MOLECULAR PROPERTIES OF POST-MORTEM MUSCLE .7. CHANGES IN NONPROTEIN NITROGEN AND FREE AMINO ACIDS OF BOVINE MUSCLE

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. Copyright © 1969, Wiley Blackwell. All rights reserved