Mechanism And Effects Of Muscle Fiber Types On Proteolysis Of Beef During Postmortem Process

Muscle fiber is the basic unit of muscle,its types are related to beef tenderness.It is generally considered that the degradation of myofibrillar protein during post-mortem aging is the main cause of the beef tenderization.The type of muscle fibers may affect the beef tenderness by affecting the degradation of myofibril protein,but the mechanism of the impact is not clear.ATPase histochemical staining and RT-qPCR were used to identify the muscle fiber type and MyHC gene type of ten muscles from forequarter,hindquarter and intermediate section of cattle carcass.The muscles with main component of type ?,type ?A and type ?B respectively were selected for further research.The index related to tenderness were tested during post-mortem aging.And the changes of myofibrillar protein degradation were studied by transmission electron microscope,western-blot and proteomics.The relationship between the muscle fiber types and the change of cathepsin activity during post-mortem were also discussed.This study revealed the influence mechanism of the muscle fiber types on the tenderization of beef after slaughter,and provided a theoretical foundation for the respectively aging of different muscles.This study contains four parts,the results and conclusions are:1.The muscle fiber types of Supraspinatus Infraspinatus,Rhomboideu,Longissimus thoracis,Latissimus dorsi,Psoas major,Biceps femoris,Semitendinosus,Gastrocnemius and Semimembranosus were identified by ATPase staining and RT-qPCR.Muscle fiber type I was the main composition of forequarter muscles in cattle carcass and for hindquarter muscles it was muscle fiber type ?B.There was a certain correlation between the two methods in identifiering muscle fiber types.Muscle fiber type ? correspond to MyHC ? gene,and muscle fiber type ?B may correspond to MyHC ?x gene.2.The pH values,the myofibrillar fragmentation index and the protein solubility of Infraspinatus,Rhomboideu,Longissimus thoracis,Psoas major,Semitendinosus,Biceps femoris and Gastrocnemius were measured at 0,1,3,7,9,11,14 d during postmortem time.The myofibrils ultrastructure,the degradation of key protein Desmin and Troponin-T of Infraspinatus,Rhomboideu and Semitendinosus were studied.The results showed that the muscle with more fiber type I was degraded faster after slaughter and was more tender.3.The changes of cathepsin B,H and L activity of 7 beef muscles were studied.Cathepsin activity was positively correlated with the proportion of muscle fiber type ? and negatively correlated with the proportion of muscle fiber type ?B.Therefore,the degradation of myofibrils caused by cathepsins in muscles with type ? as dominant component was faster than in muscles with type ?B as dominant component.The results of the relationship between muscle fiber type and postmortem tenderization is consistent with that in the second part of the research.4.The results of differential proteomics of Infraspinatus,Rhomboideu and Semitendinosus at 0 and 14 d during postmortem aging time showed that the differential proteins related to tenderness affected the tenderization of beef mainly through citric acid cycle,oxidative phosphorylation,glycolysis,calcium signaling pathway,fatty acid metabolism,apoptosis and other metabolic pathways,as well as involved in protein hydrolysis,muscle contraction and cell structure composition,cell defense and stress.The Infraspinatus,which was dominated by type ? muscle fiber,tenderizes faster than the Semitendinosus,which is dominated by type ?B muscle fiber,while the Rhomboideu,which is dominated by type ?A muscle fiber,was between the two.To sum up,the difference of muscle fiber type affected the change of tenderness during the postmortem process of beef through metabolic pathway,structure composition and endogenous enzyme activity.The muscle with higher muscle fiber type ? content had higher endogenous enzyme activity,and its metabolic pathway was more conducive to protein degradation,thus the rate of tenderization was faster.This study will provide theoretical basis for respectively aging of different beef muscles and provide target orientation for animal husbandry to produce high quality beef.