What Causes Postmortem Tenderization?

Inconsistency and variability in tenderness have been identified as a major problem facing the meat industry (Morgan et al., 1991). Tenderness increases with increasing time of postmortem storage (Calkins and Seideman, 1988; Goll et al., 1964; Wheeler and Koohmaraie, 1994), and it has generally been assumed that part of this increase is caused by postmortem proteolysis mediated primarily by the calpain system (Goll et al., 1974; 1983; 1992; Koohmaraie, 1988; 1992a; 1992b). Consequently, considerable effort has been directed at determining what myofibrillar proteins are degraded during postmortem storage and whether this degradation is related to tenderness and to calpain activity (Goll et al., 1992; Koohmaraie, 1988; 1992a; 1992b). These studies have shown that the large cytoskeletal proteins, nebulin and titin, along with troponin T and vinculin are degraded during postmortem storage (Anderson and Parrish, 1989; Ho et al., 1994; 1996; 1997; Huff-Lonergan et al., 1995. Paxhia and Parrish, 1988; Taylor et al., 1995). Degradation of nebulin, titin, and vinculin would be expected to contribute to increased tenderness. These same studies, however, have also shown that very little proteolytic degradation of these cytoskeletal proteins occurs during the first 24 hr postmortem (Anderson and Parrish, 1989; Ho et al., 1994; 1996; 1997; Taylor et al., 1995) and that proteolytic degradation of muscle proteins is minimal even up to 3 days postmortem (Huff-Lonergan et al., 1995; Taylor et al., 1995). Yet, the largest changes in tenderness occur during the first 3 days postmortem (Calkins and Seideman, 1988; Dransfield, 1992; Goll et al., 1964; Wheeler and Koohmaraie, 1994). Is the limited amount of proteolysis that occurs during the first 3 to 4 d of postmortem storage sufficient to cause the large changes in tenderness that are observed during this period or are other factors involved? If other factors are involved in postmortem tenderization, what are these factors and how important is their role? This article will summarize the evidence suggesting that postmortem changes in tenderness involve at least two different phenomena: 1) proteolytic degradation of cytoskeletal proteins, particularly nebulin, titin, and vinculin, during postmortem storage; this proteolytic degradation is caused primarily by the calpain system; and 2) changes in the nature of the actin/myosin interaction: these changes result from alterations in the manner in which myosin cross-bridges bind to the actin thin filament. We suggested several years ago that both changes in the actin/myosin interaction and calpain-induced proteolysis are involved in postmortem tenderization (Goll et al., 1995), and this article will elaborate on some of the points made in this previous paper.