The usual purpose for refrigerating and freezing foods is to preserve their quality, which technically is changing what would be natural processes of deterioration. Refrigeration may also play a role in facilitating a change in the character or chemical structure of food—called processing here. Some foods incorporating refrigeration in their processing are cheese, beverages (such as beer, wine, and juice concentrates), and instant coffee. The procedure for making cheese varies from one type to another, but all cheeses are made from milk to which is added a starting organism to cause the formation of curds and whey. Once the whey is drained, the curds form the basis of the cheese. Refrigeration serves a role in the manufacture of most cheeses during the curing step. Depending upon the type, the cheese is held at between 10°C (50°F) and 20°C (68°F) for a period ranging from several days to several months.
Although it might not be obvious to the general public, refrigeration is an important function in the bakery industry. Some applications include ingredient cooling and dough mixing; freezing of bread for later holding, thawing, and sale; refrigerating unfrozen dough products; freezing of dough for the food service industry and supermarkets; and the freezing of fried and baked products for sale to consumers for microwaving or toasting.
With beer, two main chemical reactions occur in the brewing operation: (1) converting grain starches into sugar; and (2) fermentation, which converts the sugar into alcohol and carbon dioxide. The fermentation process is exothermic, so if heat were not removed, the temperature would rise and ultimately terminate fermentation. Refrigeration maintains the fermenting mixture between 7°C and 13°C (45°F and 55°F), and also cools the storage facilities where the beer is kept for two or three months until it matures.
The production of wine also demands refrigeration. Following fermentation, the wine is held in refrigerated storage at 10°C (50°F) in insulated stainless steel tanks for six months to two years. In addition, wines require a cold stabilization process to precipitate potassium bitartrate (PB). PB is a harmless but unaesthetic crystal and would be particularly noticeable in bottles of white wine. PB will precipitate naturally at 10°C (50°F), but only over a lengthy period. To accelerate the process, the wine is chilled to -4°C (25°F) for ten days.
To concentrate some foods that are sensitive to high temperatures, water may be removed by evaporation or by freezing. One concept is to employ a refrigeration system as a heat pump with the condenser providing heat for vaporization of the water in the product and the evaporator liquefying this water vapor. Because of its high first cost this application is rare while the heat-pumping principle of thermo compression is preferred. In thermocompression the evaporated water vapor itself is pumped with a high-volume-flow compressor from the product fluid to a condenser. Of the concentrating techniques that apply refrigeration, freeze concentration4 is the most popular. Some of the products that can be concentrated by freezing include citrus fruit juices, vinegar, beer and wines, coffee and tea extracts, sugar syrups, and aroma extracts. The mostused procedure is to bring the solution in contact with refrigerated surfaces on which a thin layer of ice freezes. The ice is continuously removed by scraping and then separated from the concentrated solution.
Another example of refrigeration being used for food processing is in the drying of fish.5 Figure 1.4 schematically shows air at a low humidity and moderate temperature removing moisture from the product. Some water is then removed as the air passes through the evaporator of the refrigeration unit that provides thermal load for the refrigeration system. The discharge gas from the refrigerant compressor divides, some passing to the air-cooled condenser that warms the air and the other portion to an auxiliary condenser that essentially removes the energy introduced to the circuit by the compressor. This concept is particularly applicable in localities where the cost of electricity is moderate.