Frozen storage changes

Tofu can be kept frozen or freeze-dried to prevent microbial deterioration. However intemolecular interactions occur during frozen storage. As a result, the texture of tofu is changed from soft, smooth to sponge-like with a meat-like chewiness. 

Berg, L. Vanden. 1961. Changes in pH of milk during freezing and frozen storage. J. Dairy Sci. 44, 26-31. 

Frozen storage of meat, poultry meat and fish is one of the most important preservation methods for these foods. During frozen storage, deteriorations due to putrefaction and autolysis are decreased and the foods are satisfactorily preserved from the hygienic point of view. However, several undesirable changes still occur in the frozen stored meats. 

In order to ovecome these defects, much research has been done in an attempt to clarify the mechanisms and causes of these changes during frozen storage of meats. Such studies have included a wide variety of animals including beef animals, hogs, poultry, fish, shellfish and other invertebrate aquatic animals. The number of papers published in this area amounts to several hundred. 

Information on the molecular changes occurring during frozen storage of whole muscle or of isolated protein preparations will be reviewed here. 

Actomyosin. Frequently, the change in amount of soluble actomyo-sin is regarded as the primary criterion of freeze denaturation. It must be remembered that solubility data do not tell precisely how much protein is denatured and how much is native rather, it provides a relative measure of denaturation. Solubility decreases have been found in frozen storage experiments with either intact muscle, protein solutions or with suspensions of isolated actomyosin. 

Jarenback and Liljemark (75,76) found similar changes in cod actomyosin solution and cod muscle during frozen storage. The denatured myosin was not extracted with salt solution. 

When 0.1 M sodium glutamate was added to carp actomyosin, denaturation during frozen storage was almost eliminated, as measured by changes in solubility, viscosity, ultracentrifugal behavior, ATPase activity and electron microscopic profiles (66,72) (Figure 3). This protective effect of sodium glutamate will be discussed below. 

Changes in solubility, viscosity, ATPase activity, and ultracentrifugal and salting-out profiles were found during frozen storage at -20°C of carp myosin solutions (in 0.6 M KC1) and carp myosin suspensions (in 0.05 M KC1) (82,93). 

HMM and LMM were stored frozen at -20°C, and the changes in properties were followed for each protein (82). While there were no significant changes in the solubility curves for HMM and LMM, appreciable changes were found in other properties. ATPase activity of HMM decreased to 50% of the pre-freezing value after 1 day and was not detectable after 2 weeks. No initial increase in activity was found with HMM. The rate of decrease in ATPase activity was much faster than with myosin solutions, where about 55% of the initial activity was retained after 7 days frozen storage. The ability of HMM to bind with F-actin, as determined by electron microscopy, was lost after 2 weeks frozen storage. 

Addition of 0.1 M sodium glutamate reduced the changes occurring in HMM and LMM during frozen storage (Figure 5). 

The water-activity relations, effects of displacements of water or effects of changes in the state of water must be the most important factors to trigger and to promote the denaturation of muscle proteins during frozen storage. 

Data on myosin (50,51,82,91) and LMM (82) support side-to-side aggregation of molecules without appreciable change in conformation during frozen storage, as proposed by Connell (91). 

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