Rennet hysteresis

Heating milk increases its rennet coagulation time (RCT) to a lesser or greater extent depending on the severity of heating. However, if milk is held in the cold for extended periods after heat treatment (e.g., after pasteurisation), its RCT increases significantly and the lower the storage temperature and longer the storage time, the greater the effect. This increase in RCT on storage of heat-treated milk is known as rennet hysteresis.

Adverse effects of heating on RCT is caused mainly by denaturation of the whey protein, beta-lactoglobulin which interacts with kappa-casein at the micelle surface. However, this adverse effect is offset to some extent by beneficial effect of lower pH caused by heating (heating milk causes precipitation of calcium phosphate from soluble to colloidal states with the production of hydrogen ions, thus reducing the pH which favours rennet action). However, heat-induced changes to milk salts equilibria are partly reversible on cooling, so full adverse effects of protein-protein interaction on RCT then become apparent.