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

Bovine beta -lactoglobulin (beta -lg), alpha -lactalbumin (alpha -la) and b ovine serum albumin (BSA), dispersed in ultrafiltration permeate, that had been prepared from whey protein concentrate solution (100 g/kg, pH 6.8), we re heated at 75 degreesC. The consequent protein aggregation was studied by one-dimensional (1D) and two-dimensional (2D) polyacrylamide gel electroph oresis (PAGE). When 100 g beta -lg/kg permeate solution was heated at 75 de greesC, cooled and examined, large aggregates were observed. These aggregat es were partially dissociated in SDS solution to give monomers, disulphide- bonded dimers, trimers and larger aggregates. When mixtures of beta -lg and alpha la or BSA were heated, homopolymers of each protein as well as heter opolymers of these proteins were observed. These polymer species were also observed in a heated mixture of the three proteins. Two-dimension PAGE of m ixtures demonstrated that these polymers species contained disulphide-bonde d dimers of beta -lg, alpha -la and BSA and 1:1 disulphide-bonded adducts o f alpha -la and beta -lg, or BSA. These results are consistent with a mecha nism in which the free thiols of heat-treated beta -lg or BSA catalyse the formation of a range of monomers, dimers and higher polymers of alpha -la. It is likely that when whey protein concentrate is heated under the present conditions. BSA forms disulphide-bonded strands ahead of beta -lg and that alpha -la aggregation with beta -lg and with itself is catalysed by the he at-induced unfolded BSA and beta -lg.