Modelling post-mortem tenderisation-III: Role of calpain I in conditioning

A simple model is developed to show how proteolysis by calpain I can account for the variations in tenderness in electrically stimulated and nonstimulated beef pectoralis profundus muscles stored between 0°C and 30°C. As the pH of the muscle falls to about 6·1, calpain I is activated and causes proteolysis and tenderisation. The rate of tenderisation is then proportional to the concentration of calpain I which is autolysed slowly reducing its concentration and the rate of tenderisation. The activation energy for the inactivation (autolysis) of calpain I is slightly higher than that for its activity in tenderisation (proteolysis) and therefore, at higher temperatures, less tenderisation occurs. Proteolysis and tenderisation continue at a rate governed by the concentration of calpain I and the temperature until calpain I is depleted when tenderisation stops. Parameters for the activity and inactivation of calpain I were derived and were shown to predict 68% of the variation in toughness.