Protein frozen storage

Sych, J., Lacroix, C., Adambounou, L. T., Castaigne, F. (1990). Cryoprotective effect of lactitol, palatinit and polydextrose on cod surimi proteins during frozen storage. J. Food Sci, 55, 356-360. 

Sugar alcohols have also found application in foods containing sugars. Sorbitol is an effective cryoprotectant in surimi, preventing denaturation of the muscle protein during frozen storage. 

The effect of cooking on incurred residues of oxfendazole in cattle liver has been also investigated (88). However, the results drawn from this study are inconclusive due to several variable factors. One such factor is the unstable equilibrium between oxfendazole, oxfendazole sulfone, and fenbendazole in the incurred tissue. Other factor is the overall instability of oxfendazole and its metabolites in tissue during frozen storage. Another factor is the variable distribution of the residues within the tissue used for the study and the effect of protein binding on the extractability of the residues from the tissue. It was nevertheless... 

EL Le Blanc, RJ Le Blanc. Separation of cod (Gadus morhus) fillet proteins by electrophoresis and HPLC after various frozen storage treatments. J Food Sci 54 827-833, 1989. 

Khan, M. A. A., Hossain, M. A., Hara, K., Osatomi, K., Ishihara, T., and Nozaki, Y. 2003. Effect of enzymatic fish-scrap protein hydrolysate on gel-forming ability and denaturation of lizard fish Saurida wanieso surimi during frozen storage. Fish. Sci., 69,1271-1280. 

Chemical Deterioration of Muscle Proteins During Frozen Storage... 

Actomyosin. At high salt concentrations ( . . 0.6 M KC1), actin and myosin combine to form actomyosin filaments giving a highly viscous solution. Actomyosin retains the ATPase activity of myosin and demonstrates "super-precipitation" on the addition of ATP (24,34). As expected, there are differences between actomyosins of rabbit and fish with respect to solubility (10,22,35,36), viscosity (46) and ultracentrifugal behavior (477. Since actomyosin is the most readily available form of myofibrillar proteins from fish muscle, its behavior relative to deterioration during frozen storage has been most frequently studied. 

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. 

The above mentioned dissociation of actomyosin into actin and myosin could be due to a shift in the equilibrium, actomyosin 5= actin + myosin, by the highly concentrated salt solution of the unfrozen liquid portion in the protein-water system (22, 77). However, if this is true, the dissociated actin and myosin must re-associate immediately after thawing. This may be difficult since the ability to associate is decreased during frozen storage. 

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. 

Tropomyosin and troponin. Tropomyosin is apparently the most stable of the fish fibrillar proteins during frozen storage. It can be extracted long after actin and myosin become inextract-able however, it does denature gradually (90). 

Some enzymes and enzyme systems are still active at the temperature of frozen storage (123-132). Such enzymatic activity, especially of proteases, may cause loss of biological activity of actomyosin and other muscle proteins. Products of such enzymatic activity, e.g. free fatty acids and formaldehyde, may effect a secondary denaturation of muscle proteins. 

Among the above hypotheses, effects of lipids (4-17,59-62, 69-71,155-159), formaldehyde (160-166), and gas-solid interface TMJ appear to be very important in Gadoid fishes. Denaturation of myofibrillar proteins caused by free fatty acids and/or lipid peroxides must occur during frozen storage. To prove this, Jarenback and Liljemark have shown by electron microscopy that, in muscle stored frozen with added linoleic and linolenic hydroperoxides, myosin became resistant to extraction with salt solution (168). 

The concentrated salt solution may denature the proteins (9-17, 169-177). Whereas experiments with isolated muscle protein preparations cannot exclude the effects of salts such as NaCl or KC1 (since they are required to solubilize the proteins), denaturation during frozen storage has been decreased or prevented completely when an efficient cryoprotectant such as sodium glutamate or glucose was added (66,67,82,93,145-150). Hence, the effect of salts may not be of primary importance, though they may contribute. 

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. 

As described by Fennema (9J, several refined hypotheses such as "physical barrier and structured water hypothesis" (134,178, 179), "ice-moderator hypothesis" (180-183), and "minimum cell volume hypothesis" (184) have been proposed. However, the author will take a more naive approach in interpreting the results on denaturation of muscle proteins during frozen storage at the same time taking advantage of the basic ideas of the above hypotheses. 

Figure 9. A schematic model of denaturation of a-helical proteins during frozen storage and its prevention by cryoprotectants. The case with dianionic cryopro-...

Unfolding of globular proteins and subunits. Data on frozen storage of HMM, actin and sarcoplasmic enzymes have led us to propose that denaturation involves unfolding of the protein chain based on a decrease in enzymatic activity (myosin, HMM, and sarcoplasmic enzymes), polymerizing ability (actin) and filament forming properties (myosin) (82,99,113-116,122). 

Figure 6. The insolubitization of soybean protein during frozen storage at —5°C and their solubility behavior in urea and mercaptoethanol (ME) (10).

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