Characterization of callipyge phenotype effects on lamb skeletal muscle and meat tenderization of longissimus dorsi muscle by high ultimate pH

In order to further investigate mechanisms that cause the toughness of callipyge lamb meat, we determined the relative contributions of different chemical, biochemical and structural attributes of the meat tenderness of two different muscles, longissimus (LD) and semimembranosus (SM), from callipyge lambs at 1 d and 14 d after slaughter. Callipyge lamb LD muscle had a higher protein (P < .01) and a lower fat content (P < .01), a lower cytochrome oxidase activity ( P < .01) and a higher activity of cathepsins (P < .01). A higher (P < .01) total collagen content was found in the SM muscle of normal lambs compared to that of callipyge lambs. Light and electron microscopy showed that callipyge lamb LD muscle had a more compact structure with larger muscle fibers and fewer and smaller mitochondria than those of normal lamb LD muscles. After 14 d aging, a specific breakdown near I band Z-disk junction was observed for the normal LD muscles; whereas, random degradation of thick and thin filaments was observed for the callipyge LD muscles. The higher protein content (P < .01), lower fat (P < .01), lower cytochrome oxidase activity ( P < .01), compact structure, and reduced postmortem proteolysis and myofibril fragmentation appeared to be related to the greater toughness of LD muscle from callipyge lambs. The ultrastructural changes in callipyge lamb myofibrils during aging suggested that calpain system was inhibited by higher calpastatin activity. Increasing the muscle's ultimate pH to activate calpains was a potential strategy to tenderize the LD muscle of callipyge lambs. The effects of pH on normal and callipyge lamb LD muscle protein degradation, myofibril fragmentation, shear force values and ultrastructure were investigated during 14 d of aging. It appeared that the high ultimate pH activated the calpain system and improved the tenderness by disruption at the Z-disk-I band linkages and protein degradation after 14 d of aging of callipyge lambs. In addition to the pH, it was found that the ATP level, which determined both ultimate pH and contraction state, was more closely related to the callipyge lamb LD muscle shear values after aging.