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The metabolism of a living tissue is a function of the temperature of the environment. Low temperature is applied to retard chemical and enzymatic reactions in food. In addition, reducing temperature retards or stops growth and activity of microorganisms in the food. Lower the temperature, the slower will be the rate of above natural activities. Cooling thus slows down or stops the spoilage of foods.
Freezing and refrigeration are among the oldest methods of preservation. Mechanical ammonia refrigeration systems invented during 1875 allowed development of commercial refrigerated warehousing and freezing. Low temperatures employed can be:
- Cellar storage temperature (15°C)
It is usually used for the storage of surplus foods like root crops, potatoes, onions, apples, etc. for limited periods.
- Refrigeration/ chilling temperature (0 to 5°C) Foods kept at this temperature slow down the microbial activities and chemical changes resulting in spoilage. Mechanical refrigerator or cold storage is used for this purpose. Examples of this include meats, poultry, eggs, fish, fresh milk and milk products, fruits, vegetables, etc. which can be preserved for 2-7 days by refrigeration.
- Freezing (-18 to -40°C)
In freezing, water in food turns into ice and hence, makes the water unavailable for reactions to occur and for microorganisms to grow. Most perishable foods like poultry, meats, fish, ice-creams, peas, vegetables, juice concentrates, etc. can be preserved for several months at this temperature. In vegetables, enzyme action may still produce undesirable effects on flavour and texture during freezing. Heating, like blanching, therefore, must destroy the enzymes before the vegetables are frozen.
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