The two basic ways to achieve the freezing of foods are quick and slow freezing. Quick orfast freezing is the process by which the temperature of foods is lowered to about −20◦C within 30 minutes. This treatment may be achieved by direct immersion or indirect contact of foods with the refrigerant and the use of air blasts of frigid air blown across the foods being frozen.
Slow freezing refers to the process whereby the desired temperature is achieved within 3–72 hours. This is essentially the type of freezing utilized in the home freezer. Quick freezing possesses more advantages than slow freezing, from the standpoint of overall product quality. The two methods are compared in Exhibit 16–1.
With respect to crystal formation upon freezing, slow freezing favors large extracellular crystals, and quick freezing favors the formation of small intracellular ice crystals. Crystal growth is one of the factors that limit the freezer life of certain foods, because ice crystals grow in size and cause cell damage by disrupting membranes, cell walls, and internal structures to the point where the thawed product is quite unlike the original in texture and flavor.
Upon thawing, foods frozen by the slow freezing method tend to lose more drip (drip for meats; leakage in the case of vegetables) than quick-frozen foods held for comparable periods of time. The overall advantages of small crystal formation to frozen food quality may also be viewed from the standpoint of what takes place when a food is frozen.
During the freezing of foods, water is removed from the solution and transformed into ice crystals of a variable but high degree of purity.17 In addition, the freezing of foods is accompanied by changes in properties such as pH, titratable acidity, ionic strength, viscosity, osmotic pressure, vapor pressure, freezing point, surface and interfacial tension, and oxidation–reduction (O/R) potential (see reference below).
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