Change and stability in frozen foods

The defining step in freezing is the removal of heat. This lowers the temperature of foods so that microbial and chemical changes are prevented or minimised. By storing in the frozen state it is possible to prolong greatly the length of time that many foods may be maintained with an excellent sensory and nutritional value. It is, however, important to realise that at the typical temperatures used for industrial and domestic storage of frozen foods (typically -24°C and -18°C respectively), chemical reactions that can lead to a reduction of quality and nutrient loss may continue to occur. Many of these reactions take place in solution and even at -24°C, natural foods such as fruit, vegetables and meats may still contain 2-5% of their total water content in the liquid phase. As the temperature of natural foods is reduced below 0°C ice crystals begin to form and the solutes present in intra-and extra-cellular fluids become more concentrated in the remaining liquid water, thereby lowering the freezing point of this water. Therefore, although the rates of most reactions will be substantially reduced by the lower temperature of frozen foods, the increased solute concentration may to some extent counteract this effect. Another effect of the increased solute concentration is to move water by osmosis between compartments. The formation of ice may also rupture cell structures causing mixing and reactions between components previously held apart. The complex nature of the changes that take place when foods are frozen makes it difficult to predict effects on quality and stability.

Probably the most important reaction leading to both quality and nutrient losses in frozen foods is oxidation. The consequences of oxidative instability are the key factors that limit the storage life of frozen foods. Just as in foods kept at more normal ambient temperatures, unless they are stored in a vacuum, or in an inert gas, atmospheric oxygen will diffuse through frozen food and may react with many of the soluble and insoluble components. One consequence of oxidation on sensory quality is the development of 'off flavours' and rancidity, usually caused by oxidative breakdown of membrane and storage lipids (Erickson, 1997). Other adverse consequences of oxidation may include colour loss and/or change, and in fish and meat foods a toughening of muscle structures. Although macro-molecular components such as carbohydrates and protein may undergo limited oxidation, any influence on nutritional value is likely to be small. However, several vitamins such as ascorbate and folates are particularly susceptible to oxidative damage.

A feature of the quick freezing of foods is the formation of a large number of relatively small ice crystals that cause minimal damage to cellular and tissue structures but on prolonged frozen storage, and particularly in conditions where temperatures fluctuate, crystals of ice grow in size. Although at any temperature below 0°C, the total amount of ice in a food will remain constant, large crystals grow instead of a larger number of smaller crystals, a process known as Ostwald ripening. The growth of larger ice crystals may break delicate food structures and compress others. On thawing of frozen foods these changes may have serious effects on texture leading to poor sensory quality; vegetables and fruits may lose their characteristic crispness and meat or fish may become tougher and drier. An adverse consequence for nutritional value is the reduced water-holding capacity of structurally damaged foods, leading to increased 'drip loss'. Significant amounts of water-soluble nutrients may be discarded if this drip loss is not incorporated into the food to be consumed.

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