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Many nitrogenous compounds are involved in postmortem reactions in the muscles of animals, primarily as enzymes catalyzing the catabolysis of different tissue components. Thus, they contribute to changes in color, flavor, and rheological properties of the products. Furthermore, proteins and NPN participate also as substrates in reactions leading to the formation of products affecting the pH in the fish tissues. On the other hand, the rate and extent of changes in proteins are influenced by the products of degradation of saccharides and organic phosphates, as well as by the concentration of free ions, whereas these depend on the antemortem condition of the fish and on the temperature in the tissues after catch. A positive relationship of the enthalpy of activation of fish myofibrillar ATPase and the habitat temperature was demonstrated, from large negative values for cold-adapted species to high positive values for tropical species (Johnstone and Goldspink, 1975).

Heating, as applied in seafood technology, affects the rate of enzymatic processes, brings about denaturation and subsequent interactions of proteins, and accelerates chemical reactions of different tissue components. Therefore, heated seafoods change in color and in rheological properties, lose part of their water-retention capacity, and develop new, mostly very desirable flavor characteristics. The character and extent of these changes are affected by the rate, duration, and temperature of heating, as well as by the composition, biochemical state, and integrity of the food structure, the pH of the environment, the presence of different added substances, mainly salts, macromolecular extenders, and polyols, and the access of air (Aitken and Connell, 1979; Pan, 1990). Investigations on the effect of these factors should provide the fish processor with knowledge necessary to control the quality of cooked seafood products. Proper choice of heating rate, duration, temperature, and humidity may prevent losses in hot smoking of fish (Sikorski et al., 1984). Applying high enough time/temperature in cooking of crab may minimize leaching losses of soluble proteins and discoloration as well as facilitates the picking of the meat (Zaitsev et al., 1969; Dowdie and Biede, 1983).

Frozen fish stored several months at about −20°C may, after cooking, become tough, chewy, rubbery, stringy, or fibrous. This is accompanied by a loss in functional characteristics of the muscle proteins, mainly solubility, water retention, gelling ability, and lipid emulsifying properties. Freezing and thawing may result in lysis of mitochondria and lysosomes and a change in distribution of enzymes (Karvinen et al., 1982). A gradual decline in the activities of various muscle enzymes has also been observed during storage at freezing temperatures. The loss of ATPase activity, both in meat homogenates and in protein solutions, may reach 50–80% (Buttkus, 1967). The total solubility of proteins in neutral 5% NaC1 solution may decrease to about 30%, whereby the main loss regards the contractile proteins, mainly myosin heavy chain, M-proteins, tropomyosin, and troponins I and C in descending order (Owusu-Ansah and Hultin, 1992). It was shown by Jarenback and Liljemark (1975b), that the myosin microfibrils in fresh cod muscle could be almost totally extracted, whereas myosin in frozen muscle after prolonged storage was resistant to extraction. Significant changes in the sodium dodecyl sulfate—polyacrylamide gel electrophoresis (SDS—PAGE) and HPLC profiles of cod sarcoplasmic proteins due to frozen storage have been shown recently by LeBlanc and LeBlanc (1992b).