Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
Poult Sci. 2018 May 1;97(5):1808-1817. doi: 10.3382/ps/pex455.
During postmortem metabolism, muscle pH gradually declines to reach an ultimate pH near 5.6 across most meat species. Yet, broiler pectoralis major (P. major) muscle generates meat with high ultimate pH (pH ∼ 5.9). For better understanding of the underlying mechanism responsible for this phenomenon, we evaluated the involvement of breast muscle chilling on the extent of postmortem metabolism. Broiler breast muscles were either subjected to chilling treatment (control) or left at room temperature (RT) for 120 min. P. major muscle from the RT treatment had lower ultimate pH, greater glycogen degradation and lactate accumulation. While these findings suggest that carcass chilling can contribute to the premature termination of postmortem metabolism, chilling did not fully explain the high ultimate pH of P. major muscle. Our results also revealed that glucose-6-phosphate (G6P) was very low at 24 h, and therefore we hypothesized that G6P was limiting. To test this hypothesis, muscle samples from P. major and porcine longissimus lumborum (LL) muscle were homogenized into a reaction buffer that mimics postmortem glycolysis with or without 0.5 mg/mL isolated mitochondria. While samples containing porcine LL muscle reached the normal level of ultimate pH, P. major muscle samples reached a value similar to that observed in vivo even in the presence of excess G6P, indicating that G6P was not limiting. Mitochondria enhanced the glycolytic flux and pH decline in systems containing muscle from both species. More importantly, however, was that in vitro system containing chicken with mitochondria reached pH value similar to that of samples containing LL muscle without mitochondria. To investigate further, phosphofructokinase (PFK) activity was compared in broiler P. major and porcine LL muscle at different pH values. PFK activity was lower in P. major muscle at pH 7, 6.5, and 6.2 than LL muscle. In conclusion, carcass chilling can partially contribute to the high ultimate pH of broiler P. major muscle, while low PFK activity and mitochondria content limit the flux through glycolysis.
在死后代谢过程中，大多数肉类的肌肉 pH 值逐渐下降，最终达到接近 5.6 的 pH 值。然而，肉鸡胸大肌（P. major）产生的肉质具有较高的最终 pH 值（pH ∼ 5.9）。为了更好地理解导致这种现象的潜在机制，我们评估了胸部肌肉冷却对死后代谢程度的影响。肉鸡胸大肌要么进行冷却处理（对照），要么在室温（RT）下放置 120 分钟。来自 RT 处理的 P. major 肌肉具有较低的最终 pH 值、更大的糖原降解和乳酸积累。虽然这些发现表明胴体冷却可以促进死后代谢的过早终止，但冷却并不能完全解释 P. major 肌肉的高最终 pH 值。我们的结果还表明，葡萄糖-6-磷酸（G6P）在 24 小时时非常低，因此我们假设 G6P 是有限的。为了验证这一假设，将来自 P. major 和猪背最长肌（LL）的肌肉样本匀浆到反应缓冲液中，该缓冲液模拟死后糖酵解，有或没有 0.5 mg/mL 分离的线粒体。虽然含有猪 LL 肌肉的样本达到了正常的最终 pH 值，但即使在有过量 G6P 的情况下，P. major 肌肉样本也达到了体内观察到的值，表明 G6P 不是有限的。线粒体增强了含有两种物种肌肉的系统中的糖酵解通量和 pH 值下降。然而，更重要的是，含有鸡的体外系统在含有无线粒体的 LL 肌肉的样本中达到了相似的 pH 值。为了进一步研究，在不同 pH 值下比较了肉鸡 P. major 和猪 LL 肌肉中的磷酸果糖激酶（PFK）活性。与 LL 肌肉相比，P. major 肌肉在 pH 值为 7、6.5 和 6.2 时的 PFK 活性较低。总之，胴体冷却可以部分解释肉鸡 P. major 肌肉的高最终 pH 值，而低 PFK 活性和线粒体含量限制了糖酵解的通量。
