Re-association Behavior of Casein Submicelles in Highly Alkaline Environments

The pH-dependent dissociation and re-association behavior of casein proteins in alkaline environment was studied. In dilute aqueous solutions of pH = 12 and 13, casein was found to exist in the form of submicelles which represent the fundamental structural unit of casein micelles. Morphology and dimension of the casein submicelles were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM), revealing spherical particles of similar to 10-30 nm in diameter. When the pH was increased to 14, unexpected re-association of casein submicelles occurred, resulting in the formation of large network aggregates at pH > 14. Furthermore, calcium phosphate was found to be the main driving force behind the re-association, as verified by comparative experiments employing Ca-depleted casein. Based on an analysis of potential interactions between the proteins, this re-association is ascribed to decreased electrostatic repulsion, strengthened hydrophobic attraction and formation of calcium phosphate linkages between casein proteins.