Introduction
Providing the meat tenderness that consumers desire is one of the major problems facing the meat industry, because postmortem meat tenderization is highly variable between carcasses. Therefore, it is essential to study at the molecular level biochemical mechanisms that relate to postmortem muscle breakdown. Myofibril proteins, whose function is to maintain structural integrity of myofibrils, are involved in intermyofibrillar (desmin and vinculin) and intramyofibrillar (titin, nebulin, and troponin-T) protein. Recent findings suggest that calpain-mediated proteolysis of myofibril proteins is mainly responsible for improvement of meat tenderness during postmortem storage of carcasses (Ouali and Talmant, 1990; Goll et al., 1992; Koohmaraie, 1992; Koohmaraie, 1994). Postmortem tenderization of meat is widely hypothesized to be the result of degradation and weakening of the myofibril proteins near the Z-disks (Goll et al., 1992; Kendall et al., 1993; vanden Hemel-Grooten et al., 1997). Postmortem proteolysis, change of muscle protein, also causes change in pH, which is related to myofibril breakdown during the postmortem period, and, therefore, meat undergoes metabolism for oxidation or glycolytic activities. It is possible to explain the process of meat tenderization by analyzing myofibril proteins, because postmortem change in myofibril proteins is one of the main causes of the change in meat structure. If differences in the rate and content of proteolysis of these specific proteins account for differences in the rate and content of tenderization, then variation in the tenderness of meat at the consumer level could be controlled by genetic and biochemical methods.