Protein interactions in heat-treated milk and effect on rennet coagulation

Skim milk was heated at different pH values to cause differential association of whey proteins (WP) with the casein micelles. All of these milk treatments coagulated poorly with rennet. To understand this in more detail, the casein micelles from heated milk were redispersed in unheated serum or unheated micelles were suspended in the sera from heat-treated milk. Systems containing micelles from milk heated at pH 6.7 and 7.1 were marginally better than the heated milk, but that from milk heated at pH 6.3 was not. The sera from milk heated at pH 6.7 and 7.1 impaired the clotting of native micelles but that from the pH 6.3 milk did not. Native casein micelles were suspended in permeates or dialyzed (against unheated milk) sera from heat-treated milk. Permeate systems free of WP/κ-casein complexes produced significantly stronger rennet gels; as did dialyzed systems. The impaired rennet clotting of heat-treated milk was attributed to a synergistic effect of the casein micelles with their heat-modified surfaces, the soluble serum WP/κ-casein complexes, and other dialyzable serum components.