Effects of different protein phosphorylation levels on the tenderness of different ultimate pH beef

This study investigated the relationship between tenderness and protein phosphorylation levels of normal ultimate pH (pH_u, 5.4-5.8, NpH_u), intermediate pH_u (5.8-6.2, IpH_u) and high pH_u (≥6.2, HpH_u) Longissimus lumborum from beef. During 21 d of ageing, the HpH_u group had the lowest Warner-Bratzler shear force (WBSF) values, while the IpH_u group showed the highest and even after 21 days of ageing still had high levels. In the late stage of the 24 h post-mortem period the faster degradation rate of troponin T and earlier activation of caspase 9 in the HpH_u group were the key reasons for the lower WBSF compared with the NpH_u and IpH_u groups. The activity of caspase 3 cannot explain the tenderness differences between IpH_u and HpH_u groups, since their activities did not show any difference. At 24 h post-mortem, 17 common differential phosphorylated peptides were detected among pH_u groups, of which nine were associated with pH_u and WBSF. The higher phosphorylation level of glycogen synthase may have caused the delay of meat tenderization in the IpH_u group.