THERMAL AGGREGATION AND GELATION OF BOVINE BETA-LACTOGLOBULIN

Heat-induced aggregation and gelation of beta-lactoglobulin, dispersed in imidazole-HCl buffer containing 0.1 mol/dm3 NaCl at a concentration of 10% (w/v), was studied at different temperatures and pHs. Gel electrophoresis under non-dissociating (PAGE, in the absence of dissociating and reducing agents) and dissociating but not reducing conditions (SDS-PAGE) was used to determine the extents of aggregation. The rate of loss of beta-lactoglobulin from non-dissociating-PAGE was faster than from SDS-PAGE and the rates could be described by second-order kinetics. The loss of protein on heat treatment at 75-degrees-C increased as the pH increased from 6.0 to 9.0, under both non-dissociating and dissociatiung conditions. Dynamic shear measurements were used to determine the development of gel structure by measuring the changes in storage modulus (G') during heating. No detectable changes in G' were observed during heating at 70-degrees-C. At 75 and 80-degrees-C. G' increased with heating time, the rate of increase being greater at 80-degrees-C. The increase in G' on heat treatment at 75-degrees-C was faster at pH 7.0 than at pH 6.0, 8.0 and 9.0. Comparison of the loss of native beta-lactoglobulin structure due to the formation of non-covalently linked protein aggregates and covalently cross-linked aggregates with the changes in G' showed that no measurable changes in G' occurred until most of the protein had formed covalently cross-linked aggregates. This suggested that protein aggregates are formed as an intermediate prior to the formation of self-supporting macroscopic gel networks.