Water In Food Products
This book deals with freezing food, a process in which the temperature of the food lowered so that some of its water crystallizes as ice. This happens in freeze-drying, freeze concentration of the juices and firming of meat cutting or grinding ('hardening'). However the greatest application of freezing of food products to retain them, or to extend their shelf life. This is the basis of the huge frozen foods sector, widely developed and adopted food consumers. Low temperature (18C in the household freezers, -28C in primary wholesale refrigeration warehouses or -60C in certain food refrigerated warehouses) slow down the processes of damage (enzymic autolysis, oxidation and education bacteria)that would have taken place at room temperature, or even in cold temperatures.
Products are usually preserved by freezing
Water is a catalyst for biochemical deterioration of the quality of foodstuffs. Dry foods are more stable than wet products, because any water remaining in their low activity, aw. Freezing removes water from the food matrix on the formation of ice crystals.
Although ice crystals remain in the food, the remaining water, which is in contact with the food matrix becomes a concentration of dissolved solids and aw is getting low. Freezing so akin to drying and this is the reason for canning freezing. Most microorganisms, cease to function under the water activity of about 0.7. Usually frozen foods are those that contain significant amounts of water.
Living cells, biological materials (the tissues of plants and animals in their natural state are able to hold as a rule, 80% water by weight on a wet surface. Therefore, food products derived from them contain similar with high water content. This also applies to the " engineering " products, such as ice cream, where the water/ice mixture is required to give texture.
Influence of freezing and storing the frozen on the quality of the food
Food thawed after the cold storage should ideally be indistinguishable from the fresh product (this obviously does not apply to products such as ice cream consumed frozen). This requirement is easier to achieve in some foods than in the other. Products with a fine structure, more likely to suffer damage to cells. However, for basic food products (bread, meat, fish, vegetables) quality of the thawed product truly comparable with the fresh product (and in some cases, the use of certain criteria, for example, the content of vitamins, improves the quality of fresh products sold as fresh or chilled). The formation of ice crystals may lower the quality of the meal one of the following three mechanisms:
- (a) Mechanical damage in the structure of power. The specific volume of ice more than water (higher by approximately 10%) and, consequently, the expansion of the ice crystals compress matrix. The ice crystal expansion in some fruits, such as strawberries seriously damage them because of their fine structure (the fetus becomes 'raw', thawing). At the macroscopic level, with the rapid cryogenic freezing, thermal stresses due to expansion can hack food.
- (b) Cross-linking proteins (meat and fish). Reduction of quantity of liquid water proteins and increase in the concentration of the electrolyte in the freezing lead to aggregation and denaturation of actomyosin (Connell, 1959; Buttkus, 1970).
- (c) Limited reabsorbqiyu water from melting. It is connected with the mechanism (b). Again, we can take the example of animal tissue, in which the muscle proteins in the process of frozen storage, become " naked " their water of hydration and cross-linked. The thawing tissues cannot re-absorb the melted ice crystals completely water content, it had before freezing. This leads to undesirable release of the exudate - 'drip loss " - and endurance texture in the thawed muscles, the main features that determine the quality (Mackie, 1993).
Mechanisms (b) and (c) usually are the main causes of deterioration of the frozen products quality, which means quality deterioration, mainly due to the processes in frozen storage, and not during the initial freezing. Quick freezing is possible only for small samples, not commercial. The speed of freezing achievable for large commercial samples, so small that the quality of the products will not be much depends on the speed of freezing (extracellular ice invariably forms for all samples, other than those that are small and have stood in the laboratory by special techniques)...
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Food refrigeration and freezing