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SUMMARY Post‐mortem changes in tenderness and protein solubility were studied in bovine semitendinosus muscles. Muscles excised immediately post‐mortem were compared with muscles left attached to the skeleton. Post‐mortem times of 0, 6, 12, 24, 72, and 312 hr were studied. Sarcoplasmic protein solubility was highest immediately after slaughter and lowest in muscles left attached to the skeleton. Myofibrillar protein solubility was decreased in muscles left attached. Protein solubility changed during the first 6 hr post‐mortem but not during the 6‐ to 312‐hr aging period. Muscles left attached to the skeleton were least tender immediately after death and gradually increased in tenderness during post‐mortem aging. Excised muscles were least tender 6–12 hr post‐mortem and became progressively more tender thereafter. Even after 312 hr of aging, excised muscles were less tender than muscles still attached to the skeleton. Protein solubility did not appear to be related to tenderness. Possible relationships of muscle contraction to tenderness were discussed.

SUMMARY– Proteolysis and its relationship to tenderness were studied by measuring nonprotein nitrogen (NPN), free amino groups, and shear resistance during post‐mortem aging of bovine muscle. Both NPN and free amino groups increased during post‐mortem aging, indicating some degradation of proteins and/or peptides. However, neither the increase in NPN nor free amino groups was related to post‐mortem tenderization since these quantities increased only after most of the improvement in tenderness had occurred. Much of the increase in NPN or free amino groups may originate from degradation of sarcoplasmic proteins or peptides. It is suggested that weakening or breaks at crucial points in the sarcomere, such as at the junction of the Z‐line with the thin filaments, occur within the first 48‐72 hr post‐mortem and that this weakening or cleavage is responsible for tenderization. Cathepsin D may be responsible for this weakening but most of the available evidence is against proteolysis as the primary cause of post‐mortem tenderization.