The influence of a period of ageing on the relationship between ultimate pH and shear values of beef m. longissimus thoracis.

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.