Effect of myosin heavy chain isoform composition on tenderness of bovine muscles from carcasses of two skeletal maturities

Bovine muscles (m. semitendinosus and m. longissimus lumborum) from carcasses of A- and E-skeletal maturity (SMAT) were evaluated for percentage composition of myosin heavy chain isoforms (MHC) while in the pre-rigor state, pre-rigor calpastatin and m-calpain activity, Warner-Bratzler shear force (WBS) at 7 and 14 d of postmortem aging, and gravimetric fragmentation index (GFI) at 0, 7, and 14 d of postmortem aging.; Percentage composition of MHC-I isoform did not (P > 0.05) change between muscles of A- and E-SMAT carcasses. However, muscles of E-SMAT carcasses contained (P < 0.001) a lower percentage composition of MHC-IID compared to muscles of A-SMAT carcasses. Muscles of A-SMAT carcasses contained a lower (P < 0.0001) percentage of MHC-IIA isoform compared to muscles from carcasses of E-SMAT. The eye of round contained a lower (P < 0.0001) percentage of MHC-I isoform compared to the loin, while percentage MHC-IIA did not differ ( P > 0.0001) between the two subprimals. Percentage MHC-IID ( P < 0.0001) was higher in the eye of round compared to the loin. MHC composition can be used as an indicator of tenderization, but it is important to clarify that other factors such as connective tissue also contribute to the increased toughening exhibited by muscles of advanced skeletal maturity carcasses.; Calpastatin activity was lower (P < 0.05) in muscles from carcasses of E-SMAT, but m-calpain activity was similar (P > 0.05) in muscles of A- and E-SMAT carcasses. There was no difference ( P > 0.05) in calpastatin or m-calpain activity between subprimals. Lower calpastatin levels in carcasses of E-skeletal maturity may be due to the degenerative machinery associated with the aging process.; Results indicated that carcasses of advanced skeletal maturity produced muscle that was higher (P < 0.0001) in WBS than youthful carcasses, but that postmortem aging period of 14 d helped decrease ( P < 0.0001) the Warner-Bratzler shear force of subprimals across all skeletal maturity groups. The eye of round (m. semitendinosus ) produced a lower (P < 0.05) WBS compared to the loin (m. longissimus lumborum). It is possible that this was the result of a longer sarcomere length.