Casein micelle structure: susceptibility of various casein systems to proteolysis

Abstract

Summary: The susceptibility of the components of various caseinate systems (skim-milk, β-casein-depleted milks, colloidal phosphate-free (CPF) milk, sodium caseinate and isolated β-casein) to proteolysis was investigated. Isolated α_s1- and β-caseins were quite susceptible to proteolysis, but their susceptibility decreased in heterogeneous soluble systems and even more so in heterogeneous aggregated systems. In skim-milk and β-casein-depleted milks only about 50% of both α_s1- and β-casein was hydrolysable by high levels of rennin, and in CPF milk all α_s1- and 70% of the β-casein was hydrolysable. It is suggested that about 50% of micellar β-casein is firmly fixed within the micelle and is unavailable for proteolysis, while the remainder can dissociate from the micelle on cooling and is then readily hydrolysable.The compatibility of the data with the various published models of the casein micelle is discussed, and a modification of Rose's (1969) model is proposed.