The influence of oxidation on proteolysis in raw milk

Abstract

The link between oxidation and increased proteolysis in raw milk was studied. To accelerate oxidation, H₂O₂ (1 mM) was added to raw milk, resulting in enhanced proteolysis by up to 11·2% after 24 h incubation at 5 °C. Addition of Cu²⁺ (10 μM) to milk or exposure of milk to light (60 min) likewise increased proteolysis. To explain the mechanism responsible for increased proteolysis as a result of oxidation, the effect of lipid oxidation products on plasmin-induced proteolysis was tested. Addition of malondialdehyde to skim milk increased the formation of γ-caseins, a proteolysis product from plasmin hydrolysis of β-casein. The same observation was made in a model system containing 4·5 g β-casein/l sodium tetraborate buffer at pH 8 and plasmin. Addition of a plasmin inhibitor blocked the formation of γ-casein. The results indicate that aldehydes accumulated from lipid oxidation can modify β-casein and thereby increase susceptibility of the proteins to proteolysis. Furthermore, the data suggest that proteolysis in raw milk may be connected to oxidative processes.