UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA Changes Occurring During Freezing Storage and Thawing of Fruits and Vegetables M. A. JOSLYN AND G. L. MARSH BULLETIN 551 MAY, 1933 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA ACKNOWLEDGMENTS The investigations of the rates of temperature changes reported were initiated at the request of and sponsored by the Paperboard Industries Association. During the course of the above investigations the writers were aided in the temperature measurements by D. J. Tilgner, H. B. Wilcox, and D. S. Glenn. The data on changes in pectin were determined by George Beitzel and Piet Quin ; a part of the data on peaches was determined by Walter Richert ; and Marcus Sherrill determined some of the data on the inversion of sucrose. Thanks are due to the Fibreboard Products Company, the Mono Service Company, Inc., the Lily Tulip Paper Cup Company, and the American Can Company for furnishing the containers used in these in- vestigations. Changes Occurring During Freezing Storage and Thawing of Fruits and Vegetables 1 M. A. J0SLYN2 and G. L. MARSHs Owing to the rapidly growing interest in the preservation of fruits and vegetables by freezing storage, investigations have been conducted in the Fruit Products Laboratory, University of California, during the past three years on several important problems encountered in the in- dustry. This report presents the results of preliminary studies upon one of these problems, that of factors affecting the changes during freezing and subsequent thawing. Description of commercial methods and directions for preparing, packing, and freezing fruits and vegetables are given in Circular 320. A number of physical, chemical, and bacteriological changes occur during the freezing and subsequent thawing of fruits and vegetables. It is necessary to know the nature of these changes in order to determine what fruits and vegetables can be preserved satisfactorily by freezing and how they should be prepared, packed, frozen, and stored. The changes that occur serve as criteria of the degree of success attained in preserving these products. They also determine the suitability of the product for use by preservers, jelly and jam manufacturers, ice cream manufacturers, and by the housewife. The physical changes that occur during freezing storage depend chiefly upon ice formation and osmotic action ; they involve changes in volume, drained weight, and texture. These physical changes occur principally during freezing and during thawing, but generally not during freezing storage, especially if a constant storage temperature is maintained. The results of studies on rates of temperature change in various products under different conditions are included with physical changes. The chemical changes that occur during preparation, freezing, and thawing involve changes in composition such as hydrolysis of pectin and sucrose by the hydrolytic enzymes present ; changes in color and flavor due to oxidation, largely arising through the activity of oxidizing enzymes ; and changes in flavor due to anaerobic respiration and other i Received for publication July 25, 1932. 2 Research Assistant in Fruit Products. 3 Research Assistant in Fruit Products. 4 University of California — Experiment Station causes. These changes are especially marked during the thawing period, owing to injury to the tissues during freezing. The variability of much of the material used in these tests due to variety, degree of maturity, and growing conditions, the difficulty of accurately measuring many of the physical and chemical changes which occurred, and the fact that the simple mechanism postulated in some of these tests may not be the one operative in the particular case, intro- duced certain discrepancies in some of the data presented. Nevertheless the data is valuable for it clearly shows the trend of the results in most cases and in others it indicates the extent to which the factors enumer- ated above influence the results obtained. Much work has yet to be done before we know in detail the kind of complex changes occurring in fruits and vegetables during freezing storage and thawing and before we can fully apply this knowledge to practice. TEMPERATURE CHANGES DURING FREEZING AND THAWING* If a food product cools too slowly during freezing, it may spoil before it is completely frozen ; if it warms too rapidly on thawing, it may spoil during distribution unless rigorous care is taken to insure proper re- frigeration. It is necessary, therefore, to know how the size and type of container, the nature of the contents, the type and style of the packing case, and the kind and temperature of the surrounding medium, influ- ence these temperature changes, before definite recommendations can be made for the packing of food to be frozen and for the transportation and distribution of the frozen product. Temperature Changes in Sugar Solutions, Sweetened Fruit Juices, and Other Liquids. — In order to study the effect of the composition and nature of the product on the rate of temperature change during freezing and thawing, determinations were made on the following : water ; sugar solutions of various concentrations ; sweetened juices, and other liquids ; dry pectin ; and dry sugar. No. 10 tin cans filled to 90 per cent of their volume were used in these tests. Freezing was at 0° Fahrenheit, in air, and thawing at 70°. Copper-constantan thermocouples sheathed in Bake- lite tubes were used to measure the temperature at the center of the containers. The thermocouples were equipped with detachable leads so that connections to the potentiometer could be made through a panel board in the freezing room during cooling and freezing and through another panel board in the thawing room for observation during thaw- 4 Some of the results reported here have appeared in other publications. See refer- ence numbers 7, 9, and 10 listed in the terminal bibliography. References 9 and 10 were published subsequent to the date when this manuscript was first submitted for publication. Bul. 551] Changes During Freezing Storage and Thawing 5 ing and warming. The temperature data recorded were either checked with those of a preliminary experiment, or by duplicate determinations made at the same time. The results for sugar solutions, which are typical of the other products, are shown. in figures 1 and 2. The rate of tempera- Fig. 1. — Temperature changes in No. 10 cans of water, cane sugar, and sirups during freezing. ^40 f I -SUGAR 2- WATER 3- 10 PER CENT SIRUP 4-20 PER CENT SIRUP 5- JO PER CENT SIRUP 6- 40 PER CENT SIRUP 7- SO PER CENT SIRUP 8- 60 PER CENT SIRUP 9- 70 PER CENT SIRUP 10 - ROOM TEMPERATURE Fig. 2. — Temperature changes in No. 10 cans of water, cane sugar, and sirups during thawing. 6 University of California — Experiment Station ture change increased with increase in sugar concentration but was not appreciably affected by changes in viscosity. The rate of temperature change was not directly proportional to the sugar concentration. The chief factors that determined the rate of temperature change under the conditions of these experiments apparently were the specific heat of the solutions, their heat conductivity, the temperatures at which ice began to separate in the different solutions, and the amount of ice formed under the freezing conditions. 70 50 ?Z40 1 r 20 \\ \ \ \ \ *>•- i X. \\ \ l-WAl 2- EGC 'ER S -REMOVED FROM Sh ELLS 3- FILL 4- PEI ~T OF SOLE ? 'SIMMON PULP '"•-"""" «... 16 20 HOURS Fig. 3. — Temperature changes in No. 10 cans of water, eggs, fish, and fruit during freezing. The curves for the latter products are typical for solid and pasty foods. In figure 1, three distinct periods defined by temperature changes will be noted. This is particularly true of water. During the first period the center of the can falls in temperature fairly rapidly as the contents lose their heat to the surrounding cold air. During the second period, the zone of maximum ice formation, the center of the can 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. During the third period representing the further cooling of the frozen product, the tem- perature of the contents approaches the temperature of the refrigerant. Sugar solutions differed from water in the following respects. The rate of cooling increased with increase of sugar concentration ; the freezing zone was lower, the higher the concentration of sugar ; the period of Bul. 551] Changes During Freezing Storage and Thawing 7 fairly constant temperature was shorter, the higher the concentration of sugar ; and the cooling of frozen sirup was slower than that of ice. Temperature Changes in Various Food Products. — To determine how the rates of temperature change in pasty, semisolid, and solid products differ from those of sugar solutions, the temperature changes in certain meats, fish, vegetable and fruit products were determined by TABLE 1 Kates of Temperature Changes in Various Berry Products During Freezing and Thawing Berry product Blackberries : Plain In 40 per cent sirup Loganberries : Plain In 40 per cent sirup Raspberries: Plain In 40 per cent sirup Strawberries: With sugar, 2 : 1 With sugar, 3 : 1 With sugar, 4 : 1 With sugar, 5 : 1 In water In 20 per cent sirup In 40 per cent sirup In 50 per cent sirup In 60 per cent sirup In 70 per cent sirup Crushed Sugar, dry Water Freezing period at 2° Fahrenheit Hours to reach approxi- mate freezing zone 7K 7A 8% 834 7% VA 7V 2 7 7 7V 2 ioy 2 m 8A 9 9 8 7A PA Approximate freezing zone, degrees Fahr. 28.4-29.0 25.2-26.0 29.2-29.9 24.4-25.5 29.6-30.8 28.0-28.2 25.2-26.2 26.7-27.2 27.2-27.8 27.3-27.8 30.2-30.8 29.2-29.8 27.8-28.4 27.7-28.0 27.8-28.0 27.3-27.7 29.7-30.1 82 Hours during freezing zone 13J^ 14 16K 12H 17H 12 8 7 7 17A 13H 133^ 11 10 17 27 Total hours to reach 5° Fahr. 33 41 33% 38^ 33 40 33^ 35^ 39 36M 37% 39}^ 40J^ 40M 373^ 34M 37M 7 86% Thawing period at 68° Fahr. Hours to reach 25° Fahr. 73^ &A 534 10 934 4 6 9 9 93^ 8% 5% 2 % Hours from 25° to 31= Fahr. 5% m 7% 3 634 2A 2A \% l 134 13 9% 4 334 23^ 3 uy 2 mi* Total hours to reach 65° Fahr. 22 24 23 23 24 24 22 22^ 233/2 23 30 27 26 24 23 • 24 27 9A 25 * This period in the case of water was from 25° to 33° Fahrenheit. the method described in the foregoing section. Results typical of the various classes of products are shown in figure 3. 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 below, than did water. Where the separation of much ice occurred, the products thawed more slowly than did water. Products such as peas in brine and Royal Anne cherries in sirup did not differ appreciably in behavior from pasty products, such as prune pulp. 8 University of California — Experiment Station Temperature Changes in Berries Packed with Sugar or Sirup — Ber- ries are commonly frozen either with added sugar or in sirup. Therefore, in order to determine how the proportion of fruit to sugar or the concen- tration of sirup used affects the rate of heat transfer, tests were made as in the preceding section. The data obtained are summarized in table 1. No very marked differences were found in the rates of temperature change of the various berries in 40° Balling sirup. This confirms the work of Diehl et al. (3) who report that "there appears to be little signifi- cant difference in the rate of cooling of strawberries, raspberries, or logan or other berries." The rates of temperature change increased somewhat with increase in the proportion of sugar to fruit and with the concentration of sirup used. The increase in rate of temperature change with increase of con- centration of sirup in the presence of berries was not as great as for the sugar solutions that did not contain berries. Diehl et al. (S) also found that the fruit packed with cane sugar cooled slightly faster than fruit packed without sugar ; and the more sugar in the pack, the more rapid was the rate of cooling. Barrels of berries were used in Diehl's experiments and therefore the degree of the effect ob- served was greater than with the smaller containers used in our tests. In order to determine how the change in texture and sugar absorption resulting from the freezing and thawing affects the rate of heat trans- fer, the berries used above were refrozen and thawed. It was found that there was a slight increase in the rate of heat transfer, owing probably to increase in sugar content of the berries and to increase in surface exposed per unit volume, brought about by shrinkage. The most notice- able change was the spreading between the curves of the individual series. EFFECT OF TREATMENT ON TEMPERATURE CHANGES IN SEVERAL VARIETIES OF VEGETABLES In order to determine how the kind of vegetable and methods of pre- treatment affect the rates of temperature change, the temperature changes in asparagus, peas, string beans, and spinach were determined as before. The results obtained are summarized in table 2. It was found that the freezing zone, during which the temperature remained sensibly constant, was lower in the products having lower moisture content. The rate of cooling, however, increased as the moisture content decreased. The differences in the rate of temperature change observed in unblanched vegetables packed in 5 per cent brine, and in Bul. 551] Changes During Freezing Storage and Thawing 9 the ground vegetables were due in part to differences in ingoing weights, arising as a result of filling the cans to 90 per cent of their capacity by volume. Blanching the asparagus had but little effect on the rate of tempera- ture change, whether packed without liquid, plain, or in brine. This was TABLE 2 Kates of Temperature Changes in Various Vegetable Products During Freezing and Thawing Freezing period at 2° Fahrenheit Thawing period at 68° Fahr. Vegetables Hours to reach approxi- mate freezing zone Approximate freezing zone, degrees Fahr. Hours during freezing zone Total hours to reach 5° Fahr. Hours to reach 29° Fahr. Hours from 29° to 31° Fahr. Total hours to reach 65° Fahr. Hulled peas: Ground Plain 7 W 2 8 m VA 7 $A sa 8A 7U 7 7 sa sa SA 7M 29.3-30 29.3-29.8 29.6-30.0 30.8-30.9 29.8-30 3 29.8-30.2 30.3-30 7 29.9-30 3 30 2-30 3 29.5-30.2 29 3-30.3 30.3-30 8 30 3-30 7 31.1-31 2 30.4-30.7 30 5-30 7 31 0-31 1 30 6-31.0 12 8 \ 7\ 15H m 8 10^ 12H 5M J 12 12^ 9* 16M 27^ 28 33 32 25^ 33^ 2VA MA 2m 22 30^ 29^ 33 32 29^ 35 28^ 34 12^ 11 UA 10 9 12M VA 12M 12 7 11 12M UA $A SA im 10A 12H A 2A 1 6 3^ IA *A 2 2A IA m i 6 7H 2V 2 9 4A 26 21A 29A* Cut string beans: Ground Plain 29A* 2iA Plain in brine Blanched Blanched in brine Spinach: Ground Plain Plain in brine Blanched Blanched in brine Asparagus stalks: Ground 29A* 2VA 29A* 29Y 2 * 18 24^ 28 28 29H* Plain 28 Plain in brine Blanched Blanched in brine 29A* 29^* 29A* * After 29A hours, at the conclusion of the experiment, these samples had not reached 65° Fahrenheit. also true of string beans. The blanching of spinach had a marked effect because it wilted the leaves so that more than twice the weight of blanched spinach could be packed in the volume occupied by the raw spinach. Effect of Type, Size, and Shape of Container on Rate of Temperature Change. — The rate of heat transfer, especially where it is limited to con- duction, 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 found to 10 University of California — Experiment Station be not directly proportional to the surface exposed per unit volume of contents, but increased progressively with increase in the size of the container and with increase in surface exposed per unit volume. The extent of the difference in rate of temperature change between con- tainers of various size and kind is shown in table 3. TABLE 3 Kates of Temperature Changes in 40 Per Cent Sirup in Tin, Paper, and Glass Containers of Various Sizes Container Freezing period Hours to reach 31° Fahr. Hours from 31° to 25° Fahr. Hours to reach 5° Fahr. Thawing period Hours to reach 25° Fahr. Hours from 25° to 31° Fahr. Hours to reach 65° Fahr. 4-ounce can 8-ounce can 6-ounce flat can No. 1 Eastern oyster can. No. 1 tall can No. 2 tall can No. 2A 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-gallon can 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 IX IX IX ia \x ZA 2X 4X 4X 7 x i i 2 IX IX IX lA 2 2X X 1 IX IX IX IX 3X 5X 1 1 IX VA VA 2 lA X X X u X A X l X X A A A A IX IX X 6 SA 7X 8A UA 13 15 12 31 30 40 45 65 61 6M 7X 10H 13^ 10^ 13 5 HH 14 20 ZX 8 9 12A 9X 12 2SA 63 6M IX 2 2 2 2X 2X 2A 8 7A ioa 9A 15 16 1 lA 2X SX 2X 2X IX 3 *X 5A IX IX 2X 3A 2X 3H 8 17 IX 1 IX lA 2A 4A 5A \X 3 ZA m $A 12 SA 24 30 47 46 6 7 m 13 9 \2X 8 10M 15 19 5 7 7 10^ lA 10 22 45 4K The rates of temperature change in various paper and glass con- tainers decreased progressively as the size of the container increased. It was impossible, however, in this experiment to show whether the size, shape, or the material from which the container was constructed was the most important in determining the rate of temperature change. It was Bul. 551] Changes During Freezing Storage and Thawing 11 found that the rates of cooling and 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 freez- ing storage of fruits did not vary materially from each other. Because the small containers were of different shapes, and of different wall thickness, the effect of the materials of construction 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. — To determine the effect of the initial temperature on the rate of cooling, water, 40 per cent sirup, and prune pulp packed in No. 10 cans were brought to various initial temperatures TABLE 4 Effect of Initial Temperature on 1 Eate of Temperature Change in Water, Sirup, and Prune Pulp During Freezing Product Initial tempera- ture, degrees Fahr. Hours to reach freezing zone Freezing zone, degrees Fahr. Hours during freezing zone Hours to reach 0° Fahr. 179.0 m 32 18^ 30 139.3 8 32 19 29H Water 110.6 4% 32 19 28% 26H '< 73 1 32 18% 54.8 3% 32 19 26% 392 1% 32 20 25% 181.3 10 16.0-16.3 4^ 29 139.7 9^ 18.5-20.0 7 29% Sirup, 40 per cent 112.6 8 18.7-21 9>2 28% < 73.1 6H 18.7-21.2 12H 29 56.1 5H 18.7-21.2 123^ 28 40.2 4% 18.8-21.2 10 23% 161.9 16 15.7-16.3 4 31^ 134.3 14 15.7-16.3 6 28% Prune pulp 106.5 12 15.7-16.3 7 28 "< 73.7 11 15 7-16.3 W CM 00 rH M< UO I--. CO CO OJ "3 O CM CO 00 CM <-H CO CO CO 2 '; a £ 8 5 c 1 c = -c I 1 > c c 's (- DE 5 6 = t a 1- Qi > C c '5 y. DC £ CM 4 _ 2 ^ ^ e "5 t a > E T C , c - 4 1 ^ c ^ a 3 jd '5 -d 03 8 '3 i ' O . CM 1- CM P, 3 'S 00 '3 CD S 03 Ea >- °°- • £ G "3 "3 [3 a a a a, Q c C 13 a 02 c "3 H e & 2 S t a CL Eh 3 s a: a 03 CM O a a ce G 03 O C 1 C 5 C a a 2 e S C G C "3 b 1 a c C 00 C CM O a a c a 'E a c 3 1 4 c a CM O e E a a a c 3 a 1 % M , G a i 3 Cf ? G C 1 s 3 G 7. ■- 3 a .2 3 a '3 -a > "3 "3 | .S ^ 55 = cq ca _a _a -. A 13 Ph Ph i-l CC CD CD « 5 > 6 > 8 - 11} even though the loss in weight for the latter may be less. It has been our experience that, with the possible exception of asparagus, increasing the rate of freezing by using solid carbon dioxide does not appreciably improve the texture of frozen fruits and vegetables examined. However, the addition of sirup to fruit or of brine to vegetables to be preserved by freezing markedly decreases the degree of breakdown in texture. "Sugar Curing." — Although storing Banner strawberries and Phil- lips peaches in sirup or with sugar for 24 to 48 hours at 32° F prior to freezing in air at 0° F decreased the loss in weight, especially in the sugar-packed fruit, the texture was not materially improved. Wie- gand (12) reported that Oregon (Marshall) strawberries held in sirup or with sugar at 30° to 31° F for 24 to 72 hours before freezing were su- perior in color, flavor, and texture to those frozen without such treat- ment. His results indicate that osmotic equilibrium in sirup and sugar- packed Clark seedling strawberries is reached in about 24 hours at 30° to 31° F. We found that Banner strawberries and Phillips Cling peaches held in sirup or with sugar for 24 to 48 hours at 32° F prior to freezing were not materially improved in texture over similar fruits frozen immediately. However, the loss in weight of the sugar-cured fruit was less than that in fruit frozen without such storage. In a subsequent communication, Professor Wiegand reports that storage for at least 48 hours at 32° F prior to freezing was necessary to obtain the desired effect of sugar curing. Bul. 551] Changes During Freezing Storage and Thawing 27 CHANGES IN PECTIN CONTENT ON FREEZING The change in pectin content in blackberries during freezing was de- termined as follows. As a control or check, one lot of blackberries was crushed, heated to boiling in 8 minutes, boiled 3 minutes, cooled, brought to original weight by addition of water, pressed in a hand press, and the extracted juice pasteurized for 25 minutes at 175° F. One lot of whole and one of crushed berries were stored at 0° to 5° F for a period of 18 days, then allowed to thaw for 24 hours at room temperature and were then treated as previously described for the check lot. The pectic acid content of these samples was determined with the following results. The TABLE 15 Changes in Pectic Acid Content of Blackberries After Freezing and Thawing Sample and treatment Grams pectic acid per 25 cc of original juice* Check 079 Crushed and stored at 0° F 048 Whole, stored at 0° F 0.048 Whole, 1,500 grams with 1,000 grams water and stored at 0° F 058 Whole, 1 ,500 grams with 1 ,000 grams of 40% sirup and stored at 0° F 0.050 0.044 * This is after correcting for dilution of pectin in the original juice by the addition of water, sugar, or sirup. juice from the check lot contained 0.05 gram pectic acid per 25 cc of juice; that from whole, frozen berries, 0.049 gram; and that from crushed, frozen berries, 0.055 gram. Another set of samples was prepared and the pectic acid content of the expressed juice was determined as above after storage for one month. The results of this test are shown in table 15. They are averages of closely agreeing duplicate determinations. Owing to the difficulty of controlling the conditions of extracting the juice from the berries, the results were not entirely consistent. Appar- ently, however, the loss in pectin on freezing is not large. The jellying power of the berry juice as shown by jellying tests was not markedly affected by freezing, storage, and thawing. Diehl et aL (3) working with large quantities found that loss in pectin content of raspberries and strawberries occurred in a few instances, but believed that this loss was due to the longer period of time required to freeze the fruit mass in quantity. 28 University of California — Experiment Station INVERSION OF SUCROSE BY NATURALLY OCCURRING OR ADDED ENZYMES In order to determine the extent of inversion of sucrose by the natur- ally occurring enzymes in certain fruits, partially crushed Banner strawberries, Cuthbert raspberries, and crushed clingstone peaches were mixed with definite, known proportions of sucrose and stored at about 0° F as described below; they were then analyzed for reducing sugar before and after inversion with acid. The strawberries and raspberries were stored for a period of two years, during which time they were subjected to marked fluctuations in temperature, being exposed to as high as 25° F for several days. The TABLE 16 Extent of Inversion of Sucrose in Easpberries and Strawberries Frozen with Added Sucrose Ratio of fruit Per cent sucrose inverted Ratio of fruit Per cent sucrose inverted to sugar Rapid thawing Slow thawing to sugar Rapid thawing Slow thawing Raspberries Strawberries No sucrose added 1 : 1 100.0 14 4 32.4 15 5 24 100.0 17.2 30 48.8 50 568 N 1 2 3 4 5 i sucrose added 1 100 26.0 42.0 56.0 68.5 82.1 100.0 36.6 2 : 1 1 64.6 3 : 1 1 1 83 3 4 : 1 87.1 5 : 1 1 83 berries were removed from storage and one lot thawed by setting in boiling water and another allowed to thaw for 48 hours at room tem- perature. After thawing, the entire contents of the cans were boiled for one-half to one hour in distilled water adjusted to a pH of 6.8 to reduce inversion of sucrose by acid during extraction. The reducing and total sugars in the extract were determined after clarification by the Shaffer and Hartmann method. The results are shown in table 16. It is seen that a considerable inversion of the added sucrose occurred, being more marked with strawberries than with raspberries. A certain loss of added sucrose occurred, which might have been due to the activity of micro- organisms. The per cent of sucrose inverted increased with decrease in amount of added sugar and increased upon prolonging the thawing period. Ripe Sims clingstone and Orange Cling peaches were ground, mixed with varying amounts of sugar and stored at 0° to 5° F for a period of Bul. 551] Changes During Freezing Storage and Thawing 29 about eight months. These samples were allowed to thaw for 16 hours at room temperature ; after which they were extracted and analyzed. The results found indicate practically no inversion of added sucrose. A slight amount of inverted sucrose was found in the lot that was steri- lized by heating for one hour at 212° F prior to freezing, but this was probably formed by inversion during heating. It is worthy of note that the untreated peaches contained a large proportion of sucrose, whereas no sucrose was found in the berries. There is some evidence that persimmons contain an active invertase since no sucrose was found in extracts prepared from persimmon pulp frozen with varying amounts of added sucrose after storage for one year. The extracts were stored at 32° F and protected with added to- luene for a number of months prior to analysis. These preliminary investigations having shown that inversion of pure sucrose occurred in the presence of invertase during freezing storage, this point was investigated further. A number of samples of sucrose solution of different concentrations were prepared, chilled to about 35° F, and adjusted to a pH of approximately 4.5 with acetic acid. They were then divided into equal portions and mixed with such quantities of chilled invertase solutions that the resultant mixed samples contained respectively 0.05, 0.01, and 0.001 mg of pure dry invertase in the form known as invertase "scales" (free from melibiase), per cubic centimeter of solution. The completed samples were then quickly poured into chilled bottles, sealed at once, and immediately placed in the freezing room at about 3° F. Two sets of samples were placed in a refrigerator and cooled to about -40° F with dry ice. One complete set of samples was removed from the freezing room a few hours after sealing, made alkaline with ammonia while being thawed, and polarized in normal- weight solution. Previous investigations had shown that invertase is sufficiently inactivated by ammonia to permit polarization at room tem- perature without material error. This set of samples served as the con- trol. At various intervals of about 2, 4, and 8 weeks sets of samples were removed from storage and the sugars present determined by polariza- tion as before. It was found that sucrose was appreciably inverted at temperatures of 0° to 5° F by invertase in concentrations as low as 0.001 mg per cubic centimeter in as little as 250 hours at the optimum pH of the enzyme. The per cent of inversion increased with time and concentration of en- zyme, and increased also with decrease in concentration of sucrose in solution. 30 University of California — Experiment Station It was found that at the end of two months practically no sucrose was inverted in the samples stored at about -40° F. The rate of inversion of sucrose by invertase at low temperatures is being investigated further. ABSORPTION OF SUGAR BY FRUIT Fruits are generally frozen with sugar or sirup and, as will be shown, a portion of the sugar added as such or in the sirup is absorbed by the fruit during freezing and subsequent thawing. The amount of sugar absorbed can be shown in a number of ways. Assuming that the loss in weight after freezing and subsequent thawing is due to loss of water from, and gain in sugar by, the fruit, the amount of sugar absorbed can be calculated from an accurate estimate of loss in weight of fruit and increase in weight and change in composition of the sirup. However, this method may not be exact owing to the difficulty of completely drain- ing the fruit and to the fact that the simple mechanism postulated above may not hold. It is possible that the cells of fresh fruits are impermeable to sugars and other soluble solids in the cell sap but permeable to water so that only water is extracted by the osmotic action of the added sugar or sirup. However, this selective permeability is destroyed by the killing action of freezing temperatures and the fruit becomes osmotically indif- ferent so that loss of soluble solids as well as water occurs during thaw- ing. The gels present in the fresh fruit are probably changed to sols upon freezing and leak out as such through ruptured tissues during thawing. Assuming that juice and not water is lost, any increase in the concentration of soluble solids present in the fruit tissue is an indication of sugar absorption. Finally, an analysis of the fruit tissues washed with an isotonic solution of salt will indicate the degree of absorption of sucrose although the presence of an active invertase may interfere with this determination. An application of one of the last two methods was made as follows : Sliced Lovell peaches packed in sirups of varying concentrations were frozen at 0° to 5° F and stored for a period of eight months in sealed 8-ounce cans, then thawed at room temperature for 16 hours. The sugar content of the sirup and peach tissue, washed free of adhering sirup with a 5 per cent salt solution, 6 was determined by the Shaffer and Hart- mann method before and after acid inversion in the cold. The refractive index of the sirup, and of the juice pressed from the peach tissue, was determined, and the corresponding per cents soluble solids as sugars 6 The use of an isotonic salt solution for this purpose was suggested to us by H. C. Diehl. Bul. 551] Changes During Freezing Storage and Thawing 31 were obtained from tables. It was found that the sucrose content of the peach tissue increased markedly with increase in concentration of sirup although the reducing sugars remained practically constant, thus indi- cating definite sugar absorption. Similar results were obtained with sliced Phillips Cling peaches. The sugar concentrations determined by refractometer were of the same order of magnitude, but apparently certain discrepancies entered because in a number of samples higher per- centage of sugar was found in the flesh than in the sirup. Further work on this point is now in progress. TABLE 17 Calculated Sugar Absorption by "Sugar Cured" Banner Strawberries Before freezing: Weight of berries, grams Weight of sirup or sugar, grams After thawing: Weight of berries, grams Weight of sirup, grams Balling degree of sirup Calculated data: Weight absorbed by container, grams. Weight lost by evaporation, grams Water withdrawn from berries, grams Sugar absorbed* grams Sugar absorbed, t grams Sugar absorbed, grams (average) Per cent sugar absorbed Stored at 0° Fahr. immediately after preparation Packed in 50° Balling cane sugar sirup 242.7 161.7 144 3 235.0 30 5 5.2 17.2 104.8 9 2 6 4 7.8 3.2 Packed with dry cane sugar in 2 : 1 ratio 243 122.0 148.7 193 47.45 5.8 14 2 121 3 30 3 27 28.6 11.75 Stored at 32° Fahr. for 24 hours prior to storage at 0° Fahr. Packed in 50° Balling cane sugar sirup 241 8 161 163.3 200 6 32 3 5 34 8 95 1 15.7 16.6 16.15 6.7 Packed with dry cane sugar in 2 : 1 ratio 241 5 121.0 188.0 134.1 48.9 5 32.2 105.7 55 4 52.2 53 8 22.3 * Weight of sugar added minus weight of sugar in drained sirup. t Weight of water withdrawn from berries minus weight lost by berries. A calculation of the amount of sugar absorbed by the sliced peaches, postulating the simple mechanism described in the introductory para- graph of this section, was made. It was found that a variable amount of water was withdrawn from the fruit, increasing with increase in con- centration of sirup ; and that a loss of sugar from the peach tissues oc- curred when concentrations of sirup of 40° Balling or less were used, but an absorption occurred in sirups of higher concentration. An extended calculation of sugar absorption by Banner strawberries packed in sugar and in sirup and frozen in air at 0° F with and without preliminary storage at 32° F for 24 hours was made and a summary of the results obtained is given in table 17 to show the method of cal- 32 University op California — Experiment Station culation. The strawberries were packed in paraffin-impregnated tub- shaped paper containers and stored at 0° F for over a year. They were then removed from storage, allowed to thaw for a period of 16 hours, drained over a %-inch mesh screen and the drained weight of fruit, the weight of sirup, and the concentration of the sirup determined. The TABLE 18 Absorption of Sugar by Banner Strawberries (Calculated by "sugar-balance" method) Pack Concentra- tion of sirup, per cent Ratio of fruit to sugar* Loss in weight, per cent Water withdrawn from berries, per cent Sugar absorbed by berries, per centj f 10 37.4 27.4 - 6.2 20 34.1 28.4 - 3.7 Cane-sugar sirup J 30 I 40 27.5 29 1 24 6 27.8 - 0.9 10 50 26.5 28.4 3 7 [ 60 31.7 30.8 1.6 f 10 34 3 25 1 - 5.7 20 31.3 24.8 - 3 1 Invert-sugar sirup t 30 1 40 30.2 29.3 26.2 27.6 - 13 1.7 | 50 27.8 29.1 4.2 [ 60 28 8 31 5 5.7 ' 1 : 1 38 5 44 3 8.5 2 : 1 34 6 40 5 7.5 Cane sugar < 3 : 1 4 : 1 35 5 38.0 37.8 38.2 3 8 1.7 5 : 1 34 7 36 5 3 3 I .... 6 : 1 38.7 38.7 16 1 : 1 31 6 38.9 10.1 2 : 1 36.4 40 6 6.2 Invert sugart { :: ' 3 : 1 4 : 1 30 5 30 35.4 34 3 6 5 5.7 1 .... 5 : 1 34.5 36.6 3.9 { .... 6 : 1 37.1 37.5 2 * The ratio of fruit to sirup was maintained at \ l A '• 1. t Invert sugar tested 76.3° Balling. X Minus sign indicates loss of sugar. results presented in table 17 are averages of three determinations. An appreciable absorption of sirup by the containers took place, as they increased in weight about 5 grams. A loss of water by evaporation also occurred. It is seen that preliminary storage at 32° F before freezing practically doubled the absorption of sugar by the berries. Owing to losses due to absorption by the container and evaporation in storage which can be corrected for only approximately, the absorption of sugar calculated by a "sugar balance" did not agree exactly with that calcu- lated by a "water balance." Bul. 551] Changes During Freezing Storage and Thawing 33 The results of a more extensive investigation on the absorption of sugar by Banner strawberries as calculated by the "sugar balance" method is shown in table 18. The results shown are averages of three determinations. It is seen that both in the cane-sugar and invert-sugar sirups a loss in sugar from the berries apparently occurred in sirups of concentrations up to 40 per cent sugar ; but an absorption of sugar oc- curred at higher concentrations. The absorption of sugar was greater in the invert-sugar sirup packs than in cane-sugar sirup and a more regu- lar increase of sugar absorbed with increase in concentration of sirup occurred. Similar results were found with berries packed in sugar ; the amount of sugar absorbed decreased with increase in the ratio of fruit to sugar, was greater than that found in sirup packs, and the trend was more regular in invert-sugar series than in cane-sugar series. There was more variation in sugar absorbed than in water withdrawn. The water withdrawn increased somewhat, but not markedly and not regularly with increase in concentration of sugar or sirup. The loss in weight found was not equal to that calculated from the difference be- tween water withdrawn and sugar absorbed, although it was of the same order of magnitude. This may be due to certain inaccuracies in results such as failing to completely correct for evaporation and carton absorp- tion losses, or to errors in the postulated mechanisms. The latter is prob- ably the case. Diehl et al. (3) report that "determinations of soluble solids made on the juice taken from the center portions of the berries packed in differ- ent cane sugar concentrations, as well as that pressed from the outer portions of the fruit do not show consistent differences such as would occur if there were an actual penetration of sugar into the fruit tissue." However, in a later personal communication, Diehl reported that sugar penetration was found to take place. OXIDATION CHANGES AND THEIR CONTROL Discoloration of Fruits. — In the presence of air, darkening of the tis- sues, deterioration of color and development of unnatural flavors occur in certain fruits, even at freezing temperatures, owing to the oxidation of certain constituents. This oxidation is due in great part to the activity of the oxidases present in the fruit tissues. Although the fresh fruit is resistant to oxidation, the injury to the tissues by freezing allows mix- ing of the cell contents with consequent rapid oxidation upon exposure to air. As a result, although oxidation occurs during freezing storage, it 34 University of California — Experiment Station is more profound upon thawing. Changes in color in the presence of air during freezing storage occur at a reduced rate but become apparent in the course of two to six months. All varieties of fruit were not found to be equally subject to marked deterioration of color and flavor by oxidation. Figs, red grapes, black cherries, plums, persimmons, and melons are practically free from oxi- dative deterioration during a period of storage of six to eight months. Berries suffer more in flavor than in color. Apples, apricots, avocados, peaches, pears, light-colored cherries, plums, and grapes, are subject to marked deterioration in color and flavor when frozen in air. These fruits apparently contain active oxidases. Discoloration of frozen fruit by oxidation can be prevented (2 ' 6 - 11} by (a) destroying the enzyme by heat; (b) by excluding air by packing under vacuum or submerging in sirup ; (c) by the use of sulfurous acid ; and (d) by the use of strong acids. It was found that the heating neces- sary to destroy oxidases also adversely affected the flavor of the fruit ; and that although exclusion of air by packing fruits in sirup under vacuum or in hermetically sealed containers actually protected the fruit during storage and thawing in the sealed can, oxidation occurred when the fruit was exposed to air during consumption. Discoloration can also be minimized by selection of suitable varieties of fruits, as pointed out by Diehl. (4) To determine the severity of sulfur dioxide treatment necessary to preserve the color of Gravenstein apples, Blenheim apricots, and El- berta peaches, the sliced fruits were dipped for 1, 5, 10, 30, and 60 min- utes, in sulfurous acid solutions containing 500, 1,000, 1,500, and 2,000 p. p.m. of sulfur dioxide ; the samples were then drained and frozen in petri dishes. After 20 hours at 0° F the samples were removed and exposed at room temperature. After 24 hours, it was found that treat- ment for 1 minute in 500 p. p.m. of sulfur dioxide solution was suffi- cient to preserve the color of peaches and apricots ; but 10 minutes was necessary for apples. In another test Blenheim apricots and Elberta peaches were packed in a sirup of 40 per cent sugar containing 200 p. p.m. of sulfur dioxide and in sirup of 40 per cent sugar after being held for 5 minutes in a solution of 500 p.p.m. sulfur dioxide. Both treat- ments were found to preserve the color upon exposure to air during thawing, for over 24 hours. Packing in sirup containing a small amount of sulfur dioxide was found preferable, since the flavor of the product was better. To determine the minimum concentration of sulfur dioxide in the sirup necessary to preserve color, sliced Phillips Cling peaches were packed in hermetically sealed containers in sirup containing 50, / Btjl. 551] Changes During Freezing Storage and Thawing 35 100, 200, and 500 p. p.m. of sulfur dioxide. Sirup containing 100 p. p.m. of sulfur dioxide preserved the color and flavor without adversely affecting the flavor ; but in sirup containing more than this amount of sulfur dioxide, the preservative was detectable by taste. In order to determine the effect of method of lye peeling and of rins- ing in acid, Phillips Cling peaches were lye-peeled by immersion in 2% per cent NaOH at 212° F for y 2 minute and by immersion in 10 per cent lye at 140° F for 2 minutes. The peeled fruit was washed in water and then frozen in sirup of 40 per cent sugar content with and without rins- ing in either 2 per cent citric acid solution or 2 per cent hydrochloric acid solution. Both lye-peeling methods not accompanied by rinsing in acid resulted in marked surface discoloration of exposed peaches. Rins- ing in 2 per cent hydrochloric acid resulted in fruit that was too sour in flavor, while a 2 per cent citric acid did not check the surface darkening. Rinsing lye-peeled Elberta peaches in 3 per cent citric acid was found satisfactory. No difference such as described by Woodroof (13) for El- berta peaches was found between the two lye-peeling methods applied to Phillips Cling peaches. In the course of our studies on temperature changes in foods in 1930 we found that whole peaches frozen in solid carbon dioxide darkened more rapidly and pronouncedly than did those frozen in air at 0° F both during freezing storage and thawing. A comparison between certain clingstone and freestone varieties of yellow-fleshed California peaches was made and in some cases it was found that clingstone varieties dis- colored more than the freestone. Apricots, apples, pears, peaches, and grapes in general were found to discolor more when immature than when mature. This is in accordance with reported experiments of oxi- dase activity in fruits in which it was found that the oxidase activity decreases with maturity. Caldwell, Lutz, and Moon (2 > 11} have reported that the discoloration of peaches of all varieties was most rapid and pronounced in immature fruits and decreased in intensity with advancing ripeness. They found the best stage of maturity for freezing purposes to be one day before full eating ripeness. They found no chemical treatment wholly effective in preventing discoloration and that although exposure to the fumes of burning sulfur or the addition of sodium bisulfite to the sirup complete- ly prevented discoloration, the odor and taste of sulfur dioxide were distinctly evident. Sliced peaches frozen in solid carbon dioxide thawed more quickly than those frozen at 16° to 18° F in air. In hermetically sealed containers the quick-frozen and the slow-frozen fruit was in most varieties indistinguishable from the fresh in appearance, texture, and 36 University of California — Experiment Station flavor ; but in a few varieties the quick-frozen fruit was slightly darker than the slow-frozen. In paper containers opened while still frozen, the quick-frozen material was in all varieties distinctly darker in color than slow-frozen, and this difference became more pronounced during de- frosting. The degree of discoloration and the general quality of the product varied markedly with variety. White-fleshed varieties were found very unpromising and yellow-fleshed varieties differed widely. J. H. Hale, Reeves, Chairs, St. John, and Up-to-Date were found dis- tinctly superior to the other varieties. Flavor Changes in Vegetables. — Oxidative changes in vegetables re- sult in the development of an unnatural haylike flavor rather than dis- coloration. Preliminary scalding or blanching of the vegetables followed by rapid chilling prior to freezing not only destroys the enzymes re- sponsible for this change, but also improves the color of the product. Blanched vegetables are greener in appearance and the brine in which they are frozen is more nearly free from sediment. In preliminary investigations^' 6) it was found that blanching for about 2 minutes in steam or in boiling water, in brine, or in citric acid solution was sufficient to prevent the appearance of these haylike flavors during storage at 0° to 5° F for over eight months. Subsequently J. Barker (1) reported that partial cooking for 8 minutes prior to freezing in water prevented autolytic changes in peas during storage for over four months at 0° F. In order to determine the effect of the length of blanching and to compare blanching in steam and in water, early green peas of good quality in quantities of about 550 grams were blanched for varying lengths of time in steam and in boiling water. Accurately weighed amounts of blanched peas were placed with accurately meas- ured volumes of brine containing 2 per cent salt (sp. gr. 1.014) in one- pound paraffin-impregnated paper tubs and the samples were then frozen in air at 0° F and stored for four weeks. They were then allowed to thaw completely and the drained weight, volume, and specific gravity of brine were determined. The results are shown in table 19. It is evident that all of the samples blanched for a minute or longer either in steam or water gained in weight during freezing and subsequent thawing. It would seem from the data that it is impossible for the peas to absorb more than a certain maximum percentage of moisture. This maximum is greater in the steam-blanched peas than in water-blanched. A loss in the volume of brine occurs in the steam-blanched peas which increases rapidly with length of blanch but reaches a fairly constant amount after 3 minutes' blanching. Similar results were found for peas blanched in water. All Bul. 551] Changes During Freezing Storage and Thawing 37 of the blanched samples were practically identical in texture, but some- what more tender than the unblanched. The quality of the samples was determined after boiling the thawed vegetables in their own brine to which 200 cc of water was added. The cooking period was such that the time of blanching plus that of cooking was 12 minutes. In the opinion of 7 out of 9 persons who judged the cooked peas, those blanched in steam for 30 seconds and one minute were "good." The samples blanched in water for 3 minutes or less were judged as being "very good." A blanch TABLE 19 Effect of Blanching on Change in Weight of Peas and Brine During Freezing and Thawing Before freezing After thawing Percentage gain or loss in weight of peas Percentage Treatment Weight of peas, grams Volume of brine, cc Weight of peas, grams Volume of brine, cc Specific gravity of brine gain or loss in volume of brine No blanch 237.5 150 211 163.0 1.0240 -10.9 + 8.7 Steam blanch: 30 seconds 250.0 140 247.8 130 5 1 . 0240 - 0.9 - 7 1 minute 250.0 140 257.0 126.0 1 0245 + 3.2 -14.3 3 minutes 260.0 150 285.0 116.0 1 0235 + 9.6 -22 6 5 minutes 245.0 150 267.5 116 1 . 0220 + 9.2 -22.6 7 minutes 260.0 150 291.5 112.0 1.0225 +12.1 -25.3 10 minutes 270.0 150 298.8 115.0 1.0220 +10 4 -23 4 Water blanch: 30 seconds 250.0 150 257.0 136.0 1 0240 + 2.8 - 9 7 1 minute 260.0 150 277.3 125.5 1.0230 + 6.7 -14.6 3 minutes 270 150 289.3 122.0 1.0215 + 7.1 -18.6 5 minutes 250.0 150 270.7 1190 1.0195 + 8.3 -20.6 7 minutes 240.0 140 257.0 113.5 1.0180 + 7.0 -22.7 10 minutes 260.0 150 284.5 115.0 1.0180 + 9.4 -23.7 of over 3 minutes either in steam or boiling water was detrimental to the quality of the product. Prolonged blanching resulted in a decided leaching of soluble solids into the water bath and into the brine of those samples which were steam-blanched. Similar results were found with string beans. Subsequent investigations indicated that blanching for 60 seconds in steam retained the flavor of peas, string beans, and asparagus during freezing storage for more than a year. CHANGES IN FLAVOR OWING TO CAUSES OTHER THAN OXIDATION Some fruits and vegetables acquire during freezing and subsequent thawing a peculiar flavor and an odor which are sometimes undesirable and offensive. Weakly flavored fruits such as cling peaches apparently lose all of their characteristic flavor upon prolonged storage, even when 38 University op California — Experiment Station protected from oxidation. Grapes, apples, berries, and cherries some- times acquire a rather undesirable flavor which seems to be more appar- ent in hermetically sealed containers in which they have not been promptly cooled and frozen. It is thought that this may be due to anaero- bic respiration which occurs in the closed containers during freezing. Peaches also on long storage acquire a pronounced benzaldehyde flavor. This is more marked in freestones than in clingstones, and is more marked in exposed peaches although it occurs in those submerged in sirup. Sometimes off flavors are found as a result of the permeability of the paper containers allowing absorption of foreign cold storage odors and flavors, and often off flavors can be traced to absorption of foreign flavors from containers. These are more pronounced upon long storage. It was found that sliced pineapple, which was fair to good in flavor after three or four months' storage, suffered a complete loss in pineapple flavor and became markedly darkened after storage for ten months. This darkening and loss in flavor, however, may be due to oxidation since it was more pronounced in exposed slices of pineapple. Pineapple juice was decidedly not equal to the fresh after prolonged storage, but never- theless had more pineapple flavor than the frozen fruit. SUMMARY AND CONCLUSIONS Observations made during the course of three years' study of a num- ber of the physical and chemical changes occurring during the freezing storage and thawing of fruits and vegetables are reported here as a sur- vey of the field. It is believed that a knowledge of these changes, of the conditions affecting them, and of the agencies causing them will lead to a development of more suitable methods of pretreatment, packing, freezing, and storing these products. The chief factors that determined the rate of temperature change were the specific heat and heat conductivity of the product, the tempera- ture at which ice began to separate, the amount of ice that separated under the freezing conditions, the size and shape of the container, the initial temperature, and the temperature of the refrigerant. The effect of neighboring containers in the case was more marked than the effect of types of cases studied. The amount of expansion in volume on freezing decreased with in- crease in concentration of sugar or soluble solids. Expansion in whole fruit was less than in crushed fruit. Upon thawing, a decrease in orig- inal volume occurred. Bul. 551] Changes During Freezing Storage and Thawing 39 The loss in weight of fruit upon thawing was a function of ice forma- tion and osmotic action and depended upon the type and condition of fruit, the concentration of sugar in the sirup, the proportion of sugar and of sirup used, the kind of sugar, and the rate of freezing. The varia- tion of loss in weight with concentration of sirup was found to be irregular. Blanched vegetables as a rule increased in weight upon thawing if frozen with brine. The increase in weight increased with increase in the rate of freezing. Unblanched as well as blanched vegetables packed without brine decreased in weight. No direct relation between change in texture and loss in weight was found, but in general the greater the loss in weight, the more severe was the change in texture. There was but little loss in pectin by hydrolysis by naturally occur- ring enzymes in berries, but an appreciable inversion of sucrose was found. This was confirmed in investigations with pure invertase. Increase in the concentration of soluble solids in fruit frozen with sugar or sirup was due to penetration of sugar into the fruit as well as withdrawal of water by the osmotic action of the added sugar. Fruit exposed to air during freezing or during or after thawing will darken and discolor and develop unnatural flavors if active oxidases are present. It was impossible to inactivate the oxidases by heat without destroying the delicate fruit flavors, and permanent inhibition of oxi- dase by means of acid or reducing agents such as sulfur dioxide affected the flavor of the fruit adversely to some degree. Oxidative changes in vegetables resulted in the development of unnat- tural haylike flavors. It was found that the development of these flavors could be inhibited and the flavor and color of the vegetable improved by blanching in steam or boiling water. Changes in flavor owing to changes other than oxidation also occurred on prolonged storage. The development of benzaldehyde flavor in peaches and cherries and of off flavors, probably due to anaerobic res- piration, were especially noticeable. 40 University of California — Experiment Station LITERATURE CITED i Barker, J. 1930. Report of the director of food investigation for the year 1930. Section B. Fruit and vegetables. Department of Scientific and Industrial Research. Published by H. M. Stationery Office, London, p. 69-70. 2 Caldwell, J. S., J. M. Lutz, and H. H. Moon. 1932. How to get better quality in frozen peaches. Food Industries 4:402-405. 3 Diehl, H. C, J. R. Magness, C. R. Gross, and V. B. Bonney. 1930. The frozen pack method of preserving berries in the Pacific Northwest. U. S. Dept. Agr. Tech. Bui. 148:1-37. 4 Diehl, H. C. 1932. A physiological view of freezing preservation. Jour. Ind. Eng. Chem. 24: 661-665. s Joslyn, M. A., and W. V. Cruess. 1929. Freezing storage of fruits and vegetables for retail distribution in paraf- fined paper containers. Fruit Products Jour, and Amer. Vinegar Indus. 8(7):9-12; 8(8):9-12. 6 Joslyn, M. A. 1930. Preservation of fruits and vegetables by freezing storage. California Agr. Exp. Sta. Cir. 320:1-35. 7 Joslyn, M. A., and G. L. Marsh. 1930. Heat transfer in foods during freezing and subsequent thawing. I. Tem- perature changes in sugar solutions, sweetened fruit juices and other liquids. Jour. Ind. Eng. Chem. 22:1192-1197. s Joslyn, M. A. 1930. Why freeze fruit in sirup? Food Industries 6(8) : 350-352. 9 Joslyn, M. A., and G. L. Marsh. 1932. Heat transfer in foods. Temperature changes in fruit, vegetable, meat, and fish products during freezing and thawing. Refrigerating Engineer- ing 24:81-88. io Joslyn, M. A., and G. L. Marsh. 1932. Temperature changes in small food containers in fibreboard cases. Refrig- erating Engineering 24:214-224, 234, 236, 239. ii Lutz, J. M., J. S. Caldwell, and H. H. Moon. 1932. Frozen pack: Studies on fruit frozen in small containers. Ice and Refrig- eration 83:111-113. 12 Wiegand, E. H. 1931. The "frozen pack" method of preserving berries. Oregon Agr. Exp. Sta. Bui. 278:1-42. 13 WOODROOF, J. G. 1930. Preserving fruits by freezing. I. Peaches. Georgia Agr. Exp. Sta. Bui. 163:1-46. 14 WOODROOF, J. G. 1931. Preservation freezing. Some effects on quality of fruits and vegetables. Georgia Agr. Exp. Sta. Bui. 168:1-23.