Freezing can be thought of as a continuation of refrigeration. If we lower the food's temperature beyond a certain point, it will freeze. The freezing point for pure water is 0°C. For food, however, the freezing point is below that. You may recall from physics that the presence of solutes in water will lower its freezing point. Water contained in foods has many solutes, mainly salts and sugars.
Freezing is similar to refrigeration in that it will not destroy microorganisms or inactivate enzymes, but rather slow their deteriorative effect. In some cases, it will completely inhibit the activity of microorganisms, even though they will still be alive. Enzymes, on the other hand, will maintain a certain level of activity during freezing. This is the reason we sometimes blanch vegetables prior to freezing them. Blanching is a mild heat treatment designed to inactivate enzymes.
Freezing has been a key technology in bringing convenience foods to homes and restaurants. It causes minimal changes in the quality of food in terms of size, shape, texture, color, flavor and microbial load. This is assuming that the freezing process is carried out properly. A key factor during food freezing is how fast we freeze. It is known that when food is frozen slowly, the ice crystals formed will be large. Large ice crystals are undesirable as they will damage the cell structure of foods like meat and fish. This will result in poor texture and excessive dripping upon thawing. It is therefore important that freezing be carried out at a very fast rate. As is the case with refrigeration, proper cleaning and packaging prior to freezing will yield high quality frozen food. Frozen food must be maintained at the proper temperature at all times. Ice cream, for example, is very sensitive to fluctuations in temperature that can cause a defect called sandy texture. The ice cream will actually taste grainy, as if it contained sand granules. These granules are in fact lactose crystals. Lactose is the naturally occurring sugar in milk that is normally dissolved. Temperature fluctuation, however, will cause lactose to crystallize out, imparting sandiness.
The success of freezing technology has opened a new field for food processors. They can prepare complete meals and freeze them until the consumer is ready to thaw and heat them. Many of these meals are sold in their serving dishes. Other popular frozen foods are pot pies, fish sticks, desserts and potatoes. At present, there is no other form of food preservation that offers the convenience of frozen foods.
Food processors freeze foods to an internal temperature of -18°C. The food must be maintained at this temperature or slightly lower during transport and storage. Many fruits and vegetables will retain good quality at the above temperature for up to 12 months or even longer. The expected frozen storage will vary with temperature. For example, frozen orange juice will last 27 months at -18°C, 10 months at - 12°C, and only 4 months at -6.7°C.
The popularity of microwave technology has contributed to an increase in frozen food consumption. Both developments in food processing and preparation enable us to have convenient, tasty and nutritious meals at a reasonable price.