Characterization of heat-induced aggregates of beta-lactoglobulin, alpha-lactalbumin and bovine serum albumin in a whey protein concentrate environment

Abstract

Bovine b-lactoglobulin (b-lg), a-lactalbumin (a-la) and bovine serum albumin (BSA), dispersed in ultra®ltration 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 twodimensional (2D) polyacrylamide gel electrophoresis (PAGE). When 100 g b-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 b-lg and a-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 b-lg, a-la and BSA, and 1:1 disulphide-bonded adducts of a-la and b-lg, or BSA. These results are consistent with a mechanism in which the free thiols of heattreated b-lg or BSA catalyse the formation of a range of monomers, dimers and higher polymers of a-la. It is likely that when whey protein concentrate is heated under the present conditions, BSA forms disulphide-bonded strands ahead of b-lg and that a-la aggregation with b-lg and with itself is catalysed by the heat-induced unfolded BSA and b-lg.