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To investigate the effects of postmortem aging on beef microstructure and the underlying mechanism, we herein examined three muscle types, namely Longissimus dorsi (LD),Rectus abdominis (RA), and Semitendinosus (ST), from Qinchuan cattle carcass.Meat samples were vacuum packaged, and aged at 4℃ for 72 h.Subsequently, paraffin hematoxylin-eosin (HE) staining and atomic force microscopy were conducted to examine changes in muscle fiber and myofibril fragmentation.Environmental scanning electron microscopy was also performed to examine sarcolemma, and immunohistochemistry was employed to study localization of μ-calpain in muscle fibers.The results indicated that during postmortem aging, LD muscle fiber and myofibrils exhibited the most severe fragmentation, and ST fiber showed the least changes.No significant changes were observed in sarcolemma.Furthermore, μ-calpain was detected mainly in sarcolemma and cytoplasm;high levels of μ-calpain were found in red muscle fiber (slow muscle), whereas white muscle fiber (fast muscle) showed only low levels of expression.Therefore, μ-calpain appears to be important for beef tenderization during postmortem aging.In different parts of Qinchuan cattle carcass, the ratio of red vs.white muscle fiber could significantly affect muscle fiber microstructural changes and beef tenderness improvement during postmortem aging.