Nutritional Aspects
A wide range of frozen meat, poultry and fish products are now available for the consumer. They range from simply prepared contraction of the muscles, all the birds and safety of fish to highly processed products containing components derived from several animal tissue. From the point of view of food-meat, poultry and fish are a particularly important source of food proteins and fats, minerals, iron, zinc, magnesium and selenium (in particular, fish and other sea foods); the b vitamins, vitamin A (in the liver) and vitamin D (found in oily fish species).
Following the slaughter, muscles and other tissues of animals undergo complex biochemical changes; these can occur as a result of residual metabolic reactions, or from microbial and/or oxidative damage. The purpose of freezing to slow down or prevent post-mortem changes, which may negatively influence on the microbiological safety, the sensory quality and nutritional value. Although the freezing process in itself will not have a significant impact on the levels of nutrients in meat, poultry and fish food, less certainty, long-term frozen storage is causing a significant loss of potentially volatile nutrients.
A recent study frozen storage of protein and fat storage assessed the long-term stability of food samples collected in the course of clinical nutrition (Phillips et al., 2001).
Mixed food sample was prepared from the typical components in the American diet. Storage and thawing of samples was carried out under the " ideal " conditions - in airtight containers, without fluctuations of temperature and without any drip loss after thawing. Although these conditions do not represent a common household or commercial practice, this study confirmed that the content of proteins, fats and certain fatty acids, have not been significantly reduced due to storage at -60C to 50 months.
Studies measuring the impact of frozen storage in tissues of animals under more realistic conditions were essentially aimed to determine the impact on the sensory quality, not nutrients. Early studies on the levels of vitamins in the muscles of pork, beef and poultry have shown that the losses amounted to about 10-30% of Niacin, pyridoxine, thiamine and Riboflavin after the storage period up to one year (Westerman, 1952; Lee et al., 1954; Fennema, 1975; Engler and gazebos, 1976). In more recent systematic study of the level of thiamine, Riboflavin and pyridoxine were measured in frozen pork stored for 12 months under -12C and 24C (Mikkelsen et al., 1984). The loss compared with samples taken before freezing were 10-20% of all vitamins measure, but in this, as in previous studies, a high variability in the estimates of losses and a lack of consistency between samples and treatment was noted. Many published studies of nutrient loss on frozen storage do not distinguish between the loss of nutrients caused by chemical breakdown and physical loss of nutrients in the liquid, which exudes from meat during and after thawing.
The following long-term frozen storage of muscles of the products, it was noted that the irreversible aggregation of protein actin and myosin myofibrils can happen. This will not adversely affect the nutritional value of protein, but may lead to more meat texture and reduction in the ability of the muscle structure to hold water. As a consequence, the defrosting frozen intact muscles can lose a significant amount (2-15%) of intracellular and intercellular fluid, as so-called " drop-loss". If this liquid is not included in the food products consumed, it can represent a significant loss of water-soluble proteins, vitamins and minerals. Many factors can contribute to the determination of the amount of drip loss; these include, the age of the animal, pre-slaughter of the animal, the speed of freezing and thawing rate...
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Food refrigeration and freezing