Modelling post-mortem tenderisation-V: Inactivation of calpains.

The calpain-activity model, which allows computation of the in-situ activities of calpains, was used to predict tenderisation. Tenderisation results from the net proteolysis which is governed by the relative activities and the intramolecular inactivation of calpains. The activity increases non-interactively with increase in pH and increase in temperature. The rate of inactivation depends interactively upon pH and temperature. At high temperature, inactivation is high and almost independent of pH. The rate of inactivation decreases with decrease in temperature, but below about 10°C it increases at low pH. Rapid rigor development produces rapid activation and tenderisation but it may be short-lived, particularly in slowly-chilled meat, producing tough meat. Rapid cooling causes rapid inactivation of calpains and can give rise to very tough meat. Therefore, the calpain-activity model predicts the toughness often observed in PSE meats and rapidly-chilled meats without evoking structural changes dependent upon water-holding capacity or the degree of overlap of actin and myosin. Furthermore, the model demonstrates the known interactions of ageing with these conditions, interactions which cannot be explained by those structural changes alone. Variations in post-mortem activity of calpains therefore provide a single concept accounting for the variations in texture arising from variations in animal production, chilling and ageing and their interactions.