Effects of rate of heating and final internal temperatures on the texture of fish gels were studied using mechanically deboned fish tissues of 2 species and 2 harvest locations. Generally, rapid heating to 85°C internal temperature (using 100°C steam) produced a firmer, more springy texture in fish gels in comparison with those heated slowly to 70°C internal temperature. However, these thermal effects on gel textures were species and harvest location dependent. Degradations of tropomyosin and myosin observed in cooked fish gels were highly related to gel textural properties. Results further suggested that changes in muscle proteins during heating were caused by proteolytic factor(s) in the sarcoplasmic fraction. The proteolytic crude fraction was isolated and partially characterized. Optimal temperature for proteolytic activity was 60°C; optimal pH was between pH 8.0 and pH 8.5; calcium ion activated the proteolytic activity and the optimal calcium ion concentration for activation was 1 mM; metal chelators, EDTA and EGTA, inhibited the proteolytic activity. A thorough understanding of these proteolytic factors and their subsequent control is important for the utilization of mechanically deboned fish tissues.

Heat-induced changes in myofibrillar protein solubility were studied in samples of at-death and postmortem bovine logissimus heated at 45", 50", 55", 60", 70" and 80°C. Myofibrillar proteins were extracted with strong salt solution after the removal of sarcoplasmic proteins for each heat treatment. Changes in myotibrillar protein solubility were determined by using SDS-polyacrylamide gel electrophoresis. The proteins of thick and thin filaments and Z-disks reacted differently to heat and postmortem aging time. Alpha-actinin was the most heat labile and became insoluble at 50°C. Next, heavy and light chains of myosin became insoluble at 55°C. Actin, tropomyosin and troponin were more heat resistant, however, inasmuch as actin was insoluble between 70 and 80°C and tropomyosin and troponin became insoluble above 80°C. That the 30,OOOdalton component was more intense after heating suggests that calcium activated factor (CAF) activity is stimulated during heating. This further suggests that the effect of CAF on heated muscle is additive to its tenderization effect on postmortem aged muscle.

SEM was used to determine the effect of postmortem aging (unheated) and cooking (heated to internal temperatures of 60, 70 and 80°C by broiling) on the ultrastructural characteristics of bovine longissimus and psoas major muscles. Micrographs from postmortem aged (unheated) muscle clearly showed the fibrillar and connective tissue structures of muscle. Progressive changes with increased temperature were observed in endomisial sheath swelling, collagen fiber disintegration and myofibril fragmentation, coagulation and shrinkage. Specifically, after heating to 7O"C, banding patterns and myofibril fragmentation at Z-disks were clearly evident. Degradation of collagen fibers in the perimysium was initiated at 70°C and intense disintegration was observed at 80°C. Changes observed in psoas major were different from those in the longissimus in that intact myofibrils and tubules were observed in both the heated and unheated samples. This may be due to the "loose" packing of myofibrils unique to psoas major muscle. Furthermore, less shrinkage and coagulation of myofilaments in the A band region and wider I band regions were noted. These observations of looser packing of myoflbrils, thinner myotibril threads and wider I band regions offer additional evidence as to why steaks from psoas major muscle are more tender than those from longissimus muscle.

Protein functionalities were studied in cornminuted fish gels from mechanically deboned fish tissues of four species after storage .at -29°C up to 12 months. Differences in gel textures, which were evaluated instrumentally and by a texture profile sensory panel, were evident among fish species and time in frozen storage. Fish gel texture was not significantly related to, protein solubility of raw tissues, but was closely related to water-holding capacity and protein solubility of cooked gels. Degradation of tropomyosin and myosin probably occurred in some of the fish gels during therfnal processing.Protein extraction of raw and cooked fish gel Two grams of raw or cooked fish g&l (in duplicate) was homogenized with 20 ml of solution (either 3% NaCl or.5 mhJ Tris) in a Sorvall OmniIMixer for three -15-set periods and centrifuged at 15,000 x G for 20 min to obtain the soluble fraction. The protejn Volume 44 (7979)--JOURNAL OF FOOD SCIENCE-1087 '* .: :

Myofibrils were isolated in a buffered medium by differential centrifugation from at-death and l-, 2-and 10day postmortem bovine longissimus muscle. Following isolation, myofibrils were extracted with 1 Hasselbach-Schneider (H-S) solution, 0.6M KC1 buffered solution and 1 mM Tris, pH 8,5 solution. Postmortem changes in isolated myofibrils and their protein extracts were characterized with SDS-polyacrylamide gel electrophoresis. SDS-polyacrylamide gels showed that H-S and KC1 buffered solutions extracted thick and thin filament proteins of myofibrils isolated from either at-death or postmortem muscle samples. Changes in protein extractability wrth H-S and KC1 solutions during postmortem storage were difficult to assess. 1 mM Tris solution extracted the thin filament proteins, actin, tropomyosin and troponin, and the thick filament proteins, C-protein and myosin light chains. or-Actinin was very resistant to 1 mM Tris, pH 8.5 solution extraction from isolated myofibrils of at-death muscle, but it was readily extractable from isolated myofibrils of postmortem muscle. C-protein also seems to become more extractable during postmortem aging of muscle. The prominent and distinguishable change occurring in the strong salt-soluble proteins of the myofibril during postmortem storage of muscle at 2°C was the gradual degradation of troponin T and the concurrent appearance of a 30,000-dalton component. These results indicated that modifications occur in thick and thin filament proteins during postmortem storage and that these modifications are possibly related to calcium activated factor (CAF).