Heat-induced interactions of beta-lactoglobulin A and kappa-casein B in a model system.

The interaction of kappa-casein and beta-lactoglobulin is fundamental to all heat-induced modifications of milk product functionality, such as the heat stability of concentrated milks. Purified native kappa-casein B and beta-Ig A solutions were heated at 80 degrees C at pH 6.7 separately and in a mixture. The circular dichroism spectra in the near UV indicated irreversible changes in the disulphide bonding patterns involving both proteins. Alkaline- and SDS-PAGE of heated samples showed that, in the presence of kappa-casein, less beta-Ig was converted into beta-Ig polymers and the rate of loss of native beta-Ig was greater. When kappa-casein was added to previously heated beta-Ig and the mixture was heated, the kappa-casein reacted with the heat-induced beta-Ig polymers more readily than with the beta-Ig native monomers. The formation of beta-Ig dimers, trimers etc. was diminished. It was concluded that, when beta-Ig and kappa-casein were heated together, beta-Ig formed thiol-exposed monomers, which reacted with each other or with the native kappa-casein depending on the relative concentrations of beta-Ig and kappa-casein. The products of these reactions included some disulphide-bonded 1:1 beta-Ig:kappa-casein complexes, some monomer kappa-casein and a range of large aggregates held together by either or both disulphide bonds and hydrophobic association.