Nine Principles for Freezing Vegetables

James A. Berry, F. E. Lindquist.

Food spoilage putrefaction or fermentation is due to the activities of micro-organisms, which break down organic matter in their search for food and energy.

Decay is most marked at high temperatures. It proceeds slowly at low temperatures. It stops at hard-freezing temperatures. Plant and animal tissues deteriorate after harvest or slaughter because of the activities of enzymes. Though responsive to temperature, enzymes are more tolerant of cold than are bacteria, yeasts, and molds. Hence, in the freezing preservation of vegetables, the inactivation of enzymes by heat is a routine preparatory step.

When the natural deteriorative processes have been checked, a frozen foodstuff will keep indefinitely if steadily exposed to sufficiently low temperatures. Because freezing alters the fresh character of foodstuffs least of all the preservation methods, the freezing industry has grown rapidly in recent years.

SUCCESSFUL FREEZING preservation rests on nine well-defined principles.

The foodstuff must be naturally adapted to the method. For example, peas freeze satisfactorily, but cucumbers do not.

The variety or strain must be of proved suitability. Garden-type peas, for instance are superior to canning varieties for freezing.

The maturity or general character of the raw product must be of the best. Freezing preserves defects as well as good qualities.

Handling between field and plant and in the plant itself must be prompt. During unavoidable short delays, produce must be kept cool to check deterioration by respiration and microbial growth.

Natural enzymes, capable of lowering quality while the product is in freezing storage, must be inactivated. Scalding is thus obligatory for vegetables intended for freezing.

Freezing must be rapid enough to assure retention of quality in the product, give capacity to the plant, and yet be economical.

The plant must be kept sanitary, and the line clean. Molds, yeasts, and bacteria, if present in a frozen product in excess of reasonable limits, are regarded as proof of a poor sanitary history.

Packing must be such that practically no loss of moisture from the product occurs during a year's storage.

Storage temperature must be uniform and must not exceed 0 F.

The vegetable-freezing industries, which may be said to date from 1929, have grown rapidly. At first people were doubtful about the method. The behavior of vegetables at freezing temperatures was but little understood, and experience in fruit freezing, an older industry, offered only limited help. However, when it was proved that bad odors and tastes in frozen vegetables were due to action of enzymes that could be inhibited by a short scalding treatment before packing, the first hurdle was cleared.

Scientific work in the Department of Agriculture, State experiment stations, and private agencies has kept pace with commercial development not infrequently has it made such development possible. Mention may be made of the establishment of times and temperatures for the scalding of various vegetables, the engineering of suitable freezing systems, determination of the relationships between storage temperature and product quality, the adaptability of varieties or strains of vegetables to freezing preservation, and the development of knowledge in such important matters as packaging, distribution, and public health protection.

The pioneer packs of frozen vegetables about a million pounds in 1930 were of peas, which are still the most popular commodity. The garden, or fresh-shipment, types were recognized as superior to canning varieties, and, as their vine growth is heavy and makes good ensilage, farmers in many sections came to find pea raising for frozen pack a profitable venture. The feasibility of freezing other vegetables (such as snap beans, spinach, corn, and asparagus) had been demonstrated by 1934. New concerns quickly entered the business. Some were offshoots of already established food-processing firms. Others were newcomers to the industry. Also, with expansion and diversification of output, the industry became more widespread geographically. Operators in quest for more raw material naturally came to depend on areas more remote from their processing plants. The resulting long hauls created problems of bacterial growth and deterioration in quality through respiration, which were met by icing lug boxes or establishing precooling stations. The perishability of harvested vegetables, especially shelled peas, and the close relationship between quality of raw material and that of frozen product early impressed itself on the industry and shaped its practices.

SCALDING, or blanching, vegetables to inhibit the enzymes is a key operation in preparation for freezing. If it is not done, the enzymes will function slowly at low temperatures and cause the development of undesirable odors and flavors in the frozen vegetables. Heat, either hot water or steam, is used to destroy the enzymes. The scalding operation obviously must be adapted to the product so that the necessary enzyme destruction causes minimum sacrifice of fresh quality. Peas, for example, are scalded for about 1.25 minutes and asparagus for 3 minutes at 212 F. Incidental advantages of scalding are intensification of color in green vegetables; slight wilting, which facilitates packing; and reduction in numbers of bacteria.

THE TERM "QUICK FREEZING," often heard in connection with food freezing, has no definite meaning. Probably most commercial vegetable freezing may be called quick ; that is, the product is frozen to a temperature of about 0 F. in an hour or less. The packer is guided more by practical considerations of plant output, often paramount during harvest peaks, than by a conviction that such a freezing rate is absolutely necessary to insure good quality in the product. In vegetables like snap beans and asparagus, the rate of freezing may be reflected in the ice patterns, but such differences are of minor significance when the products are cooked and served. On the other hand, temperature of storage is recognized as being highly important. Deterioration in color and probably in eating quality takes place in frozen vegetables stored, for example, at 15 F., apparently irrespective of rate of initial freezing.

Various freezing systems have been advocated for vegetables. The basic consideration is the rapid transfer of heat to effect a hard-frozen state--about 0 F. quickly and economically.

Most vegetables now are frozen in one of three ways. (1) With the refrigerated-plate method, the packaged food is placed between adjustable refrigerated plates, and pressure is applied to insure close contact and rapid freezing. (2) The air-tunnel system consists of a woven-steel belt moving slowly in a tunnel in which refrigerated air is circulated. Products like peas, lima beans, and cut corn can be frozen loose on the belt and packaged afterwards in a low-temperature room. (3) With the tray method, the vegetables, packaged or not as necessary, are spread on shallow trays in portable racks and frozen in rapidly moving refrigerated air in an insulated chamber.