Effect of hydrolysis of casein by plasmin on the heat stability of milk

This study examined the effect of hydrolysis of casein by added plasmin (6 mg L^-1) on the heat stability of raw, pre-heated, serum protein-free or concentrated skim milk. Plasmin activity markedly affected the heat stability-pH profile of skim milk and serum protein-free milk, apparently by altering the properties of the casein micelles. It is probable that changes in the surface charge of the micelles, as a result of the hydrolysis of caseins, contributed to this effect. Hydrolysis by plasmin reduced the zeta-potential of the casein micelles from ∼-19 to ∼-16 mV. The effect of hydrolysis of casein by plasmin on the heat stability of pre-heated milk was less pronounced, shifting the heat stability-pH profile to more alkaline values; the heat stability of concentrated milk was unaffected by plasmin. A very high (50 mg L^-1) level of added plasmin resulted in clearing of the skim milk; the L^* value decreased from ∼75 to ∼35 after 24 h incubation at 37°C. Clearing was correlated with a change in casein micelle diameter from an initial value of ∼175 to ∼250 nm. It is suggested that plasmin-induced changes in zeta-potential may promote micellar aggregation or changes in micelle stucture.