and then pressing the juice out in cheeses in a rack-and-frame hydraulic filter press) is brown in color, oxidized in flavor, and may have an objectionable press-cloth taste. This juice, when strained to remove coarse particles and quickly frozen, is of acceptable flavor to some consumers, and its flavor may be further improved by deaeration before freez- ing. The cloudy juice has more apple flavor than the clear juice, but the latter is more attractive in appearance and, when clarified with pectic enzymes, does not have a tendency to separate or form jelly when frozen. The juice extracted by continuous mechanical disintegrators is apt to be too pulpy, although it shows better color and vitamin retention, particularly when deaerated and flash-pasteurized immediately following extraction. Improved disintegrators are in process of development and, when available, may produce juice of more satisfactory quality for freezing. With berries, the main problem is that of color extraction. This can be obtained by heat extraction of the anthocyanin pigments before pressing, or by freezing. Juice extracted from berries such as Boysenberries, Youngberries, or Loganberries, frozen in barrels (even without sugar), and then thawed, is of excellent color and flavor. With grapes, there are two problems: color extraction in red grapes, and detartration. The former is accomplished by heating after crushing and before pressing, and the latter by freezing in bulk, defrosting, and separating by de- cantation and filtration. Grape juice and other juices may be frozen in large containers for subsequent repacking and out-of-season distribution. In the preparation of pineapple juice, particular care should be taken to use fruit tissues from ripe pineapples, and to avoid excessive exposure to air during processing. The juice is best when it is deaerated prior to freezing. Tomato juice can be preserved by freezing, but its original flavor is so well retained that frozen tomato juice has little acceptance by consumers accus- tomed to the characteristic flavor of the canned juice. For freezing, select sound, uniformly ripe tomatoes of good color and flavor. These should be washed, chopped, and rapidly brought to 2000 F in a flash-coil tank or con- tinuous preheater. This inactivates pectin-esterase and ascorbic acid oxidase. The tomatoes are then cooled, deaerated, and rapidly frozen in hermetically sealed vacuum-closed containers. 96 CALIFORNIA EXPERIMENT STATION BULLETIN 703 The fruit juices lend themselves to the production of blended products. A mixture of grapefruit and orange in equal proportions is quite popular at present. At one time, a blend of grapefruit, orange, and lemon, was marketed. Pineapple blends well.with orange juice, and apple juice may be used for blending with more acid juices. Suitably enameled tin cans or glass containers are more satisfactory for freez- ing these juices than are paperboard containers, unless the paperboard is inert, moisture-vapor-proof, has a liquid-tight coating or liner, and can be hermet- ically sealed. As an added precaution against oxidation, containers for juice should be closed under vacuum. FREEZING SWEETENED JUICES, SIRUPS, NECTARS, PUNCHES, AND CONCENTRATES These products are easy to prepare for serving, have excellent color and flavor, and are growing in popularity with consumers. The addition of sugar markedly improves the keeping quality of fruit juices, and the resulting sirups are attractive for use in beverages. The citrus juices, berry juices (Loganberry, Youngberry, and Boysenberry), and passion-fruit juice produce acceptable sirups for use as beverages because their high acidity and pronounced flavor carry through into the finished beverage. The sweet- ened products can be prepared readily by the addition of 7 pounds of sugar per gallon of juice. The mixture is stirred, with a minimum of aeration, until the sugar is dissolved. It is then deaerated, packaged, and frozen. For best flavor retention, the citrus juices should first be deareated and flash-pasteurized. The sweetened fruit juices will not only keep well in storage, but may also be readily served by preparing the beverage without previous defrosting of the juice. One volume of this frozen fruit juice sirup is added directly to three volumes of water, and stored. The water melts the ice in the sirup and is, in turn, cooled by it. The ease with which these sirups can be prepared for serv- ing, and the excellent color and flavor of the resulting beverage led to its introduction by Joslyn (1930c). At that time, sales tests indicated that the con- sumer acceptance for these sirups was better than for the fruit juices. The fruit juice sirups may be fortified with added color and flavor, but are even better when prepared as mixtures of individual sirups and frozen with small pieces of fruit. Pineapple and orange, grapefruit and orange, and grape- fruit, orange, and lemon blends are very attractive; many others will suggest themselves. In 1930, a punch sirup was prepared which met with some sales success, and now the market is even more receptive to such products. The punch sirup was prepared by mixing equal parts of the following: 1. Orange sirup, prepared by adding 7 pounds of sugar to one gal- lon of extracted orange juice. COMMERCIAL FREEZING OF FRUIT PRODUCTS 97 2. Lemon sirup, prepared by adding 7 pounds of sugar to one gal- lon of burred lemon juice. 3. Red grape concentrate, prepared by concentrating unsulfured Alicante Bouschet, Carignane, or Petit Sir ah grape juice to 70° Bal., in glass-lined vacuum pans. This concentrate, which is used as a color base should not have any objectionable flavors and should be a rich red, not brown, color. Grape concentrate, pro- duced by concentrating grape juice by freezing storage, yields a product of excellent quality. Cruess and Glazewski (1946) recently reported that pulpy fruit juices or nectars can be well preserved by freezing individually or as blends. Apricot nectar was prepared by steam-blanching halved, pitted, ripe Blenheim apri- cots for 3 1/4 to 4 minutes to inactivate enzymes concerned in oxidative discolor- ation. The fruit was then pureed by sieving in a conical-screw, conical-screen continuous extractor, mixed with an equal volume of 150 Bal. sugar solution, and frozen in hermetically sealed containers. This frozen nectar is very at- tractive in color, aroma, and flavor, and can be used not only as a beverage, but also as a base for fruit ice, or in milk shakes, whips, gelatin desserts, etc. The addition of fresh lemon juice (6 per cent by volume) markedly improved the flavor of the apricot nectar, as did also the addition of 0.2 per cent citric acid. Several blends of apricot with other fruit juices were prepared. Of these, the following were most acceptable: 2 parts apricot nectar and 1 part of apple juice, or 1 part apricot puree with 2 parts apple juice; 2 parts Valencia orange juice and 1 part apricot nectar; pineapple and apricot, after acidification. Pulpy fruit juice beverages can be prepared from other fruits but, as a rule, require acidification to be palatable. A blend of pureed peach from highly flavored, yellow freestone peaches, such as the J. H. Hale, Elberta, or Rio Oso Gem, with an equal volume of 150 Bal. sugar solution and 0.3 per cent added citric acid, is excellent in flavor. The ripe peaches were lye-peeled in 2 per cent boiling lye solution, washed thoroughly in cold water, dipped into a dilute citric acid solution (0.5 per cent), halved, pitted, steam-blanched 8 to 10 min- utes, and pureed in a nonaerating, screw-expeller press. Bartlett pears, ripened to optimum canning conditions, peeled, halved, cored, steam-blanched until cooked through, and sieved as above, produced a satisfactory beverage when mixed with an equal volume of 150 Bal. sugar solution and acidified with 0.3 per cent citric acid. Equal volumes of the pear puree and filtered Gravenstein apple juice were found superior in flavor to a blend of the pear nectar with the juice. Blends of apricot and pear, pear and peach, pear and Santa Rosa plum, and pear and pineapple nectars were found to be satisfactory. Plum beverages were made in various ways from red-colored, strongly flavored varie- ties, such as Duarte, Santa Rosa, and Satsuma. Usually, two volumes of 150 sugar solution to one of puree were found best, as addition of only one volume of sugar solution made the resulting beverage too sour and somewhat too thick. 98 CALIFORNIA EXPERIMENT STATION BULLETIN 703 The tropical fruits, such as mangoes, guavas, and papayas can be used indi- vidually in the preparation of frozen pulpy beverages or blended with acid fruit juices, such as citrus, pineapple, or passion fruit. Unheated, pulped mangoes were used in early beverage studies, and later observations were made on papaya beverages. Cruess and Glazewski (1946) prepared guava nectar by cutting guavas in half, steaming till soft, and blending with two to three volumes of 150 Bal. sugar solution. The nectars from varieties of naturally high acidity required no additional acid; those of medium to low acidity re- quired 0.2 to 0.4 grams of citric acid per 100 cc for proper balance in flavor. Blends of guava puree (1 part) with Valencia orange juice (3 parts) or grape- fruit juice (3 parts) were found to be attractive. FROZEN CITRUS JUICE CONCENTRATES The two new processes discussed in this section are suitable for all fruit juices, and further improvement in the quality of such products is expected. While it has long been recognized that fruit juices concentrated by evapora- tion in vacuum have but short storage life at room temperature and above, and that heat injury and flavor loss are most severe in later stages of concentra- tion, effective means to overcome these conditions were not available until recently. Fruit juices, particularly citrus juices, concentrated sufficiently to prevent microbial spoilage (about 700 Bal. or 7:1 concentrates) were subject not only to flavor changes during concentration in the best of the available vacuum pans, but also underwent chemical deterioration (both oxidative and nonoxidative in nature) in color and flavor at room temperature. Storage of concentrates under refrigeration, at 300 F or below, in sealed containers, will prevent changes in color and flavor (Irish, 1925). To avoid changes in flavor that occur when juice in concentrated form is heated, Joslyn and Marsh (1937) recommended that the extent of concentration be reduced (to 4: 1 from 7:1), and the resulting light concentrate be preserved by freezing storage. One of the first successful attempts to obtain better flavor retention during concentration was made by Johnson (1938) and Meinzer (1940a and 19406), who found that by separating the chromatophores from the steamed juice by centrifuging before concentration, flavor losses in concentration would be pre- vented. The clear citrus juice was concentrated to about 72 ° F and stored in large tin containers or barrels, at about 32 ° F, and the chromatophores were preserved by quick-freezing and storage at o° F. Just prior to distribution, the frozen chromatophores were defrosted and blended with the concentrated serum. The pre-separation of chromatophores also facilitated concentration by freezing. Recently two methods have been developed for the production of partly concentrated citrus juices of good flavor. In the first (Stahl and Jordan, 1946), concentrated citrus juices were prepared by a modification of the Gore process COMMERCIAL FREEZING OF FRUIT PRODUCTS 99 of concentration by freezing. Juice was extracted, by reaming, with minimum contamination by peel oil, strained on shaker screens to separate rag and remove most of the chromatophores, deaerated, precooled, and frozen, either in cans or in continuous freezers. The solidly frozen juice was put through an ice shaver or crusher, then fed into a high-speed basket centrifuge in which the partially concentrated juice was separated. The larger the ice crystals, the better was the separation. The freezing and centrifuging cycle was repeated until the desired degree of concentration was obtained; each freezing cycle was reported to triple the concentration. The concentrate was then deaerated and brought to the desired degree of concentration by blending with freshly extracted juice or overconcentrated juice, and incorporated with juice sacs saved from the second screen of the shaker-strainer. The concentrate was frozen at 440 Bal. (4 : 1) at which it was ideal for preservation freezing at o° F. At -50 F, the concentrate was solid, at +50 F, soft solid, and at 1 50 F, liquid. Its vitamin C content was best retained at -50 F, but flavor retention was satisfactory in the range of-50 to +150. This concentrate was readily reconstituted from o° F by addition of water, or could be readily dispensed at soda fountains as a beverage base at 150 F. The loss in solids on freezing-centrifuging was 2 per cent in can freezing, and 5 to 10 per cent in flake ice machine or continuous freezing. In the second method the extracted and strained citrus juice was concen- trated by evaporation in vacuum to 4 : 1, and then mixed with an equal volume of freshly extracted juice. The latter improved the flavor of the concentrate and diluted it so that a 3: 1 concentrate was obtained. This was preserved by freezing, and prepared for use by addition of 2 volumes of water to 1 of con- centrate. Both concentrates are of good flavor and, in time, will displace the frozen single-strength juices. These processes are applicable to all fruit juices. Further improvements in the quality of the juices concentrated in vacuum will be made by the use of recent improvements in fractional condensation of the esters and other volatile constituents evolved during concentration, and then returned to the con- centrate. 100 CALIFORNIA EXPERIMENT STATION BULLETIN 703 LITERATURE CITED Adriano, F. T., A. Valenzuela, and L. G. Miranda. 1933. Studies on the quick freezing of Philippine fruits and utilization of the frozen pack products. Philippine Jour. Agr. 4(1): 41-59. AlKEEN, W. H. 1946. New plastic films— their packaging qualities. Modern Packaging 19 (9): 141-44, 172. Anonymous. 1934. Frozen foods. West. Canner and Packer 10(5): 7-1 9. 1945a. Commercial preparation and freezing preservation of sliced apples. Western Regional Research Lab. U. S. Dept. Agr. Information Sheet A1C-57. Also: Food Indus. 16:805-6, 862-63; and The Canner 100(19):20, 22, 24, 36. (1915) . 1945b. Experiments in freezing tropical fruits. Quick Frozen Foods 7(12):52-53. 1946a. Dimensions of frozen food packages. Quick Frozen Foods 8(10):84. 1946b. Well organized line speeds applesauce freezing at Merced. West. Canner and Packer. 38(13):75-77. 1946c. New Arizona plant freezes dessert grapefruit segments. West. Canner and Packer 38(7):51-53. 1946