Storage Temperatures

Because the purpose of freezing is to extend the shelf life of food products, the influence of storage temperature on storage life and thawed quality is a concern. Traditionally, it has been assumed that storage at - 18°C (0°F) is adequate for all products. Although it is true that storage at lower temperatures is preferred to storage at higher temperatures, - 18°C does not always result in an acceptable shelf life. Some products require lower storage temperatures for extended storage. Ice cream and fish, for example, are much better stored at temperatures below — 30°C if unacceptable changes in quality are to be avoided. The influence of temperature on storage life is now much better understood. At temperatures close to — 5°C, change tends to be accelerated, compared to temperatures above or below. This is because of the reaction-enhancing effect of increased solute concentration in the unfrozen phase outweighing the general slowing effect of lowered temperature (18). This is of special significance in the careful handling of frozen foods. Beyond the zone of maximum ice formation, the influence of temperature is the primary determinant of reaction rates. However, it is now understood that it is the relative temperature, rather than some absolute temperature, which is significant. As a system freezes and ice forms, the properties of the unfrozen aqueous phase change. Increasing concentration and decreasing temperature lower the mobility of the solute molecules. At some characteristic temperature, solute mobility becomes severely constrained, and reaction rates decrease markedly. Although it may not be economically feasible to use temperatures below this characteristic temperature for storage, it also serves as a reference temperature against which the actual temperature of storage can be compared. For products where this temperature is lower, storage temperatures also should be lower to achieve extended shelf life.

As previously indicated, freezing is necessarily followed by thawing. The thermal properties of water and ice are such that the freezing process and the thawing process are not mirror images. Thawing follows a different profile (18). Because ice is a better conductor of heat, heat transfer into a frozen product is more rapid than heat removal from a similar unfrozen product. As ice begins to melt at the surface, a thermal barrier is formed, because liquid water is a less effective heat transfer medium. As a result, on thawing the temperature rapidly rises into the zone around

— 5°C, and then heat transfer becomes less effective. The latent heat plateau takes longer to traverse than in freezing, where the heat transfer is through an external layer of ice. As a consequence of this property, exposure of frozen products to higher temperatures even for a short time can lead to significant quality loss. The thawing process, too, if not well controlled, can result in loss in quality, as a consequence of the potential for significant reaction at

Freezing is an effective method for food preservation. The deceptively simple technology requires care to be exercised in temperature control for optimum quality of product to be maintained to consumption.

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