Storage in the frozen state enables products to be kept for longer than maintaining chilled conditions. Freezing reduces bacterial degradation reactions to a very low level but causes structural change in the product due to the formation of ice crystals. The cells of animal and vegetable products contain a water solution of salts and sugars. When this solution starts to freeze, surplus water will freeze out until the eutectic mixture is reached (see Section 12.5). If freezing is not carried out quickly, the ice crystals will grow and pierce the cell walls; then when the product thaws out, the cells will leak and the texture will be spoiled. This is of no great consequence with the meats, whose texture is changed by cooking, but fresh fruit and vegetables need to be frozen quickly.
The texture and moisture content of the product after thawing will differ from that of the fresh product, and for some products it also results in weight loss in the form of ‘ drip loss ’ . Different freezing methods are used to minimize these effects.
As a general rule, any product which will be eaten without cooking, or only very brief cooking (such as green peas, strawberries and beans), should be quick- frozen in a blast-freezing tunnel or similar device. Other foodstuffs need not be frozen so quickly, and placing food items in large refrigerated rooms is the most common method of freezing. For meat and poultry there is no clearly defined optimum freezing rate. Many factors such as final product quality (tenderness, flavour), weight loss, drip loss, and uniformity of texture have been investigated. The most recent comparison (Sundsten et al., 2001) revealed some commercial advantages of fast freezing, but no quality advantages. During industrial processing, frozen raw material is often thawed or tempered before being turned into the final product which is subsequently frozen. Meat-based products, i. e. pies, convenience meals, burgers, etc., often include meat which has been frozen twice.
Frozen confections such as ice-cream rely on speed of freezing to obtain a certain consistency and texture, and they require special treatment (see Chapter 16).
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