Relationship between proteolytic changes and tenderness in prerigor lactic acid marinated beef

A meat tenderising procedure involving injection of a lactic acid solution into prerigor muscle was investigated using beef M pectoralis profundus. The distribution of lysosomal enzymes in subcellular fractions, densities of myofibrillar protein bands after SDS-PAGE and shear force were measured in non-injected, 0.5 M and 1.0 M lactic-acid-injected samples during a 21 days ageing period. The activities of cathepsin B + L and β-glucuronidase in the soluble fraction increased with level of lactic acid and with time post-mortem (P < 0.001). Lactic acid and storage decreased densities of SDS-PAGE bands migrating at the position of myosin heavy chain (MHC) and α-actinin and increased densities of a 150 kDa band (P < 0.01). SDS-PAGE of isolated perimysium cleaved with CNBr showed proteolytic cleavage of collagen after prolonged storage. Lactic acid injection significantly reduced shear force (P < 0.001). The cathepsin B + L activity in the soluble fraction correlated to shear force (r = −0.8), the degradation of MHC and α-actinin (r = −0.88 and −0.90) and the generation of the 150 kDa fragment (r = 0.90) but not to the generation of a 31 kDa fragment (r = 0.05). A major part of the tenderness improvement after lactic acid injection was complete at 24 h post-mortem, and was therefore due to a rapid process, eg pH-induced swelling of the muscle structure. The data on enzyme activities and protein degradation, however, suggested that the action of lysosomal cathepsins also contributed to textural changes.