Trends in skeletal muscle biology and the understanding of toughness in beef

This review focuses on recent developments in the field of muscle biology that reflect on the problem of toughness in beef. Meat science has shown that post-mortem processing can make a large contribution to beef tenderness. However live-animal factors such as growth path and genotype also influence the toughness of beef either directly or indirectly through interactions with processing technologies. This review sets out to integrate recent developments in the field of meat science into a mechanistic overview of toughness, while still highlighting the biology of some important contributors. These contributors are discussed at several levels of order between the molecular and the whole animal. The myofibrillar component of muscle is identified as the major contributor to initial toughness particularly through the effects of variation in sarcomere length. Muscle fibre-types whilst important to the growth and development of the animal are yet to be linked convincingly with toughness. Connective tissue is seen to play a dominant role in the sensation of toughness in muscles where its content is high. In muscles that are generally used for table beef, the contribution of connective tissue is less significant. In either case its contribution to measurable toughness cannot be easily separated from that of the myofibrillar component and the review discusses various levels of interaction between these 2 major components of beef. The review covers aspects of muscle ultrastructure as far as they are pertinent to the problem of beef toughness. In particular it deals with current knowledge of post-mortem metabolism of muscle and the degradation of costameric structures. Molecules are considered that are likely to propagate tensional forces from sarcomeres across the sarcolemma to the extracellular molecules of the endomysium. While much of the research around these molecules has not been performed by meat scientists, the insights developed are likely to be important to our understanding of beef toughness. Technological approaches to the objective measurement of toughness are discussed, as well as recent developments in the field. The review takes an integrative approach to features of the life of the bovine that might impact on the toughness of beef derived from its carcass. Features of the animal's pre-slaughter experience, including stress and physical activity, have been shown to influence markedly the toughness of beef through mechanisms that are described at the tissue level. Features of the growth path that the animal followed during its development have also recently been shown to significantly reduce the toughness of beef and properties of the connective tissue component have been implicated. Areas of strategic research are identified that, in the author's opinion, will facilitate commercial-scale improvements in the tenderness of beef.