Difference Between Freezing And Freeze Drying
Freezing prevents spoilage by stopping the growth of microorganisms, which cannot function at very cold temperatures. Freezing dates back to the early 1900s, when the first commercial methods for freezing meat and fish were developed. But the process did not begin to compete with canning until after World War II, when large numbers of consumers purchased their first electric refrigerators with freezing compartments. Today, many consumers frequently prefer frozen foods over their canned counterparts because, when cooked, frozen foods more closely approximate fresh foods in appearance and flavor.
There are two methods of freezing foods. The older, now seldom-used method involved putting the fresh foods in a refrigerated room and leaving them until they were frozen solid. This is a long, slow process, particularly for animal carcasses and large containers of food. Because food’s temperature drops slowly, water in food forms large ice crystals that break the cell walls in the food. Then, when food is defrosted, it may be mushy.
Today, quick freezing is the preferred method. Food temperatures are rapidly lowered to 0°F (−18°C). This keeps the ice crystals small; upon defrosting, the food is not mushy. Three methods are used: air blast, indirect contact, and immersion.
Blast freezing uses a blast of cold air to chill food. Indirect contact involves placing food on hollow plates through which a cooling liquid, or refrigerant, circulates. Immersion in a very cold liquid, usually a solution of salt and water, is probably the fastest method of freezing food. To prevent contact between the food and the liquid, the food is packaged prior to freezing.
Some foods do not lend themselves to freezing. Lettuce, tomatoes, and other foods with a high water content do not freeze well. Meat and fish with a high fat content freeze well but cannot be stored for extremely long periods of time after freezing because the fat becomes rancid.
This technique involves freezing food in a vacuum and then dehydrating it by sublimation (a process in which ice turns directly into water vapor). The food retains its original size, shape, and cellular structure. It is lightweight and may be stored for long periods without refrigeration. To use the food, a consumer simply adds water, or rehydrates it. The end product looks much like fresh food. Coffee, meat, and fruits are often freeze-dried.