Relationship between muscle microstructure, the calpain system, and shear force in bovine longissimus dorsi muscle.

The objective of this study was to examine the occurrence of microstructural changes in aged LM from Norwegian Red cattle, and to investigate how these changes relate to pH decline, calpain and calpastatin activities, and tenderness (Warner-Bratzler shear force; WBSF). Samples of the LM from 403 Norwegian Red dual-purpose bulls were collected over a 4-yr period and analyzed for muscle pH, protease activity, and WBSF. Microstructural analysis of fiber-fiber detachment, muscle fiber-perimysium detachment, contracted muscle fibers, and fractured muscle fibers were performed on a subset of 50 animals. The occurrence of fractured muscle fibers was negatively correlated with WBSF (r = -0.33, P < 0.05) and calpastatin activity (r = -0.51, P < 0.01) and was positively correlated with the ratio of μ-calpain:calpastatin activity (r = 0.66, P < 0.001), strongly indicating that these fractures are important in the development of meat tenderness and that they are a result of calpain-mediated proteolysis. In contrast, detachments between individual muscle fibers and between muscle fibers and the perimysium did not play an important role in determining variation in LM tenderness in these animals. Moreover, both pH decline and what is considered a normal ultimate pH range in beef were positively correlated with calpastatin activity and WBSF and were negatively correlated with fractured muscle fibers. Thus, an accelerated muscle pH decline and a low ultimate pH seem to increase calpain-mediated proteolysis, causing fractures in muscle fibers and thereby giving rise to more tender meat.