Individual pre-slaughter muscle proteolysis levels correlated with postmortem taste-related amino acid concentrations in broiler chickens

Pre-slaughter fasting is an important practice in the poultry industry that reduces microbial contamination. However, fasting-induced skeletal muscle proteolysis can occur, which may influence meat quality because it increases the concentration of free amino acids that contribute to the umami taste during postmortem aging. In a previous study, we found that antemortem proteolysis levels induced by pre-slaughter fasting (0, 8, 16, and 24 h) were positively correlated with the free glutamic acid (Glu) content in the pectoralis major muscle after 48 h of aging. In this study, we investigated the relationship between individual differences in antemortem proteolysis levels and meat quality, especially free amino acids content and taste sensor value, in the pectoralis major muscle subjected to the same duration of 16-h pre-slaughter fasting, and the mechanisms of muscle free Glu accumulation during postmortem aging in broiler chickens. Antemortem skeletal muscle proteolysis levels, evaluated as changes in plasma N^τ-methylhistidine concentrations, ranged from -1.0 to 12.7 nmol/mL (mean ± SD: 3.6 ± 3.01). Free Glu content in the pectoralis major muscle after 48 h of postmortem aging ranged from 9.7 to 45.8 mg/100 g (18.0 ± 7.34). A significant positive correlation was observed between antemortem proteolysis levels and postmortem free Glu content in the pectoralis major muscle after 48 h of aging (r = 0.42, P < 0.01). Postmortem free Glu content was positively correlated with the mRNA expression of ATP-independent proteolytic enzymes, including Calpain 11 (r = 0.27, P < 0.1), Calpain 2 (r = 0.45, P < 0.01), Cathepsin L-like (r = 0.55, P < 0.0001), and Cathepsin H (r = 0.35, P < 0.05). We measured the levels of troponin-T (TnT), which releases free Glu upon degradation, to investigate the cause of free Glu accumulation during aging. However, no correlation was observed between TnT and postmortem free Glu content. In contrast, specific low-molecular-weight proteins (approximately 12-15 kDa) exhibited associations with antemortem skeletal muscle proteolysis levels and postmortem free Glu acid content in the pectoralis major muscle. These findings suggest that individual pre-slaughter proteolysis levels influence postmortem muscle-free Glu accumulation by enhancing the expression of specific proteolytic enzymes or myofibrillar protein degradation.