UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CIRCULAR 320 Preservation of Fruits and Vegetables by Freezing Storage 1 M. A. JOSLYN 2 The outstanding feature in the preservation and distribution of small fruits in recent years has been the development of freezing storage. 3 The frozen product more nearly approximates the fresh in color and flavor than does the product preserved by other common means; hence, it is being used extensively in preserves, jellies, jams, marmalades, soda fountain supplies, ice cream, and in pies and other pastries. Distribution of the frozen fruit in small containers by the retail trade has proved successful. Owing to the popularity of the frozen fruits, the interest of manufacturers and growers has been aroused to the possibilities of this method for utilizing surplus fruit and in marketing fresh fruit. Not only the whole fruit but also fruit juices, crushed fruit, vegetables, meat and fish have been preserved by this method, both experimentally and under commercial conditions. In response to numerous inquiries received at this Station this cir- cular has been prepared to present the principles of the method, the extent of the industry, its possibilities and limitations, the present commercial methods used and improved methods developed in the Fruit Products Laboratory of the University of California. Investiga- tions at present under way wilL be reported later in another publi- cation. 1 Supersedes Bulletin 324 of this Station. 2 Research Assistant in Fruit Products. 3 Storage at and below freezing temperatures. University of California — Experiment Station DEFINITIONS AND REGULATIONS The process of preserving- fruit by storage at freezing temperatures is commonly termed the "cold pack" process and the product as cold packed fruit. However, Diehl 4 has suggested '"frozen pack" as being a more acceptable term. This product is defined by the U. S. Food, Drug and Insecticide Administration (1926) as, "The clean, sound product obtained by packing in a suitable container, properly pre- pared fresh fruit, with or without the addition of sugar, and maintain- ing it at a temperature sufficiently low to insure its preservation. ' ' Recently the Secretary of Agriculture has issued regulations 5 whereby frozen-pack fruits are now included under the scope of the U. S. Warehouse Act. In order to be accepted, the fruit must be inspected and approved at the packing plants by licensed examiners and not more than 12 hours may elapse from the time of packing to placing in sharp storage at a temperature not to exceed 0° F. Here the fruit must remain for at least 48 hours after which it may be removed to regular cold storage at not above 20° F. The National Preservers Association has prepared the following recommendations for the guidance of packers : "1. The barrel shall be standard new cooperage, odorless, paraffin lined. 2. The fruit when packed shall be the entire edible portion, ripe, firm and sound, clean, free from dirt, sand, stems, caps and other foreign material. 3. The sugar, when employed, shall be dry, clean, refined standard white granulated sugar. 4. The fruit and sugar, and other substances when employed, shall be handled and packed under sanitary conditions. 5. The tare weight on the barrel shall be the weight of the entire barrel, including both heads. 6. The fruit and sugar shall be evenly packed and distributed throughout the barrel. 7. No substances other than the fruit and the sugar described above shall be packed with them unle'ss specified by the buyer. * Diehl, H. C, J. B, Magness, 0. E. Gross and V. B. Bonney. The frozen pack method of preserving berries in the Pacific Northwest. U. S. Dept, Agr. Tech. Bui. 148:1-37, figs. 1-15, tables 1-10. 1930. s Bureau Agr. Econ. U. S. Dept. Agr. Service and Regulatory Announcement No. 111:1-3 1928. Cm. 320] Preservation of Fruits and Vegetables by Freezing 3 8. If washed, the berries shall be thoroughly drained before packing". 9. The packed and headed barrel shall be placed in freezing stor- age at not over 15 degrees F within 24 hours after the fruit has been picked in the field. 10. The following information must be conspicuously stated on each and every barrel packed : (a) Name and address of the packer. (&) Kind of fruit, name of variety if straight variety pack; other- wise the words 'mixed varieties' must be stated. (c) The proportion by weight of the fruit and sugar employed. (d) Gross, tare and net weights of the packed fruit. (e) Date packed." LOCATION OF THE INDUSTRY The frozen pack method has developed in Oregon and Washington more rapidly than elsewhere because of conditions favorable to berry growing and because increasing production and great distance from Eastern markets have forced the growers to utilize all possible out- lets. Strawberries and raspberries are packed to a greater extent than other fruits. Moderate quantities of loganberries and blackberries and small quantities of black raspberries, currants, gooseberries, prunes, rhubarb and huckleberries are also packed in the Oregon- Washington District. Second in the production of frozen packed fruits is the tri-state district of Delaware, Maryland and Virginia. The principal district is near Norfolk and Baltimore. Some strawberries are frozen in North Carolina, Michigan, Arkansas, Missouri, New York, Tennessee, Ohio, and Louisiana. Red sour pitted cherries are packed in New York, Wisconsin, Michigan and Colorado as well as in Oregon and Washington for the baking trade. In California, considerable amounts of grape juice and orange juice and in Florida, orange juice, are preserved by freezing. Small commercial quantities of peaches have been preserved in this manner in California and Georgia. University of California — Experiment Station EXTENT OF THE INDUSTRY Beginning" in the Pacific Northwest as an industry which utilized fruit unsuited to canning purposes, the frozen pack industry now successfully competes with canning for the finest berries. The quan- tity of fruit handled has increased from about 3,000 fifty-gallon barrels in 1918 to over 75,000 in 1928. Its most rapid growth has been in the last five years. TABLE 1 Production of Fruit Frozen in Pacific Northwest Fruit Gallons of fruit packed 1926 1927 1928 1929 1,658,225 1,803,385 234,425 30,840 423,950 19,850 1,000 43,000 500 2,851,148 557,100 111,958 18,080 112,840 12,500 4,100 31,735 4,788,939 1,499,015 147,340 63,516 106,951 16,097 11,673 46,740 3,521,268 900,027 Loganberries Black raspberries Blackberries 191,342 63,552 363,932 29,958 5,427 63,648 1,700 400 3,525 200 14,503 26,596 1,165 Total 4,215,175 3,703,386 6,655,204 5,168,615 TABLE 2 Production of Frozen Eed Sour Cherries State Number of 50-gallon barrels 1926 1927 1928 21,800 6,500 14,000 3,059 940 8,600 2,000 3,000 2,200 634 16,000 17,000 17,000 400 Oregon and Washington 900 The volume of fruit frozen in the Pacific Northwest is given in table l. 6 Over 5,000,000 gallons of fruit were frozen in 1929, and in s Table 1 has been prepared from data furnished by M. J. Burns, Secretary of the Pacific Northwest Barrelers' Association, giving the quantities of fruit of various kinds, type of package and kind of pack used commercially during the sea- sons of 1926, 1927, 1928 and 1929 in Oregon and Washington. For purposes of comparison eight pounds of fruit were taken to be equivalent to a gallon. Cm. 320] Preservation of Fruits and Vegetables by Freezing 5 that same year about 20,000,000 7 gallons of the same kinds of fruit were canned. The total fruit canned in California, in 1929 amounted to 15,585,734 7 cases or about 93,500,000 gallons. These figures repre- sent roughly the present relative magnitudes of the two industries. In table 2 is given the production of frozen red sour cherries in recent years. In 1928, New York, Michigan and Wisconsin produced most of the pack. PRINCIPLES OF FREEZING STORAGE Methods of Food Preservation. — The commonly used methods of food preservation fall into two general classes. In the first, the micro- organisms capable of causing spoiling of the product are killed and the access of others prevented. In the second, conditions are made unfavorable to the growth and activity of microorganisms. Products preserved by the first general method, that of sterilization, are per- manently preserved. Products preserved by the second method are temporarily and not permanently preserved ; they are liable to spoil if favorable conditions prevail either during packing or distribution. The preservation of fruits, vegetables and their products by freezing storage falls in the second class. Frozen fruits and vegetables are perishable and under suitable conditions will spoil in the same manner as if they had not been preserved originally by freezing. The micro- organisms present in the product are not killed by freezing and are preserved in the same manner as the fruit or vegetable. Role of Temperature. — The deterioration of fruits and vegetables is caused either by rapid softening and decay brought about by the action of certain ripening enzymes or by the action of molds, yeasts and bacteria. The ripening, respiration and normal life processes of the fruit or vegetable and of the microorganisms with which they may be infected are most active at certain optimum or favorable tempera- tures. At lower temperatures the enzyme action and growth of micro- organisms are retarded and if the temperature is low enough they may be entirely inhibited. The temperature used will depend not only upon the product but also upon the desired storage period. At a tem- perature of 10° F low enough to freeze the fruit, molding, fermenta- tion and bacterial spoiling does not occur and the normal ripening and respiration processes are effectively arrested. 7 From: Oanners League of California Bulletin 1021a, 1930. The cases of fruit canned were converted into gallons of fruit by multiplying by 6. 6 University of California — Experiment Station Although freezing temperatures are required to actually stop the development of microorganisms in fruits, Diehl 8 has found that reduc- ing the temperature of the fruit to 40° F as quickly as possibly will very markedly retard the growth and multiplication of organisms and reduce the danger of fermentation and spoilage during freezing. After the fruit mass is frozen, a temperature of 15° to 20° F has been found satisfactory for long storage. According to Diehl, although "the rapid cooling that takes place in barrels of frozen pack berries when exposed to temperatures around 0° F is often desirable for quick retardation of fermentative processes and the development of organ- isms," it is unnecessary to hold the product at such temperatures throughout the storage period. Rapid chilling is necessary, for otherwise a very considerable growth of microorganisms may occur, and there may be deterioration in quality owing to normal ripening processes and anaerobic respira- tion. In large containers where the chilling of the entire mass is slow, marked spoiling may develop under these conditions. Temperature Changes Daring Freezing and Thawing. — The tem- perature changes in the product at the center of the container are a measure of the suitability of conditions to the preservation of the product. If the product cools too slowly it may spoil before it is com- pletely frozen ; if it warms too rapidly it may spoil during distribution. Experiments in the Fruit Products Laboratory have shown that the following factors influence the rate of temperature change during freezing and thawing: 1. Condition and composition of the product: Tests have shown that the heat conductivities of most of the watery fruits are about the same. The differences observed in the rate of heat transfer are prob- ably due to the size and shape of the pieces, as these determine whether the product is loosely or solidly packed, and to their sugar content. 2. The nature of the packing medium : It has been observed that when dry sugar is added the heat transfer is more rapid the higher the ratio of sugar to fruit. On this point the experiments made at this University substantiate those of Diehl. When syrup is added, heat transfer is more rapid in the syrup of higher concentration. In pure sugar solutions, the same relationship holds. 3. The size, nature and shape of the container: For containers of the same material and shape, heat transfer is slower in larger than in smaller containers. In containers made of material which conducts s See footnote 4. Cir. 320] Preservation of Fruits and Vegetables by Freezing 7 heat slowly temperature changes are slower. Since in most foods heat transfer during freezing and thawing takes place chiefly by conduction rather than by convection, the shape of the container, the thickness of container walls and the conductivity of these walls are important in determining the rate of temperature changes. Other factors being equal the container having the greater surface exposed per unit volume is the more desirable in so far as the rate of cooling is concerned. 4. The temperature and nature of the cooling medium : The lower the temperature of cold air surrounding the product and the more rapidly the liberated heat is removed by convection currents the faster is the cooling. Cold air in motion is a better cooling agent than sta- tionary air. Cooling in brine, which has a higher heat capacity than air, is probably faster. In the cooling of 50-gallon barrels of fruit the temperature may remain above 40° for over 36 hours. Loss by fermentation occasionally occurs in fruits packed in this manner, especially when no sugar or syrup is used. Temperature measurements show that the temperature changes in the center of a container during freezing are naturally divided into three intervals. During the first interval the temperature is lowered gradually to the freezing temperature. During the first stage the con- tents lose their heat to the surrounding cold air, the material at the outside of the container losing its heat directly through the walls of the container and that at the center in turn losing its heat to that at the outside. The rate of heat transfer of that portion of the material next to the container walls depends upon the thickness and heat con- ductivity of these walls, and upon the temperature and movement of the surrounding air. The rate of heat transfer from the center to the outside depends upon the total heat conductivity of the food. During cooling the temperature at the center is always higher than that at other portions of the container and decreases as the outside is approached. As cooling proceeds, a temperature is finally reached at which ice separates. This is the freezing point. For water this is 32° F. The freezing point of various fruits and vegetables is shown in table 3 taken from U. S. D. A. Department Bulletin 1133 9 . It has been found that the freezing point of fruit packed in sugar or in syrup is lower than that of the unsweetened fruit, generally 9 Wright, K. C. and G. F. Taylor. The freezing' temperatures of some fruits vegetables and cut flowers. U. S. Dept. Agr. Dept. Bui. 1133:1-8, tables 1-3. 1929. 8 University of California — Experiment Station about 23° to 25° F. The freezing" point of grape juice has been found to be 25° to 27° F. As the sugar content of the juice increases the freezing point decreases. The freezing point of berries packed with different amounts of sugar is shown in table 4. 10 TABLE 3 Average and Extreme Freezing Points of Fruits and Vegetables* Product and varieties Temperatures, deg. Fahr. Aver- age Extremes Mini- mum Maxi- mum Product and varieties Temperatures, deg. Fahr. Extremes Mini- mum Maxi- mum Apples: Summer varieties Fall and winter Bananas (Jamaica) : Green (Peel \Pulp Ripe fPeel \Pulp Blackberries: Black varieties White varieties Logan (Loganberry). Cherries Cranberries Currants Gooseberries Grapes: American European 28.44 28.51 29.84 30.22 29.36 26 00 29.15 28.40 29.51 27.81 27.16 30 21 28.91 28.12 28.21 29.76 30.10 29.15 25.45 28.73 28.12 29.32 27.56 26.28 30.18 28.70 27.85 24.35 28.62 28.87 29.92 30.58 29.53 26.50 29.42 28.63 29.75 28.25 26.93 30.25 29.18 28.37 24.78 Grapefruit Lemons Oranges Peaches (hard ripe) Pears (Bartlett): Hard ripe Soft ripe Pears (unknown variety) Japanese persimmons (Tan- enashi) Plums Raspberries: Red varieties Black varieties Strawberries Chestnuts (Italian) Walnuts (Persian or so-called English) 28.36 28.14 28.03 29.41 28.46 27.83 29.39 28.33 28.53 30.41 28.76 29.93 23.80 20 00 28.00 27.89 27.86 29.09 28.06 27.20 2934 28.07 28.20 30.12 28.24 29.56 23.00 28.50 28.47 28.34 29.74 28.70 28 00 29.53 28.63 28.85 30.50 28.79 30.13 24.20 22.10 Beans (snap) Cabbage (Early Jersey Wakefield) Carrots Cauliflower Celery Corn, sweet Eggplant Kohl-rabi 29.74 31.18 29.57 30 08 29.73 28.95 30.41 30 02 29.65 31.06 29.42 29.95 29.70 28.65 30 .17 29.74 30.06 31 34 29.68 3015 30 00 29.22 30.69 30 22 Lettuce Onions (dry) Onion sets (Yellow Globe). Peas (green) Potatoes Potatoes, sweet Tomatoes (ripe) Turnips 31 20 30.09 29.50 30 03 28.92 28.44 30 38 30 23 31 03 29.69 29 00 29.67 28.80 28 10 30 20 30 16 31 38 30.24 29.90 3025 29.03 28.72 30.67 3048 * Data from U. S. D. A. Dept. Bui. 1133. During the second interval of the cooling period, which is at the freezing point, water separates as ice, first on the bottom and sides of the container. As the ice forms, latent heat is liberated without any temperature change. During this period the center of the container is at a fairly constant temperature, the latent heat liberated being 10 Diehl, H. C, and Lon A. Hawkins. Frozen pack method of storing berries. Siebel Technical Eeview 4(2) : 60-61. 1929. Cm. 320] Preservation of Fruits and Vegetables by Freezing 9 removed by heat transfer almost as rapidly as it is liberated. During this interval the cooling" curve is nearly flat. As ice continues to form, the soluble solids become more concentrated and lower the freezing point so that no more water separates as ice without a further decrease in temperature. As freezing progresses the ice mass grows toward the center and the soluble solids are concentrated in that section. In small containers it has been found that freezing may be so rapid that the liquid is scattered as a mother liquor in a matrix of ice crystals. The period of constant temperature depends upon the freezing point and water content of the product on the amount of water separated and on the ' ' latent heat. ' The temperature of water remains constant until the entire mass is solidly frozen. In fruits, vegetables and fruit products, only a portion of water separates as ice and the period of constant temperature is relatively shorter than for water. TAB*LE 4 Approximate Average Freezing Points of Berries Packed with Different Proportions of Sugar* Proportion of sugar Degrees, Fahrenheit No sugar 3 : 1 2: 1 1 : 1 27^ to 2Sy 2 25 to 26 23 to 25 20 to 22 * Data from Diehl and Hawkins. After ice formation has ceased, the temperature is gradually low- ered to cold storage temperature. This period represents the cooling of the frozen product and is generally slower than that of the fresh product above its freezing point. Role of Air Oxidation. — If fruits are improperly packed, darken- ing of the tissues, deterioration of color and the development of unnat- ural flavors occur even at freezing temperature owing to oxidation of certain constituents of the fruit. This oxidation is due in great part to the activity of the oxidase enzyme system present in the fruit tis- sues, which is not inhibited by storage at temperatures as low as 0° F. While the fresh fruit is resistant to oxidation, the injury to the tissues on freezing allows mixing of the cell contents with consequent rapid oxidation upon exposure to air. Discoloration of frozen fruit by oxidation can be prevented by heating the fruit and thus destroying the enzyme but such a high tem- perature is required that the fruit acquires a cooked taste and is soft- 10 University of California — Experiment Station ened. A better plan for fruits that darken badly is to exclude air by packing the fruit in vacuum or at ordinary pressures in syrup. Since the fruit floats on the syrup some mechanical means should be used to keep it completely submerged if the cover does not accomplish this purpose. Bole of Sugar. — Frozen berries are generally packed with sugar in the Pacific Northwest. Cane sugar is generally used and investiga- tions in this laboratory have shown it to be preferable to Cerelose. The added sugar retards the development of yeasts and molds, reduces the danger of fermentation, and preserves the color, flavor and aroma of the berries. If sugar is used it must be thoroughly mixed with the berries as otherwise it settles out and loses value as a preserving agent. As will be shown later, syrup is preferable to sugar for this purpose. Changes Occurring During Freezing and Thawing} 1 — The physical changes that occur during freezing storage are due to the separation of water as ice during freezing, and to osmotic action, and involve changes in texture, volume, and drained weight. They occur principally dur- ing freezing and during thawing and generally not during freezing storage. Enzymes probably cause the chief chemical changes during preparation, freezing, or thawing. These changes are especially marked during thawing, owing to injury to the tissues from freezing. They do not usually occur during the storage period. Changes in Texture on Freezing. — The separation of water in the form of ice in and between the cells, that is not absorbed on thawing, causes a soft and limp or soggy texture. During freezing, water is withdrawn from the cells to the intercellular spaces (air filled spaces between the cells) where ice first forms. As freezing proceeds, these spaces become filled with ice, the cells are forced apart by the pressure of the ice, and 'death' of the cell occurs. If the temperature is low enough, the entire fruit tissue freezes, the ice crystals being formed within as well as between the cells. The investigations of Chandler, 12 Maximow, and others, have shown that the widespread view that the water freezes within the cells and that its consequent expansion bursts the cell walls is entirely false. Splitting occurs only when the ice which forms here and there in com- paratively large quantities pushes the cells apart. n See the excellent discussion: Glennie, A. E. Index to the literature of food investigation. Department of Scientific and Industrial Research, His Majesty's Printing Office. London, 1929. p. 1-9. 12 Chandler, W. H. The killing of plant tissue by low temperature. Missouri Agr. Expt. Sta. Research Bui 8:143-309, figs. 1-3, tables 1-52. 1913. Cm. 320] Preservation of Fruits and Vegetables by Freezing 11 Quick Freezing. — It has been found that when meats are slowly frozen, water leaves the muscle fibres to form ice between them. The fibres which constitute the substance of the muscle are therefore highly desiccated and on thawing" the water lost during freezing is only par- tially reabsorbed. Injury to the texture on freezing is more than merely a mechanical disrupture of the tissues by the ice crystals. When the rate of freezing is sufficiently rapid, ice is not formed between the muscle fibres but only within them. Such tissues on thaw- ing are much less disorganized than when ice forms between the fibres. This is the principle of the quick freezing process which has recently been applied with such success to the preservation of meats and fish. Owing to the difference in structure between fruits and meats, it is doubtful whether the same process could be successfully used for fruits. In fact, no marked difference in the texture of berries and other fruit was found when frozen in air at 0° F and when frozen very quickly in solid carbon dioxide at a much lower temperature in tests made in this laboratory. Changes in Weight on Thaiving. — A decrease in the weight of frozen fruit and vegetables occurs after thawing. This decrease in weight is due to the water or juice extracted from the fruit during freezing and thawing by the osmotic action of the sugar. Where the product is frozen unsweetened the water separated as ice during freez- ing and not reabsorbed on thawing leaks out through the ruptured tissues and causes a decrease in the drained weight. When sugar is added, water is also drawn from the fruit tissues by osmosis, the water passing out through the fruit and dissolving the sugar to form a con- centrated syrup. Where heavy syrup is used, water will be extracted as with sugar but in lesser amounts. It is theoretically possible by using a syrup of the proper concentration and osmotic pressure to prevent all extraction of water or juice but this is not attained per- fectly in practice because several uncontrollable factors may influence the osmotic pressure in the fruits. The factors that determine the change in drained weight of the frozen foods on thawing may be classified as follows : 1. The nature of the product — whether tender, or tough and fib- rous, or of high or fairly low moisture content. 2. The method of packing — whether in sugar, or syrup, and the relative amounts and concentration of syrup, and the relative amount of sugar added as such or in syrup. 12 University of California — Experiment Station 3. The size of container — since the weight of the fruit above ex- presses juice from that below, this weight varying with the depth of the container. The loss in weight after freezing is less in the 40° syrup pack than in the sugar pack and the loss increases with increase in sugar con- centration or with increase in the ratio of sugar to fruit. In tests made in this laboratory it was also evident that the concentration and amount of syrup in the 2 + 1 pack are similar to those of a 50° syrup pack. Typical drained weights for sugar packed berries as published by Diehl are given in table 5. TABLE 5 Drained Weights of Raspberries and Strawberries Packed with Sugar* Before freezing After thawing Fruit and pack Weight of fruit, pounds Weight of sugar pounds Weight of fruit, pounds Weight of syrup, pounds Degree Balling of syrup Raspberries: 2+1 300 150 242 201 50 2+1 300 150 238 210 50 5 2+1 300 150 297 155 56.4 Strawberries : 2+1 300 150 192 258 49.0 2+1 300 150 164.5 282.5 45.3 2+1 300 150 157 293 43.7 * From U. S. D. A. Tech. Bui. 148. Change in Volume on Freezing. — Water unlike most other liquids expands upon freezing. The increase in volume amounts to about 9 per cent. When water, fruit, fruit juices and syrups are frozen in sealed containers it is necessary to allow for the increase in volume on freezing to avoid the bursting of the container. The increase in volume of syrups of various concentrations when packed at room temperature and frozen at 0°-5° F is less than that of water, becoming according to tests made in this laboratory, prac- tically per cent increase at 60° Balling. Fresh fruits generally contain air in the intercellular spaces, in which ice is first formed. When frozen at high temperatures the fruits contract rather than expand in volume. However, when they are frozen at temperatures at which the entire fruit is rapidly frozen an expansion in volume occurs. Cm. 320] Preservation of Fruits and Vegetables by Freezing 13 The increase in volume of strawberries and raspberries packed untreated when frozen at 0.5° F ranged from 3 to 4 per cent and when crushed ranged from 6.3 to 8.2 per cent in tests made in this •laboratory. Owing to the air liberated in freezing fruits and to air which is present in containers of fruit juices, it is necessary to allow more headspace than is shown by the above figures, especially for fruit juices. Role of Enzyme Activity. — Enzymatic changes in composition such as the inversion of sucrose by invertase, or the hydrolysis of pectin by pectinase may take place during freezing or thawing. The action of invertase, a rapidly acting enzyme, is now being investigated. It has been found that loss in pectin on freezing is not large. The jellying ability of the pectin was also not markedly affected by freez- ing as shown by jellying tests. Diehl in a few cases has found some loss in pectin in frozen berries which he thought was due to the long period of time required to freeze the fruit in large containers. He has not found any other significant changes in the chemical constitu- ents on freezing. Several commercial jelly makers have found it necessary to finish jellies from frozen berries at a higher temperature owing to a possible change in the pectin. However, this difficulty may be due to other causes. Changes in Vitamin C Content of Frozen Orange Juice. — It is well known that vitamin C is readily destroyed by oxidation, especially in alkaline media. It was thought that some loss of this vitamin in orange juice might occur during freezing. However, according to Dr. H. Goss of the Division of Animal Husbandry, the rate of oxidation at 0°-10° F is so slow that unless abnormal conditions are present, no loss occurs during freezing or storage. Recent investigations of the U. S. D. A. Bureau of Chemistry and Soils 13 have shown that storage of foods at low temperatures has but little effect on vitamins A and B although vitamin C is reduced to some extent. Delf 14 has found that orange and lemon juice stored at -10 to -14° C retains half its original antiscorbutic potency when stored for five years and retains all of it when stored for 17 months. For this reason it seems desirable to subject citrus juice to a vacuumization process is Anonymous. Cold storage does not destroy important vitamins in foods. Siebel Technical Review, 5(1) : 9. 1930. i* Delf, Ellen Marian. The influence of storage on the antiscurvy value of fruits and vegetables. Biochem. Journal 19: 141-152. 1925. 14 University of California — Experiment Station prior to freezing. The juice should be prepared in a manner in which it is subjected to' the minimum exposure to air and either sealed and frozen in vacuo or evacuated and frozen in tightly sealed containers. Role of the Container. — Storage tests show that the flavor of the fruit stored in open containers is often rendered objectionable by odors absorbed from fish and other sources. A closed container is desirable not only to eliminate absorption of cold storage odors but also to facilitate shipment. It has been found that the paraffined paper containers are not entirely air tight and should not be used for apples, apricots, cherries, and other products that darken rapidly, since discoloration can occur even in the frozen products. Such fruits are preferably vacuum packed in vacuum closed enameled tin cans. In order to avoid imparting objectionable tastes and odors to the fruit, paper containers should be made from pulp free from natural resins, pitches and tars as well as from other foreign material ; animal glue should not be used in the manufacture of the cartons and the paraffin coating should be made of odorless, tasteless paraffin of high melting point resistant to fruit juices and acids. Comparison of the Sugar Pack and Syrup Pack Methods. — The methods of freezing berries with sugar as commonly used commer- cially is not altogether satisfactory. In many cases the product has been found to be unattractive in appearance because the sugar added often forms a heavy syrup which settles to the bottom, carrying with it the unabsorbed sugar and leaving the upper berries exposed. The exposed berries are generally discolored, are less sweet and possess a disagreeable oxidized flavor. The bottom berries are of better color and fresher flavor but are often too sweet and sometimes too tough owing to the action of the thick syrup. The frozen fruit tends to settle in the container when it begins to thaw. This would give the appearance of a slack-filled container when opened by the consumer although the container had been well filled when packed. The settling is due to the limp texture of the thawed berries and to the extraction of water by the added sugar. This method of freezing used for berries is not sufficiently flexible to be satisfactory for other fruits, especially figs, apricots, and grapes. For this reason it was believed desirable to develop a more general method of packing in which the deterioration in flavor and color by oxidation and the loss in texture would be controlled or entirely eliminated. Cm. 320] Preservation of Fruits and Vegetables by Freezing 15 To avoid some of these difficulties a syrup pack method was devel- oped in the Fruit Products Laboratory in 1919 by Cruess, Overholser and Bjarnason. 15 This method has been found to be superior to and of wider application than the sugar pack method. The following advantages of this method over the sugar pack method have been demonstrated in investigations carried out during 1928 and 1929 : 1. Air discoloration is reduced to a minimum. 2. The syrup 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 syrup 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 caused 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 syrup is a better aid to preservation during freezing than the sugar. It can be chilled before use and acts as a pre-cooling agent. 6. The texture of the thawed fruit is better. 7. The syrup pack is applicable to all fruits. While the syrup pack method is not in general use the results of other investigators have recently confirmed the findings of this lab- oratory and show the superiority of the syrup over the sugar pack method. It has been used successfully by two plants in the Northwest. Behavior of Various Fruits and Fruit Products on Freezing. — It was found that apricots, avocados, berries, cherries, figs, grapes, peaches, plums, persimmons and pineapple as well as apples, pears and melons could be preserved by freezing in syrup either whole or sliced. Frozen figs, grapes and pineapple have been found equal to the fresh product. Although berries retain their full color and flavor, they suffer a loss in texture, as do the other fruits. Apricots, Royal Ann cherries, avocados, apples and pears, because of their tendency to brown, have to be handled in such a manner as to exclude air. Melons become soft and rubbery on thawing. It is believed that plums, persimmons and avocados could best be packed in the crushed form. It has been found that oxidation in fruit such as apples, pears, peaches and apricots occurs only where the fruit is exposed to air. The oxygen or air absorbed in the fruit itself generally does not cause oxidation. Darkening occurs more rapidly when the fruit is exposed to air during thawing than during freezing or storage. Paper containers !5 Cruess, W. V., E. L. Overholser and S. A. Bjarnason. Storage of perishable fruits at freezing temperatures. California Agr. Exp. Sta. Bui. 324:1-20. 1920. 16 University of California — Experiment Station have proved unsatisfactory for such fruits because of ready penetration of air. For this reason these fruits to be sold in the retail trade will be most satisfactory when packed under vacuum in syrup. Fruit which is apt to darken and which is packed in ordinary containers should be consumed in the frozen state. Vacuum-packed fruit is preferable if it is used after thawing. When used for preserves, canned fruit salad, etc., it should be rapidly thawed in hot water and immediately heated to inactivate the oxidase which may otherwise cause darkening". Investigations in the Fruit Products Laboratory by W. V. Cruess and J. H. Irish as well as those carried out by the writer have shown that crushed fruits and fruit juices can be successfully preserved by freezing storage. The color and flavor of such products is indefinitely preserved by freezing storage. Fruit juices have been satisfactorily preserved for more than two years by the writer and for more than five years by Cruess. Keeping Quality of Frozen Fruits. — Small containers of frozen fruits from storage at 0° F, when removed and stored at 68° F showed appreciable molding at the end of three days when berries were packed plain, in water, and in 10° and 20° syrup. The fruit packed in 30°, 40° and 50° syrup molded in four to five days and the fruit packed in 60°, 70° and 2 + 1 molded at the end of seven days. The chief spoil- age in this case was molding. In other cases fermentation occurred followed by souring by acetic bacteria. Fermentation occurred in syrup concentrations up to 30° in three days, at 30° and 40° in four days, at 50°, 60° and 70° in five to seven days. Raspberries have been found slightly more resistant to spoiling than strawberries. Sliced apricots and peaches ferment as readily as the berries. These observations will indicate the keeping quality of the thawed products. COMMERCIAL PRACTICE The commercial methods of freezing fruits and fruit juices are as follows : Packing Berries in the Pacific Northwest. — The process consists of the following steps, some of which are omitted by some packers or in handling certain fruits: (1) washing; (2) sorting; (3) grading for size; (4) conveying to container; (5) addition of sugar; (6) closing of container; (7) transportation to cold storage; (8) chilling in the 'sharp' room; (9) storing at 15-20° F until shipped in refrigera- tor cars. Cir. 320] Preservation of Fruits and Vegetables by Freezing 17 -Barrels used in the Pacific Northwest for freezing berries, 50, 30-, 10- and 5-gallon sizes. Fig. 2. — Cans used for freezing fruit in the Pacific Northwest. Left to right they are the 5-gallon, 30-pound, 15-pound, 10-pound, No. 10 and 1-pound cans. Containers. — A number of kinds and sizes of containers are used (see fig's. 1, 2 and 3). The most common container is a 50-gallon paraffin-coated fir barrel. The 30-gallon barrel and 5 and 10-gallon kegs are also used. Considerable quantities are packed in 5-gallon and in 10, 15, 30 and 50-pound cans. Small paraffined paper cartons and one-pound cans have been used during the past two seasons. Although 18 University of California — Experiment Station Fig. 3. — Small containers used for freezing fruits. most of the fruit is packed in 50-gallon barrels there is a distinct trend toward smaller containers particularly for strawberries as shown in table 6. Although wooden containers, especially the 50-gallon barrel, are still extensively used, the use of lacquered tin cans is increasing. A special 5-gallon can (see fig. 4) fitted with a 6-inch friction seal open- ing in the top and coated inside and out with a protective enamel that prevents rusting and reduces corrosion was first used. With proper care it can be used for several seasons. However, it was not entirely satisfactory for fruit because it was so tightly sealed that the gases given off by the fruit caused bulging of the can making it difficult to TABLE 6 Kinds of Containers Used for Frozen Strawberries in the Pacific Northwest Gallons of fruit Kind of container 1926 1927 1928 1929 1,616,600 11,550 4,110 4,150 2,691,400 18,240 3,720 16,310 3,827,200 20,640 5,430 2,865 608 20,085 910,190 41,138 28,293 2,669,100 15,030 6,000 3,185 30,738 21,815 26,220 95,258 22,375 301,811 29,138 35,800 604 1,053 54,252 139,357 69,178 Cm. 320] Preservation of Fruits and Vegetables by Freezing 19 stack the cans properly in the sharp rooms. For this reason some packers release the gases through a nail hole punched through the lid, the hold being soldered before shipping. Also owing to the shoulder in the top of the can the frozen berries could not be removed without crushing or waiting until the entire mass had thawed. This proved a serious objection where the berries were bought for repacking. Fig. 4. — Closures for the 30-pound and 5-gallon can developed for freezing fruit. The 30-pound enameled can with slip-over cover (shown at the left in figure 4) is replacing the container described above. It permits escape of air and expansion but cannot be stored top side down during shipment. Cans of 15 and 10-pound size have a similar closure. The methods of packing in 50-gallon barrels, known as 'barreling,' are different from those used for the one-pound containers. Proportion of Fruit to Sugar. — At present most frozen pack fruits are packed with dry sugar or without the addition of sugar or liquid. 20 University of California — Experiment Station The proportion of sugar used varies with the fruit packed and the expected storage period. For long storage more sugar is used. The ratio of fruit to sugar is generally expressed thus, 2 + 1, meaning two parts of fruit to one part of sugar. There is a growing tendency to use less sugar than formerly. Over 85 per cent of the strawberries in 50-gallon barrels are packed 2 + 1 and less than 15 per cent as 3 + 1. Although in the 2+1 pack the flavor of the fruit and the flavor and consistency of the syrup are best the berries are larger, firmer and of better color in the 3 + 1 pack in which less syrup is formed. Very few strawberries are packed in the ratios 1 + 1, 4 + 3, 1%+1, 4 + 1 and 5 + 1 and unsweetened or 'straight.' For strawberries in smaller containers the 3 + 1 pack predominates. Raspberries are generally packed either without sugar or 2 + 1 in 50-gallon barrels, the tendency being to increase the amount packed without sugar. In 1926, about equal quantities of raspberries were packed unsweetened and 2 + 1 ; in 1929 about 80 per cent of the pack was unsweetened and the re- mainder 2 + 1. In smaller containers of raspberries the 2 + 1 pack predominates. Loganberries and blackberries are generally packed like raspberries. The 3 + 1 pack predominates for other fruits. The Barreling of Strawberries. — The chief barreling sections are located in Salem, Woodburn and Portland, Oregon. The packing season for strawberries starts under favorable conditions the latter part of May or the first of June and generally lasts five weeks. The Marshall is the principal variety frozen as it is cheaper, sweeter, better flavored, more intensely colored than the Ettersburg which is used for canning. It is readily hulled in the field and is a heavy bearer. Its defects are its irregularity in size and shape and its softness. Preservers find the variation in size and shape a serious defect and are beginning to demand a closely graded pack. For use in ice cream the size and shape of the berry are of little consequence as the berries are crushed before used. Occasionally, Wilson and other varieties of strawberries are used. The berries are generally picked during the day (see fig. 5) and transferred the same evening to the barreling plants. They are usually hulled at the time of picking principally because this practice adds to the growers revenue, is somewhat cheaper than hulling at the plant and tends to relieve congestion at the packing plants. In the field the hulled berries are packed in wooden baskets, 5x5x2 inches, holding one pound of berries. Twelve baskets are packed in a crate (see fig. 6). In this way injury to the berries during transportation is minimized. Cir. 320] Preservation of Fruits and Vegetables by Freezing 21 Fig. 5. — Picking berries in Washington. Fig. 6. — Field crates used for berries. 22 University of California — Experiment Station From the receiving platform the berries are trucked to the washer and a crate at a time is dumped into an agitating hopper filled with cold water from which the berries are carried up an incline by the force of sprays of water. A tank of cold water may be substituted for the hopper and from the tank the berries may be carried slowly up an inclined belt conveyor past water sprays to the inspection belt, The berries are fairly well drained by the time they reach the inspec- Fig. 7. — The Allen slat type grader for small fruits. tion or sorting belt. In sorting, unhulled, green, mashed, molded or wormy berries, and all foreign matter such as leaves, hulls, etc., are removed. The washing cleanses the berries and cools them to about 60° F and thus acts as a precooling agent. After the berries are washed and sorted they are usually graded before packing into barrels, although the former practice of packing without grading still survives. Discriminating buyers are willing to pay a premium for graded berries. Two types of graders are generally used. The older and more pre- valent type is the common screen grader. In recent years the Allen Cm. 320] Preservation of Fruits and Vegetables by Freezing 23 grader, developed by Mr. W. G. Allen of Hunt Brothers Packing Corporation has been installed in the newer and more modern plants (see fig. 7). This grader consists of wooden slat grading screens which cause minimum injury to the fruit. The Allen grader has the best feature of the belt grader with the addition of several new fea- tures which increase its capacity. The following size grades are generally used : y% to % inch in diameter. % to 1 inch in diameter. 1 to 1% inches in diameter. 1% to 1% inches in diameter. ig. -A mechanical jolting platform for mixing berries and sugar in the barrel. Note the tubs of sugar in the background. Some packers grade to sizes different from those given. Field run strawberries vary from % to l 1 /^ inches in diameter. From the grader, the berries are conveyed to the barreling crew. One of the most common methods of barreling consists in adding ber- ries and sugar to the barrel on a mechanical jolting platform upon which the barrel stands, beneath the end of the graded fruit delivery belt. The sugar is added continuously or at frequent intervals to the berries during jolting so that a thorough mixing of the materials is insured. In the absence of a mechanical jolting platform, the barrels are jolted by rocking them back and forth by hand (see figs. 8 and 9). Granulated cane sugar (berry grade) is generally used; but some packers prefer to use confectioners' coarse grade sugar because the crystals adhere to the berries better and do not settle out so much. 24 University of California — Experiment Station Fig. 9. — An example of the continuous sugar admixturing hoppers which are coming into wide use. In the better equipped plants, the sugar is added continuously from a hopper. First the barrel is filled about one-third full of ber- ries. Then the sugar is allowed to run slowly into the barrel and the remainder of the berries become coated by falling through the spray of sugar. As the sugar falls into the barrel it sifts through the unsweetened berries in the bottom and dissolves rapidly. The barrels are brought to the desired weight, headed up, stored in the plant until a truck load has accumulated and are then trucked to the cold storage (figs. 10 and 11). As a rule, in Oregon especially, Fig. 10, — Barreled berries in the sharp room. Cm. 320] Preservation of Fruits and Vegetables by Freezing 25 the barreling stations are generally not connected with freezing" stor- age plants, although they are usually located near commercial cold storage warehouses. Fig. 11. — Another view of the sharp room. The barrels are so filled as to allow about a 3 or 4-inch headspace for expansion on freezing. In addition there is additional air space in and around the packed berries; where the berries are not packed very solidly this may amount to about % fluid ounces of air space for each 12 ounces of berries. 16 The net weight of contents varies with the sugar added and the practice of different packers. It is generally that given in table 7. TABLE 7 Weight of Strawberries Packed in 50-Gallon Barrels Type of pack Net weight of contents, pounds Straight 2+1 3+1 4+1 375-390 450 430-440 400-410 On a certain lot of 2 -f- 1 barrels examined, the gross weight varied from 502 to 508 pounds, the tare from 49 to 53 pounds and the net weight from 452 to 460 pounds. The Barreling of Raspberries. — Raspberries are generally not washed prior to barreling unless they are very dusty. Washing fills the cups with water and it is claimed that it also injures the texture and causes a loss in flavor. For this reason buyers usually insist that raspberries to be frozen shall not be washed in spite of the fact that a state law requires that they be washed. is According to W. G. Allen of the Hunt Brothers Packing Co. 26 University of California — Experiment Station The berries are usually sorted on belts (fig. 12) although some packers sort the berries from the field baskets into dish pans. It is claimed that sorting in this manner is more effective, as any clumping from molding can be easily detected by pouring the contents of a bas- ket over the hand. Raspberries are usually not graded for size unless the packing plant is operated in conjunction with a cannery. The remainder of the barreling procedure is similar to that described for strawberries. Packing Berries in Small Containers. — In 1929, the following two types of small containers for retail distribution were first used corn- Fig. 12. — Sorting belt and jolting platform used for raspberries. mercially in the Pacific Northwest: the 1-pound Mono Service Co. Kleen Kup "tub" (or "cup") ; and the 1-pound vacuum closing tin can introduced by J. E. McConkie of the American Can Co. One method of packing berries in the Kleen Kup type of container was as follows : The berries were washed, sorted and graded into small, medium and large sizes. Only the large size was packed in this container. Large berries 1 inch and over in diameter were conveyed to filling tables where they were dropped automatically into tubs. The cups were brought to weight by hand. Twelve ounces of berries filled the cups to overflowing. Then four ounces of sugar was poured by hand on top of the berries. The berries were then pressed below the lip of the cup and tamped by hand by means of a disk of wood. Cm. 320] Preservation of Fruits and Vegetables by Freezing 27 The bottom of the cup was wiped to remove juice and sugar and the cap was slipped in place by an automatic lidder. The top and sides were then wiped clean. The cups were then packed in wooden cases, 24 to the case. The cases were closed and trucked to cold storage where they were frozen and held at 11° F. 17 Vacuum-packed frozen berries were handled almost entirely auto- matically. At first some difficulty was experienced in the addition of sugar to the canned berries. If the sugar was placed on top of the berries, the proper weight of berries could not be packed in the can and most of the sugar was lost during vacuumizing ; if the sugar was placed on the bottom it generally remained undissolved and unmixed with the fruit when the can was frozen under commercial conditions. The process developed by McConkie was to half -fill the can with ber- ries, add the sugar and then add the remainder of the berries. A 4 -f- 1 or a 5 -j- 1 pack was used for the berries frozen in vacuum. The Cold Packing of Cherries. — The Montmorency cherry, a red sour cherry, is frozen in New York, Wisconsin, Colorado, Michigan, Oregon and Washington for pie bakers' use. The fruit is stripped from the tree, the cherry being pulled free from the stem, a method of picking which greatly reduces picking cost. The fruit is hauled to the packing plant where it is immersed in tanks of cold water for several hours for both chilling and cleansing. The cherries are drawn from the tanks as needed and pass on a wide rubber belt between sorters who remove any bruised cherries, leaves or stems. The sound fruit is then elevated to the automatic pitting machine, the sharp plungers of which, descending at regular intervals into the holes in the surface of the slowly revolving cylinder, cut the pits out of the cherries and force them into a trough beneath. The clean pitted fruit falls from the pitting cylinders into a chute that delivers them to the barrels with a given amount of sugar. Pie makers prefer a 4 + 1 pack while the preserve manufacturer desires a 3 + 1. The barreled cherries are then frozen as described for berries. Frozen Packed Grape Juice. — Methods of freezing storage of fruit juices are not yet standardized. The following method is in use in one factory. The juice is extracted by crushing, stemming and press- ing the grapes. The hydraulic press yields a better product than the continuous screw press. The extracted juice is strained to remove coarse particles and is then run into storage and blending tanks 17 As stated elsewhere, syrup is recommended in preference to dry sugar for small containers for reasons previously given. 28 University of California — Experiment Station where it is rapidly precooled by means of a refrigerator coil in which brine is circulated. The juice can also be precooled to about 30° F by the use of open cooling coils such as the Baudelot cooler. This con- sists of several vertical coils of interconnected pipes, each pipe being placed nearly horizontal. The juice circulates through these pipes and is cooled by recirculated brine which is distributed in sprays or streams over the coils from the top. Thus the juice is subjected to but little exposure until cooled. The precooled juice is generally stored for 24 or 30 hours in a refrigerated storage tank to insure partial clarification by settling and removal of some of the cream of tartar. However, in the best commercial procedure, the juice precooled by the Baudelot coolers is run directly into cans for freezing. If the juice is not precooled but stored immediately in barrels, fermentation may occur since the freezing of the entire mass is slow. The cold, comparatively clear juice is barreled and placed at 0-10° F, where it is allowed to freeze. In filling the barrels care must be taken to allow for expansion of the juice on freezing as otherwise the barrel will burst. This clearance should not be less than 10 per cent of the volume ; i.e., a 50-gallon barrel should not contain more than 45 gallons. Ten-gallon enameled tin cans are also used in California. The juice after freezing in paraffined barrels or tightly sealed tin cans can be stored at 15-20° F. In order to conserve flavor and to prevent deterioration it is important to insure rapid precooling and freezing of the juice. Freezing Orange Juice. — The freezing of orange juice and other citrus juices is rapidly developing as an industry in California. Of the two commercially grown varieties of oranges, the Navel and Valen- cia, the latter is preferred for juice as it yields a juice that is of bet- ter flavor and quality and which does not deteriorate as rapidly as that expressed from Navels. Freshly picked sound oranges yield a better flavored juice than split, partially decayed or moldy fruit or fruit that has been held too long in cold storage. At the plant, decayed or moldy fruit is removed by sorting. The sorted fruit is then generally washed to remove mold spores, dust, smut or other foreign material adhering to the skin. In the majority of plants the fruit is sorted after washing. The oranges are then conveyed down a V-shaped trough to a revolving stainless steel fruit slicer or cutting disc and cut in half. The juice is extracted by burring on a revolving cone extractor made of monel metal, cast aluminum or other corrosion- resistant alloy. Horizontal burring units with an enclosed, vertical, direct-connected motor are preferable to those connected to a shaft Cm. 320] Preservation of Fruits and Vegetables by Freezing 29 or vertical burring" units. The coarse particles of pulp and seeds are removed by straining through a coarse screen, which may be revolv- ing or stationary and the remaining fibers are sometimes brought into a more homogeneous suspension by passing the juice through a tomato products 'finisher' such as the Kern brush finisher. The Kern brush finisher is often used without a preliminary straining screen. Since it is necessary for proper retention of flavor to extract the juice with the minimum exposure to air, a revolving screen strainer is superior to a brush finisher. It is necessary to cool and freeze the juice as rapidly as possible after extraction with as little metallic contamination as possible and with the minimum exposure to air. In one plant the strained juice is rapidly chilled to a mushy consistency in a vertical type direct expansion ice cream freezer having only the outer scraping paddles, and with the inner beaters removed. However, the scrapers tend to beat some air into the product unless the freezer is full. Cooling the juice by passing it through the coils of a Baudelot cooler chills the product with a minimum exposure to air. The cooled juice is then usually packed in enameled cans with "slip on" lids and stored at 0-10° F. Lemon juice and grapefruit juice are prepared and frozen in a similar manner. DIRECTIONS FOR FREEZING FRUITS Syrup Pack. — The treatment of the fruit for freezing will vary with the variety, seasonal effects and other conditions. Therefore the directions given must be applied with discretion and modified to suit conditions. The following directions are for fruits to be used in the condition in which they are packed : 1. Use fully ripe, deep colored, strongly flavored varieties of fruit. 2. Grade the fruit for size, and sort for quality, removing all green, immature, or decayed fruit and all foreign material. 3. Peel, halve or slice such fruits as pears and peaches. Pit apri- cots and peaches and remove the core from apples. That is, prepare the fruit in the form in which it will be consumed. 4. Wash the fruit thoroughly with water. 5. Drain the fruit to remove excess water. 6. Pack in the size and style of container desired. 7. Cover the fruit with a 40° Balling syrup (5.55 pounds of sugar per gallon of water) . Fresh fruit has a tendency to float on the sur- 30 University of California — Experiment Station face of the syrup. To keep the fruit entirely submerged in the syrup it will be necessary, except for small containers with suitable lids, to use some form of perforated false tops. This is especially necessary for larger containers. A perforated cone has been used in 5-gallon cans. In larger containers it will be desirable to precool the syrup used. An ice cold syrup is recommended. 8. Store the fruit in the freezing room at 0° F, or less, directly after packing and leave for 48 hours or longer, depending on the size of container. For reasons of economy and because prolonged hard freezing is deleterious to the texture of the fruit, store at 15°-20° F. Variations in Procedure with Syrup Pack. — Peaches and mealy apples and pears for pie bakers' use, and most other uses, should be blanched in steam for 2 to 5 minutes in order to inactivate the oxidase. This treatment will eliminate browning and does not materially affect the texture and flavor of these fruits if they are cooled thoroughly immediately after blanching. Dipping the fruit in 0.1 per cent sul- furous acid solution for about 60 minutes, or in a 0.3 per cent solution for 10 minutes prior to storage will also tend to prevent oxidation. This treatment is now being used for apples preserved by freezing for use in pies. Where frozen fruit is to be used in canned fruit salads, jams, or jellies, no browning will occur if the frozen syrup-packed fruit is rapidly thawed with steam or in hot water and immediately heated to inactivate the enzyme. Although a 40° syrup is generally satisfactory it may not be sweet enough for some fruits, may be too sweet for others and in certain cases the texture will be adversely affected. A 25° Balling syrup has been found satisfactory for freez- ing cling peaches for later use in canned fruit salads, and it is also best for grapes. In packing large containers it is recommended that not only pre- cooling of syrup and fruit be applied, but also that the containers be stacked in such a manner as to facilitate circulation of cold air about them during cooling. DIRECTIONS FOR FREEZING FRUIT JUICES Fruit juices are most attractive when first expressed from the fresh fruit and will rapidly deteriorate after extraction unless imme- diately preserved. The frozen juice will retain its characteristic fresh flavor and odor, and suffer no apparent deterioration during storage if it is protected from foreign odors and flavors. The expressed juice must be chilled and frozen as soon as possible after extraction. Cut. 320] Preservation of Fruits and Vegetables by Freezing 31 1. Extract the juice in the manner best suited to the fruit. Cut oranges, lemons and grapefruit in half and extract the juice by means of a '"burring" machine such as is used in soda fountains. Crush and press white grapes and apples. Crush berries and red grapes, heat to 160° F and press hot. Avoid contact of red juices with iron and keep all juices from contact with zinc. 18 2. Strain to remove coarse particles. 3. Add sugar to taste, if required. 4. Precool juices if they are to be stored in large containers. 5. Fill into containers allowing 10 per cent or more of head space to permit expansion. 6. Then treat as directed for fruits in syrup. DIRECTIONS FOR FREEZING SYRUPS In recent sales tests a frozen punch syrup has met with greater sales success than fresh juice. The punch syrup was prepared by mixing equal parts of the following : 1. Orange syrup prepared by adding 7 pounds of sugar to one gallon of burred orange juice. 2. Lemon syrup 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 Sirah grape juice to 70° Ball- ing, in glass-lined vacuum pans. This concentrate which is used as a color base should not have any objectionable flavors and should be of rich red and not brown color. Grape concentrate produced by con- centrating grape juice by freezing storage yields a product of excel- lent quality. 19 The mixed punch syrup can be frozen in pint or quart paraffined paper cartons and other suitable containers. The consumer places the frozen punch syrup in a suitable basin or pitcher with enough water to dilute the syrup to drinking strength, generally 3 parts of water to 1 of syrup. The ice in the punch syrup cools the added water and in turn is melted. A pleasing cold drink rapidly results. !8 Further directions for extracting fruit juices are given in: Irish, John H. Fruit juices and fruit juice beverages. California Agr. Exp. Sta. Oir. 313:1-64. 1928. 19 For details of process see: Irish, John H. Fruit juice concentrates. Cali- fornia Agr. Exp. Sta. Bui. 392:1-20. 1925. 32 University of California — Experiment Station DIRECTIONS FOR CRUSHED FRUITS Certain fruits can best be frozen in the crushed or pulped condi- tion either because they lose in texture when frozen whole or sliced or because they may best be distributed in crushed form. Where the fruit is used for ice cream, jam and general soda fountain use it is preferred in this form. This is especially true of avocados and per- simmons. By this means a product of good quality can be secured from blemished fruit unsuitable for packing whole or sliced. The prepared fruit can be ground, crushed or passed through a pulping machine. The resulting product retains the characteristic flavor and odor of the fruit. Avocados and persimmons should be peeled before crushing. Since pulping in a 'tomato cyclone' beats too much air into the product, grinding is preferable for fruit that is easily oxidized such as avocados, apples, peaches and apricots. The crushed or pulped fruit can be frozen without sugar in a tightly sealed container but the color and flavor is retained better when sugar is added. Some fruits should be packed with 3 parts of fruit pulp to 1 of sugar and others with 6 of fruit to 1 of sugar. The 6 + 1 or 5 + 1 pack is preferred by ice cream manufacturers. For dessert purposes a larger propor- tion of sugar is desirable. The crushed fruit and sugar are mixed until the sugar dissolves and the mixture is then added to the storage container. THE FREEZING STORAGE OF VEGETABLES The characteristic color and flavor of artichokes, asparagus, corn on the cob, peas, string beans and spinach can be preserved by freez- ing, although the frozen asparagus, string beans and corn on the cob are rather limp and soggy. The stalks of asparagus readily collapse unless packed unblanched in liquid. The frozen artichokes, peas and spinach are excellent in texture. Cucumbers and tomatoes are markedly soggy and limp in texture on thawing. The cucumbers, especially, are tough and not crisp. In most cases it has been found desirable to blanch in steam or water, to cool, and then pack in cold brine. Vegetables packed in brine are better in texture and flavor than those packed in water or without liquid. Most unblanched non-acid vegetables are unattractive when frozen, even when frozen in brine; as they contain more sedi- ment and are not as bright in color as the blanched vegetables. They are subject to deterioration in flavor through enzymatic action with Cm. 320] Preservation of Fruits and Vegetables by Freezing 33 the production upon storage of objectionable "hay-like" off flavors, especially in peas and string" beans. They are, however, superior in texture. This is especially true of asparagus. Various methods of blanching were tried, such as in water, steam, brine, citric acid solu- tions, and in vinegar. Although artichokes were better when blanched in % P er cen t citric acid solution, blanching in water or brine was satisfactory for other vegetables. The drained weights of vegetables depends upon the kind of vege- table and the packing treatment. Loss in weight is greatest for aspara- gus and spinach, and least for peas and artichokes. Blanching increases the loss in weight on thawing after freezing, although blanching in acid tends to toughen the vegetable. The spoilage in frozen vegetables when stored at room temperature was found to be chiefly bacterial putrefaction, the brine becoming cloudy and the vegetables becoming putrid. Unblanched peas, spin- ach, corn and string beans were more susceptible to spoiling than the blanched vegetables. Incipient spoiling occurred within thre,e to four days and became marked at the end of seven days at room tempera- ture. The blanched vegetables, especially the corn and spinach, started to spoil in four to five days and at the end of seven days when the experiment was discontinued they were distinctly spoiling. Further tests are now in progress concerning the type, kind and character of spoiling of vegetables during freezing storage. Although the vegetables can be preserved in the frozen form care has to be taken in the freezing and distribution of these products. These vegetables being low in acid are subject to the growth of Clos- tridium botulinum with the consequent production of deadly botulinus toxin or poison unless they are promptly and rapidly frozen and unless they are prepared and consumed directly after thawing. They should never be eaten or even tasted before cooking and should be destroyed if any evidence of spoilage appears before or during cooking. Owing to the danger of development of CI. botulinum in non-acid vegetables in the absence of air at a suitable temperature, extreme care has to be taken in using a vacuum closing tin can to assure prompt freezing and use immediately after thawing. A paper carton may be used which on thawing allows the brine to leak and the carton to become soft and unattractive in appearance providing absorption of foreign odors during storage is minimized. The appearance of the package will then serve as an indication of the condition of the con- tents and warn against spoiled thawed vegetables. Because of its tendency to pack closely and thus exclude air, spinach is more dan- gerous than other vegetables. 34 University of California — Experiment Station Anyone contemplating the freezing storage of vegetables is advised to communicate with the Fruit Products Laboratory, University of California, and with Dr. K. F. Meyer of the Hooper Research Founda- tion, San Francisco. THE DISTRIBUTION OF FROZEN FRUITS AND VEGETABLES Unlike most other food products, the quality of the frozen food may be very seriously injured by even slight negligence during distribution to the consumer. It is necessary that the product be kept frozen during this period. In this respect the distribution of frozen fruit and vegetables is similar to that of ice cream and other frozen foods. It is necessary to educate the retail merchant to protect the frozen product by proper cold storage. The consumer who buys the product must also be taught to keep it properly until it is consumed. Educa- tional advertising in this direction is necessary. All containers should be clearly labeled in some such manner as follows: Caution. Keep frozen or use immediately. Storage under refrigeration during distribution is far more impor- tant for frozen vegetable products than for frozen fruit products. Frozen vegetable products when stored or displayed at room tempera- ture are subject to bacterial decomposition in a manner similar to that which frozen meats and fish undergo under like conditions. The con- sumer who partakes of spoiled vegetables improperly packed or dis- tributed may be poisoned if bacterial toxins, particularly that of botulism, are present. If the vegetables are frozen quickly and kept frozen until they reach the consumer's kitchen and are then cooked promptly there is no danger ; the danger lies in tasting before cooking or in careless delay before cooking. As in the case of fish and meat, special care must be taken in the preservation of non-acid vegetables in any type of retail container since they are susceptible to spoilage by CI. botulinum. Until methods for freezing and distributing vegetables are properly standardized and distribution properly safeguarded it is advised that the freezing storage of non-acid vegetables be undertaken with the greatest caution and only under advice of the State Board of Health. Cm. 320] Preservation of Fruits and Vegetables by Freezing 35 POSSIBILITIES AND LIMITATIONS The fact that large quantities of fruit can be packed rapidly, simply and with relatively small investment for machinery and con- tainers permits the use of freezing storage in years of large crops as a means of preserving some of the surplus. However, while the methods and equipment are relatively simple, extensive cold storage space is required. The cost of storage naturally increases with the length of storage. Not only the storage but also the distribution of the product requires refrigeration. The product must not only be kept frozen during transit and distribution to the retailer but must also be kept frozen by the retailer. Moreover housewives have to be instructed in the proper use of the frozen product. Thus the develop- ment of the industry may be limited not only by the cost of refrigera- tion facilities but also by their availability. The chief justification of freezing as a means of preserving foods, lies in the better preservation of color and flavor. In many cases this will amply justify the increased cost of packing and distribution. ACKNOWLEDGMENTS The investigations on which recommendations have been based were carried out under the general supervision of Professor W. V. Cruess, who also aided in preparing the manuscript. Dr. K. F. Meyer and Dr. J. Russel Esty kindly reviewed the sections on frozen vege- tables. Thanks are also due to P. F. Nichols for suggestions given during preparation of the manuscript. Thanks are due to the American Can Company for furnishing the tin cans used in these investigations and to the Lily-Tulip Cup Corpo- ration, Mono Service Company, Inc., and the Zellerbach Paper Com- pany, for furnishing the paper containers used. 15m-10,'30