The biological basis for toughness in beef
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The aim of this thesis was to elucidate role of small heat shock proteins (sHSPs) in the development of tenderness in beef. Data in this thesis were predominantly obtained from bull M. longissimus dorsi aged at -1.5°C for up to 28 days post mortem. High (pH ≥ 6.2) and low (≤ 5.79) ultimate pH (pHu) at were acceptably tender within 1 and 7 days post mortem, respectively. Tenderness in these pHu groups is attributed to the extensive degradation of titin, nebulin, filamin and desmin by endogenous proteases. Rapid μ-calpain autolysis in high pHu meat was concurrent with the faster degradation of titin, nebulin and filamin in this pHu group. Conversely, faster degradation of desmin was concomitant with increasing cathepsin B activity in low pHu meat. It is hypothesised that meat tenderness is pHu compartmentalised by the variable degradation rates of myofibrillar proteins, which are in turn regulated by μ-calpain and cathepsin B activities. Intermediate pHu (5.8 ≤ pHu ≤ 6.19) meat attained acceptable tenderness at 28 days post mortem. However, due to the high variance of shear force in this group, some intermediate pHu meat will be unacceptably tough even after ageing. With up to 19% of bull beef attaining intermediate pHu, understanding the mechanisms for the inconsistent tenderness in this group is advantageous to the meat industry. Myofibrillar protein degradation was less extensive in intermediate pHu meat, partly owing to the sub-optimal activities of μ-calpain and cathepsin B in this group. Exogenous αβ-crystallin reduced the extent of desmin and titin degradation induced by μ-calpain in vitro. Furthermore, αβ-crystallin inhibited μ-calpain activity, possibly by acting an alternative substrate for the enzyme. In post mortem muscle, the increasing association of αβ-crystallin to muscle myofibrils was concomitant with the declining levels of HSP20, HSP27 and αβ-crystallin in muscle sarcoplasm as pH declined. This suggests the translocation of sHSPs from the sarcoplasm to the myofibrils and optimal binding of sHSP to the myofibrils in intermediate pHu meat. It is hypothesised that the high chaperoning potential and μ-calpain inhibition of sHSP combined with the low proteolytic activity in intermediate pHu meat results in the less extensive degradation of myofibrillar proteins leading to meat toughness as observed in intermediate pHu meat.