Biophysical aspects of meat tenderness.

Biophysical aspects of meat tenderness is reviewed, where the meat structural origin of variation in meat tenderness is tried to be elucidated. Processes, such as rigor development and ageing, known to influence the properties of the structural components, is covered, and variables that influence those processes, such as chilling, electrical stimulation and stress ante-mortem, are discussed. Meat tenderness can be evaluated both by sensory and instrumental methods. The relationships between mechanical and sensory assessments tend to be non-linear, which can be due to non-linearity in the sensory evaluation and that muscle fibre orientation is easier to control in instrumental than in sensory evaluation. Structural changes of the meat occuring during rigor development are both longitudinal and lateral contraction of the myofibrillar mass. Other structural events, based on the proteolytic action, are the loosing up of the myofibrils held together laterally, weakening of the myofibrillar length and myofibril fragmentation. Using instrumental recordings of meat toughness (Warner-Bratzler (W-B) peak force), it decreases significantly with degree of contraction, when raw, but the reverse is found, when meat is cooked above 60 °C. A structural explanation to this behaviour is suggested to be the following. When meat is raw the lateral contraction of the meat fiber increases with shorter sarcomeres, giving rise to a larger viscous component and hence a lower W-B peak force. On heating, however, with a larger extracellular space, when shortened, there is more room for the connective tissue to contract without being restricted by the myofibrillar mass. This in turn gives a higher number of fibers per unit cross-sectional area, hence a larger elastic modulus and a higher W-B peak force, when cooked. When chilling of muscle during rigor both warm- and cold-shortening occur. Minimal shortening region is for beef M. longissimus dorsi (LD) 10-15 °C and for M. semimembranosus (SM) 7-13 °C. For the SM muscle there is a high correlation between percentage shortening and ultimate tenderness both in the warm- and cold-shortening region. But for the LD muscle this is only the case in the cold-shortening region. This observation suggests that the LD muscle is a more enzymatically active muscle than SM. The influence of low-voltage electrical stimulation (ES) was followed in the cold-shortening region for muscles LD and SM. A significant effect on tenderness 15 days post-mortem was only observed for LD at 1 °C and 4 °C, but not for SM. It was suggested that enhanced proteolysis could be the reason for the improved tenderness on ES of LD, as colds-hortening was not prevented by ES. Long-term and short-term stress ante-mortem can give rise to DFD (dark, firm and dry)-and PSE (pale, soft and exudative)-meat, respectively. DFD-meat (pH(u) > 6.0 in LD) has relatively short sarcomere lengths, but still it is swollen laterally and has consequently a small extracellular space. Therefore DFD-meat usually is tender. PSE-meat has a large variation in sarcomere length. The long sarcomeres of PSE-meat is suggested to be caused by reduced shortening, due to the denaturation of the sarcoplasmic proteins during rigor. The short sarcomeres can be caused by a higher percentage of rigor development in the warm-shortening region and that the denaturation of the myosin heads cause both longitudinal and lateral contraction of the myofibrillar mass. There is also a large variation in tenderness of PSE-meat, but it has been found that this variation is positively correlated to the sarcomere length (r = 0.52**), as has been shown for the other variables that governs the rigor process.