The objective of this study was to evaluate the effects of chilling rate on porcine meat quality from the perspective of proteome using data independent acquisition (DIA)-based quantitative proteomic strategy. M. longissimus thoracis et lumborum (n = 9) was assigned randomly to the control group (3.72 ℃/h), very fast chilling-I group (VFC-I, 9.31 ℃/h) and VFC-II group (14.43 ℃/h). The DIA was used to analyze the difference in proteins under different chilling rates. Results showed that tenderness was improved significantly in meat at the chilling rate of 14.43 ℃/h. Seventy-nine differential abundant proteins (fold change > 1.5, P < 0.05), including 46 up-regulated and 33 down-regulated proteins, were identified and mainly involved in carbon metabolism, pyruvate metabolism and proteasome pathways. These pathways indicated that VFC delayed cell metabolism and glycolysis by down-regulating the expression of metabolic enzymes. The tenderness was improved by up-regulating the expression of proteasome and m-calpain.

The degradation of titin could make the myofibrillar fragmentation to improve meat tenderization during postmortem. This study aimed to investigate effect of phosphorylation on titin degradation. Protein kinase A (PKA) and alkaline phosphatase (AP) were added to crude titin extracted from ovine longissimus lumborum (LL) muscles. Phosphorylated/dephosphorylated titin were incubated with μ-calpain at 4 ℃ for 2 days. Results showed titin in AP group started degradation earlier than that in PKA and control groups. There were 20, 16 and 12 phosphorylated sites identified by iTRAQ in the PKA, control and AP group, respectively. 3D structure of dephosphorylated titin fragment was simulated and its molecular dynamics trajectory analysis was performed using Discovery Studio^TM. The dihedral angle in AP group was less and the dephosphorylated fragment had a higher kinetic energy and total energy. We suggested that changes caused by AP treatment might make titin unstable, which easily degraded by μ-calpain.