Ttt

The concept that the rate of quality loss, as influenced by temperature, in most frozen foods during storage broadly follows the Arrhenius equation, as postulated in the T-TT studies (4), has stood the test of time. One refinement has been the investigation of the glass transition phenomenon in many frozen products (11). As temperature is reduced well below the freezing point, the ice crystals grow until the remaining unfrozen moisture, the freeze-concentrated unfrozen phase, becomes an increasingly highly concentrated and viscous solution. If temperature is lowered further, the point is reached (Tg) at which it becomes a "glass"—an amorphous solid, devoid of structure and totally unlike the highly ordered structure of ice crystals. Below the glass temperature (Tg) the rate of molecular mobility is markedly reduced and hence the rate of quality loss brought almost to a standstill. The kinetics of reactions associated with quality loss at temperatures close to the glass transition temperature (Tg) can be described by the Williams-Landel-Ferry (WLF) model, in which reaction rates (and hence quality loss) are a function of the temperature difference between the storage temperature and Tg (12). Shelf-life modeling of frozen foods is well described by Fu and Labuza (13). While the Tg of most frozen foods may be so cold as to render it too expensive (and unnecessary in view of the short storage periods—a few months—necessary to span the seasons or transport food across the world) to store at or near the Tg these findings open up new avenues of research to breed food plants and formulate prepared dishes with a "warm" Tg, thus increasing their PSL at conventional storage temperatures. Perhaps this research will even extend the range of products that can be successfully frozen.

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