Changes in myofibrillar proteins of bovine longissimus and semitendinosus muscles were examined during 14 days of postmortem storage at 2°C by SDS-Polyacrylamide gel electrophoresis. Major changes in both muscles were: (1) appearance of a 95,000-dalton component;(2) gradual disappearance of troponin-T and gradual appearance of a 30,000-dalton component; (3) gradual increase in intensity of a protein around 55,000-daltons; and (4) gradual appearance of a 110,OOOdalton component. With the exception of the 95,000-dalton component, the other changes have been reported previously by other authors. At the present time we have no knowledge of the origin of the 95,000-dalton component, but it is known that calcium-activated factor (CAF) is responsible for degradation of troponin-T. Appearance of the 95,000-dalton component during postmortem storage suggests involvement of CAF for increasing tenderness during postmortem storage.

Proteolytic enzymes in the sarcoplasmic fluid of Pacific whiting (Merluccius productus) were studied to determine which proteases might be involved in the severe textural defect associated with this species. The results were compared to those obtained for true cod (Gadus macrocephalus), a fish having firm texture. Two pH optima of enzymic activity were noted for whiting, pH 3.5-3.9 and 7.1-7.2, Two pH optima were also observed for cod, pH 3.2-3.6 and 7.8-8.0. Whiting but not true cod contained enzymic activity similar to cathepsin B. Cathepsin C activity was greater in whiting than in cod.

Crude calcium‐activated factor (CAF) was prepared from pre‐rigor pressurized (PRP) and corresponding control (CON) bovine muscles at 0, 1, and 3 days of postmortem times. Throughout these periods, PRP samples had consistently lower total CAF activity than CON. Yet, it seems probable that PRP caused a transient increase in Ca++ sufficient to activate CAF for a brief period of time because (1) early postmortem, PRP muscle had quantities of a 95,000‐dalton component presumably produced by CAF; (2) Z‐lines from at‐death PRP muscles were highly degraded; (3) very early postmortem storage, the SDS‐electrophoretic patterns of PRP myofibrils resembled that of CAF‐treated myotibrils. CAF autolysis may account for the low CAF activity observed in PRP muscles at later postmortem storage times.

Myofibrils were isolated from the longissimus (L) muscle of control (CON) and cold-shortened (CS) muscles after 0, 1,3, 7, and 10 days of postmortem storage at 2°C. Isolated myofibrils were then examined by SDS-polyacrylamide gel electrophoresis to monitor the changes in the myofibrillar proteins during postmortem storage. The main changes in CS muscles were the gradual appearance of llO,OOO-, 95,000-, and 30,OOOdalton components and the disappearance of desmin and troponin-T components of myofibrils. In addition, there was a gradual increase in the intensity of a protein around 55,OOOdaltons. CON samples showed similar changes to those of CS samples. It appears that myofibrillar proteins of coldshortened muscles are affected by postmortem aging in a manner similar to that of the normally chilled muscles.