Muscle protein extractability as influenced by conditions of post-mortem glycolysis

Muscle protein extractability at the time of death has been used as an indicator of in vivo protein composition(1). Mirsky(2) suggested that a large proportion of muscle myosin becomes insoluble as rigor mortis develops. Bendall and Wismer-Pedersen(3) have stated that loss of myofibrillar protein solubility under conditions of rapid postmortem glycolysis was the result of precipitation of denatured sarcoplasmic proteins on the fibrillar surface thereby markedly decreasing the extractability of muscle proteins. The present study was initiated to investigate the changes in the extractability of sarcoplasmic and myofibrillar proteins under various conditions of rigor mortis and glycolysis. Porcine longissimus dorsi samples were frozen by immersion in a dry ice acetone bath immediately after death, at onset and completion of rigor mortis(4), and 24-hr, postmortem. These samples were extracted according to the method of Helander(5) to determine solubility of sarcoplasmic and myofibrillar proteins. Results were grouped according to the conditions of temperature and pH in the muscle at 2 hours post-mortem. Fig. 1 shows that although sarcoplasmic protein solubility at 24 hours post-mortem was decreased from the initial solubility in all groups, it had decreased approximately 30-35% in Group A (> 35°) at the onset of rigor mortis and only 5-10% in Groups B, C and D (< 35°) at the same period. This suggested that under the pH conditions encountered in the muscle at 2 hours postmortem the sarcoplasmic protein solubility remained high as long as the muscle temperature was below 35°C at 2 hours post-mortem. However, at the completion of rigor in Group B (< 35° 5.7 pH) the sarcoplasmic solubility had significantly decreased. Fig. 2 indicates that there were no changes in myofibrillar solubility at various postmortem periods as long as the pH was medium to high (> 5.8; Groups C and D) and temperature low (