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)-lg A solutions were heated at 80 (degree)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)-lg was converted into (beta)-lg polymers and the rate of loss of native (beta)-lg was greater. When (kappa)-casein was added to previously heated (beta)-lg and the mixture was heated, the (kappa)-casein reacted with the heat-induced (beta)-lg polymers more readily than with the (beta)-lg native monomers. The formation of (beta)-lg dimers, trimers etc. was diminished. It was concluded that, when (beta)-lg and (kappa)-casein were heated together, (beta)-lg formed thiol-exposed monomers, which reacted with each other or with the native (kappa)-casein depending on the relative concentrations of (beta)-lg and (kappa)-casein. The products of these reactions included some disulphide-bonded 1:1 (beta)-lg:(kappa)casein complexes, some monomer (kappa)-casein and a range of large aggregates held together by either or both disulphide bonds and hydrophobic association.