The interaction of κ-casein and β-lactoglobulin is fundamental to all heat-induced modifications of milk product functionality, such as the heat stability of concentrated milks. Purified native κ-casein B and β-lg A solutions were heated at 80 °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 κ-casein, less β-lg was converted into β-lg polymers and the rate of loss of native β-lg was greater. When κ-casein was added to previously heated β-lg and the mixture was heated, the κ-casein reacted with the heat-induced β-lg polymers more readily than with the β-lg native monomers. The formation of β-lg dimers, trimers etc. was diminished. It was concluded that, when β-lg and κ-casein were heated together, β-lg formed thiol-exposed monomers, which reacted with each other or with the native κ-casein depending on the relative concentrations of β-lg and κ-casein. The products of these reactions included some disulphide-bonded 1[ratio ]1 β-lg[ratio ]κ-casein complexes, some monomer κ-casein and a range of large aggregates held together by either or both disulphide bonds and hydrophobic association.

Bovine β-lactoglobulin (β-lg), α-lactalbumin (α-la) and bovine serum albumin (BSA), dispersed in ultrafiltration permeate, that had been prepared from whey protein concentrate solution (100 g/kg, pH 6·8), were heated at 75 °C. The consequent protein aggregation was studied by one-dimensional (1D) and two-dimensional (2D) polyacrylamide gel electrophoresis (PAGE). When 100 g β-lg/kg permeate solution was heated at 75 °C, cooled and examined, large aggregates were observed. These aggregates were partially dissociated in SDS solution to give monomers, disulphide-bonded dimers, trimers and larger aggregates. When mixtures of β-lg and α-la or BSA were heated, homopolymers of each protein as well as heteropolymers of these proteins were observed. These polymer species were also observed in a heated mixture of the three proteins. Two-dimensional PAGE of mixtures demonstrated that these polymers species contained disulphide-bonded dimers of β-lg, α-la and BSA, and 1:1 disulphide-bonded adducts of α-la and β-lg, or BSA. These results are consistent with a mechanism in which the free thiols of heat-treated β-lg or BSA catalyse the formation of a range of monomers, dimers and higher polymers of α-la. It is likely that when whey protein concentrate is heated under the present conditions, BSA forms disulphide-bonded strands ahead of β-lg and that α-la aggregation with β-lg and with itself is catalysed by the heat-induced unfolded BSA and β-lg.