Small heat shock proteins and toughness in intermediate pHu beef

•Intermediate pHu meat was tougher compared with low and high pHu meat after ageing.•Small heat shock protein decline in muscle postmortem was pHu related.•Myofibrillar protein degradation was less extensive in intermediate pHu meat.•Proteolytic activity was sub-optimal in intermediate pHu meat.

Bull M. longissimus dorsi (n = 94) categorised into high (n = 28), intermediate (n = 14) and low (n = 52) ultimate pH (pHu) were aged at − 1.5 °C for 28 days. Shear force was higher and more variable (p < 0.05) in intermediate pHu samples during ageing. Titin, filamin and desmin degradation was also less extensive in intermediate pHu samples compared to the other two pH categories. The extent of the decline of HSP20, HSP27 and αβ-crystallin concentrations during post mortem ageing was pHu related such that high pHu meat maintained the highest concentration of small heat shock proteins followed by intermediate and low pHu meat. μ-Calpain autolysis was slowest in intermediate pHu and cathepsin B activities remained consistently low during ageing in this group (p < 0.05). Meat toughness in the intermediate pHu group may be attributed to the combination of a larger pool of sHSP with a sub-optimal cathepsin B activity and intermediary μ-calpain activities.