Proteolytic activity and calcium effect in dark-firm-dry and pale-soft-exudative meat

Changes in myofibrillar proteins and pH in dark-firm-dry (DFD) and normal beef, and in pale-soft-exudative (PSE), normal and DFD pork were monitored during post mortem storage (2°C). The DFD beef exhibited an overall increase in pH during storage; the pH changes in other muscle samples were inconsistent. No detectable post mortem proteolysis was observed in myosin and actin from the SDS-PAGE analysis. In beef, the gradual appearance of a 30kDa component after day 3 was the only definitive post mortem proteolytic change in both DFD and normal muscle. This proteolytic change was more pronounced in DFD and normal pork than in beef, and in pork, the occurrence of the 30kDa polypeptide coincided with the disappearance of troponin-T/tropomyosin. The rate of these changes was generally greater in DFD than in normal pork. Compared with DFD and normal pork muscle, PSE pork had a delayed emergence of the 30kDa component. Injection of CaCl₂ solution (0.3mol/L, at 5g solution per 100g muscle) did not appreciably alter proteolysis in all pork samples. Differences observed in the 30kDa polypeptide between normal, PSE and DFD pork may be part of the reason why the anomalous pork differs from normal pork in textural quality.