the treatment of food products to prevent spoilage when the food is stored for periods. Spoilage is caused primarily by the vital activities of microorganisms and by certain enzymes in the food itself. For this reason, all preserving methods either destroy the microbes and enzymes or create conditions unfavorable for their activities. The principal methods used are sterilization, pasteurization, freezing, drying, pickling (or salting, soaking), smoking, dry-curing, and treatment with sugar or chemical additives. With all of these methods, the food is usually first sorted and washed, and its inedible parts are removed (the rind and seeds in fruits and vegetables; the bones, viscera, and connective tissues in meats; and the scales and viscera in fish); removing these parts increases the food’s nutritive value. The food is often blanched as well.
Sterilization consists of heating the food to 100°-140°C or higher in hermetic containers for as long as is necessary to destroy completely all of the microorganisms capable of causing spoilage. Sterilization and pasteurization (heating to temperatures lower than 100°C) are the basic and most common methods of preserving food.
Freezing is based on the principle that the vital activities of microorganisms and the activity of the enzymes in food decrease with lowered temperature and virtually cease at temperatures of — 18° to — 25°C. Freezing is the most progressive method of preserving food, since the food’s taste, smell, texture, and nutritional value are retained thereby to the greatest extent possible. The disadvantage of the method is the necessity of maintaining the low temperatures constantly during storage. Freezing is used for almost all types of animal and plant foodstuffs.
Drying is the removal of water from the food. Drying increases the concentration of the dry substances (and, consequently, the osmotic pressure) to a point at which it becomes impossible for unicellular microorganisms to absorb or assimilate them. The method is universal, used for the majority of foods. The prolonged action of high temperatures in the old methods of drying food with hot air in ovens or driers (cabinet or tunnel types) leads to a significant loss of vitamins and other valuable nutritive substances. More progressive methods shorten the heating time by first pulverizing, rolling, or foaming the food. (The last is suitable for liquid and pureed products.) The most advanced method is sublimation drying, by which the water is evaporated from the frozen food in a chamber with an extremely low residual vapor pressure (on the order of 100 newtons per sq m; that is, 1 mm Hg). Sun-drying is widely used for fruits (mainly grapes, apricots, pears, and apples) in the southern republics of the USSR.
In pickling, salting, and soaking, lactic-acid microorganisms ferment the sugars in vegetables and fruits to lactic acid, which, in concentrations of 0.7 percent and greater, has a preservative effect, retarding or suspending the vital activities of all microbes. Pure cultures of lactic-acid bacteria are sometimes used for pickling, but the fermentation is more frequently caused naturally by the microflora contained in the fruits and vegetables themselves. Pickled foods should be kept at temperatures of 0 ° -5°C.
Smoke-curing makes use of the antiseptic action of products formed in smoke during the sublimation of wood: phenols, formaldehyde, creosote, and acetic acid. Smoke-curing is used for meats and fish, which are usually salted first. There are two methods of smoke-curing: hot and cold. Dry-curing (primarily of fish) is the drying of salted fish in open air.
High concentrations of sugar (not less than 60-65 percent, depending upon the product) create a high osmotic pressure in a solution. This fact makes it impossible for microbes to absorb the nutritive substances and subjects the microbial cells themselves to plasmolysis (involving the loss of water). Sugaring is used to make preserves, jam, and jellies.
Food preserving with chemical additives includes the methods of marinating and salting and the addition of sulfites, benzoic acid, or sorbic acid. Marinating is the use of acetic acid, which is a preservative for fruits and vegetables at concentrations of 1.2-1.8 percent; fish and sometimes meat are also marinated. Meat, fish, and vegetables are preserved with table salt at high concentrations (up to 10-12 percent for meat, 14 percent for fish, and 10 percent for salted tomato paste).
Sulfites are used in the form of sulfurous anhydride and sulfurous acid and its salts in preserving fruits and acidic vegetables (for example, tomatoes). Sulfurous anhydride is toxic to human beings, but it volatilizes easily when heated and can be removed from the food by boiling. Other chemical preservatives include benzoic acid, sodium benzoate, and sorbic acid and its slats, which are harmless to human beings. Certain antibiotics are also used (mainly nisin and tylosin).
Modern industry uses production-line methods in preserving green peas, sweet corn, tomato purees and pastes, fruit and berry juices and purees, and meat, milk, and fish. Continuous-action sterilizers (rotary and hydrostatic) with a capacity of 400-1, 200 and more containers per minute have begun to replace autoclaves in many branches of the preserving industry. Aseptic preserving is being developed, in which liquid and pureed products are first sterilized in special apparatus at high temperatures for very short periods of time (usually no longer than 1-2 min) and then cooled and packed in presterilized hermetic containers. The quality of the preserves obtained in this manner is significantly higher than those obtained with ordinary sterilization.
Packaging for preserves is also being improved. There is increasing usage of new types of tin (electrolytically plated with a differentiated coating, or chrome plated) and light-gauge aluminum and aluminum alloys. The use of polymer materials, including polymer films, is projected for the individual packaging of many types of preserves. Metal and glass containers have been structurally improved, substantially increasing the productivity of the preserving equipment and the convenience to the consumer. The outer appearance of the packaging is also being improved; using color lithography on tin and applying various labels.
Scientific experiments have helped to lengthen the period of time a food product can be stored after being treated with ionizing radiation (primarily radioactive isotopes). Using this method, the food remains virtually fresh in hermetic containers for long periods of time, even without refrigeration.
REFERENCESFan-lung, A. F., B. L. Flaumenbaum, and A. K. Izotov. Tekhnologiia konservirovaniia plodov i ovoshchei, 3rd ed. Moscow, 1969.
Namestnikov, A. F. Khimiia v konservnoi promyshlennosti. Moscow, 1965.
Spravochnik po proizvodstvu konservov, vols. 1-3. Moscow, 1965-71.
A. F. NAMESTNIKOV