Freezing of Food Products

a method of preserving products that involves lowering the temperature of the product to below the freezing point of its juices. This point, the so-called cryoscopic point, depends on the concentration of soluble substances in the cellular fluid and on the average is -0.6° to 1.2°C for meat, -0.55°C for milk, -0.5°C for eggs, and -0.6° to -2°C for fish. During further cooling, temperatures are reduced from -18° to -25°C, and sometimes to -50° or -60°C and below. In this case almost all the water in the products freezes. There is virtually complete cessation of the activity of microflora and enzymes, as a result of which products acquire the capacity for long-term preservation of their original quality if the temperature is maintained at the same low level.

Fluid contained in the intercellular area of tissues usually freezes more rapidly than intracellular fluid. The more rapidly food products freeze (with intensive admission of a cooling agent), the greater the number of centers of crystallization formed simultaneously; as a result of this process, even with almost complete freezing, numerous ice crystals are produced in cells and intercellular areas that cannot essentially damage the integrity of fine and delicate cell membranes of the product’s tissues. Therefore, the structure of tissues is insignificantly altered, and during subsequent de-frosting before use as food, such products better preserve their nutritive and organoleptic properties. Fluid losses are negligible.

The cold required for freezing of food products is produced in refrigerators. Freezing compartments are cooled with liquid Freon or ammonia circulating in sets of pipes placed along walls and under the ceiling of the chamber or in a separate location. Directed air streams are created by blowers in order to increase the efficiency of freezing in the chamber. The products, mainly carcasses and sides of meat, are hung in the compartments. Wet freezing of food products, in which the products are immersed in liquid or sprayed with streams of a nonfreezing cooling agent (for example, a solution of sodium chloride), is used in a number of cases. Freezing generally lasts one to three days.

Multiplate freezing units consist of a number of parallel hollow plates inside which cooled ammonia or brine is circulating. Boxes or trays holding the product are mounted on the plates, which are moved together by a special device, thus providing contact between the product and the cold sur-faces of the plates. Such units make it possible to reduce the period of freezing of food products to 2-3 hours (with a layer of products up to 50 mm thick between the plates). The shortcomings of such units include batch operation and substantial expenditures of time in loading and unloading the product. Air-blast quick-freezing units, with an intensive flow of cooled air blown through the area occupied by the product, are more advanced. The newest modern quick-freezing units, which are suitable for freezing bulk and small products, operate according to the principle of fluidization in a so-called fluidized bed. The product is placed on the top-most of a set of vibrating screens placed at a slight incline. An intensive stream of cold air is directed from below onto the screen. At a certain minimum critical velocity of the air stream, the particles of the product are raised slightly above the surface of the screen and are maintained in a suspended state, forming a “fluidized” mass. The total surface of the particles that are in contact with the cooling air increases sharply, and the freezing time is reduced to dozens of minutes. Other new methods of freezing, consisting in direct immersion in liquid nitrogen, Freon, nitric oxide, and other coolants, make possible the production of low freezing temperatures (in liquid nitrogen, as low as -195°C). Freezing of food products in turbine cooling apparatus has been successfully tested; air serves as a cooling agent and provides freezing temperatures below -100°C.

The wrapping of products, which prevents direct contact with the compartment air during storage, is important in producing high-quality frozen products. Such contact causes not only oxidizing processes, which lead to loss of taste qualities, but also substantial weight losses as a consequence of evap-oration of ice. The vapor that is produced condenses in the form of an insulating layer of snow on the pipes of the cooling coils and on the walls and ceiling of the compartment; as a result, the operating conditions of the refrigerating equipment deteriorate markedly. High-quality freezing of food products also requires the maintenance of a constant temperature during storage. Any fluctuations cause partial recrystallization of the ice, frequently with an increase in the size of the crystals and damage to the structure of the tissues during defrosting.

In the USSR there is widespread industrial freezing of meat and meat products, egg melange (the body of the egg without the shell), and fish. Whole carcasses, sides, and quarters of meat, as well as meat from which the bones and inferior connective tissue parts have been removed, are frozen in blocks of standard size and shape. Various meat by-products and culinary semifinished products are also frozen in blocks. Fish is frozen whole and in fillet and block form.

The freezing of berries, fruits, and vegetables is of particular importance, since no other method of preserving food makes it possible to retain to such a degree the basic qualitative indicators of products—taste, smell, exterior appearance, and consistency—as well as vitamins, particularly vitamin C, the main sources of which in the human diet are vegetables and fruits.

Almost all types of vegetables (except the garden radish, lettuce, and certain others), fruits, and berries may be frozen. Vegetables and fruits are washed in advance, and thick skin, seeds, and inedible and marginally edible parts are removed. Certain large vegetables and fruits (beets, carrots, cabbage, and apples) are cut into lobules, slices, and disks to speed up freezing and for convenience in subsequent use. Prepared berries, fruits, and vegetables are blanched to destroy enzymes that later may facilitate oxidization processes, darkening of the finished product, and the appearance of foreign tastes. They are then packaged in small (250-1,000 g) boxes of thin waxed cardboard with linings of parchment, cellophane, polyethylene, or other moistureproof material and frozen together with the boxes in multiplate or other units. The faster method of bulk freezing with subsequent packaging in frozen form is also widespread. Vegetables are gener-ally frozen in their natural state (individually or in mixtures, such as those for soups); fruits are also frozen in a natural state or with sugar added. A small amount of ascorbic acid is sometimes added when freezing apricots, peaches, and apples to better preserve their natural color, since ascorbic acid has an antioxidizing effect. Frozen vegetables and vegetable mixtures kept at temperatures not above -18°C and frozen fruits and berries at temperatures not above -12°C may be stored for periods up to 12 months, depending on the kind of products.

In freezing the creation of a continuous refrigeration chain from the manufacturing plant to the consumer is important. The thawing of products sharply reduces their quality and causes the destruction of tissue structure and great loss of juices. Therefore, frozen vegetables and fruits are transported in refrigerated railroad cars or trailer trains; they are stored in refrigerators until the moment of delivery to commercial establishments and are subsequently kept in refrigerated store counters. Frozen vegetables are not defrosted but are immediately immersed in boiling water and cooked until ready (several minutes). Fruits are thawed.

A new trend in the freezing of food products is the production of quick-frozen dishes of prepared vegetables, vegetables and meats, and fruits, as well as processed semifinished products—soups, vegetables, vegetables and meats, and fruits. Dishes are brought to a state of near-readiness and are then frozen in small packaging in the form of individual portions or in blocks for a specific number of portions (6-10-20). Preparing such dishes requires only brief heating or boiling (3-5 min).