BIOPHYSICAL ASPECTS OF MEAT TENDERNESS

Biophysical aspects of meat tenderness is reviewed, where the meat str uctural origin of variation in meat tenderness is tried to be elucidat ed. Processes, such as rigor development and ageing, known to influenc e the properties of the structural components, is covered, and variabl es that influence those processes, such as chilling, electrical stimul ation and stress ante-mortem, ave discussed. Meat tenderness can be ev aluated both by sensory and instrumental methods. The relationships be tween mechanical and sensory assessments tend to be non-linear, which can be clue to non-linearity in the sensory evaluation and that muscle fibre orientation is easier to control in instrumental than in sensor y evaluation. Structural changes of the meat occuring during rigor dev elopment are both longitudinal and lateral contraction of the myofibri llar mass. Other structural events, based on the proteolytic action, a re the loosing up of the myofibrils held together laterally, weakening of the myofibrillar length and myofibril fragmentation. Using instrum ental 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 degrees C. A struct ural Explanation to this behaviour is suggested to be the following. W hen meat is raw the lateral contraction of the meat fiber increases wi th shorter sarcomeres, giving rise to a larger viscous component and h ence a lower W-B peak force. On heating, however, with a larger extrac ellular space, when shortened, there is more room for the connective t issue to contract without being restricted by the myofibrillar mass. T his in turn gives a higher number of fibers per unit cross-sectional a rea, hence a larger elastic modulus and a higher W-B peak force, when cooked. When chilling of muscle during rigor both warm- and cold-short ening occur. Minimal shortening region is for beef M. longissimus dors i (LD) 10-15 degrees C and for M. semimembranosus (SM) 7-13 degrees C. For the SM muscle there is a high correlation between percentage shor tening and ultimate tenderness both in the warm- and cold-shortening r egion. But for the LD muscle this is only the case in the cold-shorten ing legion. This observation suggests that the LD muscle is a more enz ymatically active muscle than SM. The influence of low-voltage electri cal stimulation (ES) was followed in the cold-shortening region for mu scles LD and SM. A significant effect on tenderness 15 days post-morte m was only observed for LD at 1 degrees C and 4 degrees C, but not for SM. It was suggested that enhanced proteolysis could be the reason fo r the improved tenderness on ES of LD, as cold-shortening was not prev ented by ES. Long-term and short-term sti ess ante-mortem can give ris e to DFD (dark, firm and dry) - and PSE (pale, soft and exudative) -me at, respectively. DFD-meat (pH(u) > 6.0 in LD) has relatively short sa rcomere lengths, but still it is swollen laterally and has consequentl y a small extracellular space. Therefore DFD-meat usually is tender. P SE-meat has a large variation in sarcomere length. The long sarcomeres of PSE-meat is suggested to be caused by reduced shortening, due to t he denaturation of the sarcoplasmic proteins during rigor. The short s arcomeres can be caused by a higher percentage of rigor development in the warm-shortening region and that the denaturation of the myosin he ads cause both longitudinal and lateral contraction of the myofibrilla r 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 variab les that governs the rigor process. Copyright (C) 1996 Elsevier Scienc e Ltd