Role of the Neutral Proteinases in Postmortem Muscle Protein Degradation and Meat Tenderness

Consumers consider tenderness to be the single most important component of meat quality. Recently, it has finally become clear that a lack of consistency in meat tenderness is one of the main problems facing the meat industry (Morgan et al., 1991b; Smith, 1992). A West coast supermarket chain (annual beef sales of $130 million) requests that its customers return any meat purchased, if they are not satisfied. Over the last 3 years, customers have returned meat valued at $364,00O/year, of which $286,000 (Le., 78%) were related to inadequate or inconsistent beef tenderness, even after blade tenderization (Morgan, 1992). The magnitude of the tenderness problem is realized when one considers the following facts as stated by Wilks (1992), “Data reveal that: a) only one-tenth of one percent of tough, dry or bland steaks are returned for replacement or to get money back, b) for every one complaint that is vocalized, ten complaints are never heard, and c) most consumers who have such experiences don’t complain-they just don’t return.” Additionally, consumers are also demanding meat with a minimal quantity of visible fat. To satisfy this consumer demand, retailers have begun to trim excess fat. Clearly, this practice contributes to inefficient production and, therefore, methods of producing lean carcasses is an intensive area of research. In many cases, lean carcasses are produced at the expense of tenderness (such as dietary administration of some O-adrenergic agonist) or inconsistent meat tenderness (intact male; Field, 1971; Seideman et al., 1982; Koohmaraie, 1988a). Therefore, i t is important to identify factors regulating meat tenderness so that methodology can be developed to manipulate the process advantageously. Such methodology may also be used to supplement the current U.S. quality grading system which is based primarily on the quantity of intramuscular fat. The inability of marbling to explain a large portion of total variation in tenderness is well documented (for review, see Parrish, 1974). Of all the observed variation in meat tenderness (obtained from animals of similar age), approximately 15% is explained by differences in marbling and connective tissue. I believe that differences in postmortem events that lead to improvement in meat tenderness probably explain the majority, if not all, of the 85% of the variation in tenderness not