Preservation of Fruits by Freezing

William Rabak.

Preserving fruits by freezing is a practical and desirable way to retain most of their natural food qualities and characteristics. It is a simple and an effective method, and suited for use in the home.

The first research on the process in the Department of Agriculture was with raspberries and strawberries and was conducted in 1904 and 1905 by S. H. Fulton, who recorded the results of his research in Bulletin 108, issued in 1907. He gave this glimpse of the start of a new industry:

"Frozen strawberries for ice cream have been in use in a limited way by confectioners for some time (before 1905), while frozen blackcap raspberries, currants, blackberries, huckleberries, and other small fruits are now being used successfully for pies and other pastries by a few restaurateurs and bakers. A large pie bakery in a central western city is successfully using frozen blackcaps, gooseberries, blackberries, currants, and huckleberries in large quantities. When made into pies the flavor of the frozen fruit is said to be practically equal to that of fresh fruit. Considerable quantities of cherries and damson plums are also frozen by this company for use in pies.

Frozen huckleberries and currants can be easily and successfully held for many months. In 1905 the firm referred to froze 14 carloads of huckleberries, using the fruit for pies months after the fresh fruit had disappeared from the markets."

H. S. Baker, once a druggist in Denver, Colo., established the freezing preservation of fruit as a continuing independent industry in 1909 in Salem, Oreg. From that beginning, the industry (particularly the freezing of berries in the Pacific Northwest ) grew to importance. In 1928, a total of 60 million pounds was frozen. The pack before then was entirely in large containers for use by manufacturers of preserves and ice cream.

The early processors found by trial and error that to avoid fermentation before freezing the berries had to be handled quickly, kept cool, and stored promptly at refrigerating temperatures. The need for careful washing, inspecting, and grading and the importance of good varieties of fruit became apparent later. Subsequent experiments indicated the best proportions for packing with sugar or sugar sirup, and the need for prompt freezing and constant low-temperature storage. Fluctuating temperatures may cause loss of quality because of the acceleration of chemical changes, excessive moisture losses, and crystallization of sugar. Shipping tests have demonstrated the importance of proper refrigeration during transit to guard against quality impairment and economic losses.

Since 1929 the packing of frozen fruits for retail or household use in small containers has developed steadily. Our total pack of frozen fruit in 1949 was almost 500 million pounds. About one-third of the pack was citrus concentrate (primarily orange), followed by strawberries, other berries, and red sour cherries. Freezing is the newest of the four main ways (drying, canning, freezing, and pickling) of preserving fruits. Dehydrofreezing, a combination of partial dehydration and freezing, has not yet been introduced into commercial practice.

PRECAUTIONS must be taken to retain inherent quality after preparation and during storage. The quality of a product when it is eaten is the standard by which any method of preservation must be judged.

Freezing maintains many natural characteristics of fruits, but changes in quality do occur. Laboratory research and the industry have done much to obviate the changes in quality, but, because of the many and diverse causes, continuing investigations are essential.

Chemical changes, although of many types, fall into two general categories ordinary and involved.

Ordinary chemical changes result from the combination or splitting of natural compounds, with the production of different compounds. Examples are the reaction between metals and fruit acids (organic salts) ; the splitting of common sugar to form simple sugars, such as glucose and fructose (hydrolysis) ; and the union of compounds that results in a new and different substance, such as production of starch from simpler components by synthesis, that is.

Unusual color development in fruits, from red through purple to blue, can be prevented by avoiding undue contact with certain metals during processing and storage. Tin and iron are the worst. The color developed is due to the formation of metallic salts with the natural ingredients of fruits, such as tannins or anthocyanin pigments.

The occasional change of the brilliant red of strawberries (anthocyanin pigment) to brown during long storage is probably due to chemical changes that occur in packages that are not airtight.

In 1939 I began examining strawberries that had been packaged and stored for 7 years at 15 F. in both nonairtight (paperboard) and airtight containers (machine-sealed tin cans). I found that the red color of the fruit packed in the nonairtight containers had changed to varying shades of cinnamon brown, while the normal red color was retained in the sealed cans. I also found that the ascorbic acid (vitamin C) was well retained in the enamel-lined tin cans but was practically lost in the paperboard.

Involved chemical changes are induced by the natural enzymes of fruits, an example of which is the browning of cut surfaces of fresh fruits like apples, peaches, and apricots. In this instance, the oxidative enzyme effects a reaction between (catalyzes) the oxygen of the air and complex constituents of the plant cells, so that the tissues are darkened. This oxidative reaction also takes place in improperly processed frozen fruit after thawing and in fruit only partly frozen, that is, above or near 20 F.

A GREAT DEAL of research has been directed toward overcoming this undesirable development. Early experimenters partly solved the problem by quickly immersing the cut fruit in sugar sirup to minimize contact with air or by rinsing in a dilute acid solution, such as 0.5 to 1.0 percent citric acid.

Experiments carried out in 1933 by M. A. Joslyn and G. L. Marsh at the University of California showed that the judicious use of sulfur dioxide would effectively inhibit oxidative discoloration. Immersion of the fruit in a solution of sodium sulfite is the usual method of application. More recently.. D. K. Tressler and C. W. DuBois discovered that ascorbic acid is effective in the prevention of oxidative browning. The use of ascorbic acid for the purpose is advantageous because, unlike sulfur dioxide, it does not affect flavor and adds to the nutritive value of the fruit.

SOME OF THE IMPAIRMENTS during the freezing of plant tissues are physical rather than chemical. Because plant juices are essentially water solutions of soluble materials, expansion occurs during freezing as a result of the formation of ice. Earlier investigators believed that the ice crystals ruptured the plant cells. Whether or not the cells are torn by crystal formation, the properties of the cells are so modified by freezing that they no longer can hold the cell juices. Even if the cells are not ruptured, as happens during quick freezing, there will still be leakage of juices. This phenomenon is less striking in some frozen plant tissues than in others; the differences probably are due to variation in cell constituents and plant structures.