Purchas R W, Yan X, Hartley D G
Department of Animal Science, Massey University, Palmerston North, New Zealand.
Meat Sci. 1999 Feb;51(2):135-41. doi: 10.1016/s0309-1740(98)00111-9.
Previous studies have shown that, provided cold-shortening conditions are avoided, increases in ultimate pH from a normal of about 5.5 for beef M. longissimus thoracis (LT) are associated with increased cooked meat shear forces to an intermediate pHu of about 6 and then decreases in shear force with further increases to 7. The current study with LT samples from 156 cattle (40 steers, 116 bulls) investigated this relationship following 1 and 20 days ageing at 1-2°C. The patterns of change in shear values with increasing pHu were similar for both ageing times with a peak at just under 6, but the difference between 20 and 1 day samples (1-20 days) varied so that the relationship between pHu and this difference also showed a peak at intermediate pHu values. In contrast to some previous studies, sarcomere length did not decrease as pHu increased from 5.5 to 6, indicating that it is not necessarily an important determinant of toughening over this pHu range. A myofibrillar fragmentation index (MFI) increased from 1 to 20 day ageing, but the extent of the change was similar as pHu increased from 5.5 to 6.2. Over the wider range in pHu from 5.5 to 7 MFI at 1 day remained essentially constant while that at 20 days, and consequently the increase from 1 to 20 days, increased. These results do not provide evidence that any one factor is responsible for the greater toughness of beef with an intermediate pH, and suggest that factors other than degree of muscle fibre contraction and extent of myofibrillar protein breakdown may be involved.
以往研究表明，若避免冷收缩情况，牛肉胸最长肌（LT）的最终pH值从正常的约5.5升高时，熟肉剪切力会增加，直至中间pHu值约为6，然后随着pH值进一步升高至7，剪切力下降。本研究使用来自156头牛（40头阉牛，116头公牛）的LT样本，在1-2°C下老化1天和20天后研究了这种关系。两种老化时间下，随着pHu升高，剪切值的变化模式相似，在略低于6时达到峰值，但20天和1天样本之间的差异（1-20天）有所不同，因此pHu与这种差异之间的关系在中间pHu值时也出现峰值。与之前的一些研究不同，当pHu从5.5升高到6时，肌节长度并未减少，这表明在该pHu范围内，它不一定是导致肉质变硬的重要决定因素。肌原纤维破碎指数（MFI）从老化1天增加到20天，但随着pHu从5.5升高到6.2，变化程度相似。在pHu从5.5到7的更宽范围内，1天时的MFI基本保持不变，而20天时的MFI以及因此从1天到20天的增加量则增加。这些结果并未提供证据表明任何一个因素是导致中等pH值牛肉更坚韧的原因，并且表明可能涉及肌肉纤维收缩程度和肌原纤维蛋白分解程度以外的因素。
