Kinetics of denaturation and aggregation of whey proteins in skim milk heated in an ultra-high temperature (UHT) pilot plant

Skim milk was subjected to various temperature-time (70-130 degrees C for 3-1800 s) treatments in a pilot-scale ultra-high temperature (UHT) plant using a direct steam injection (DSI) system. The rates of denaturation and aggregation of beta-lactoglobulin (beta-lg) and alpha-lactalbumin (alpha-la) were determined by quantitative polyacrylamide gel electrophoresis while that of immunoglobulin G (IgG) was determined by affinity chromatography. The order of reaction, the rate constant and the activation energy were determined for the denaturation of beta-lg, alpha-la and IgG using non-linear regression methods. The kinetic parameters obtained for beta-lg and alpha-la showed a marked temperature dependence. Aggregation of beta-lg mainly involved the formation of intermolecular disulphide linkages at all temperatures studied, while the aggregation of alpha-la, particularly below 80 degrees C, appeared to also involve hydrophobic interactions. The kinetic parameters obtained for beta-lg aggregation, due to disulphide bond formation, were similar to those observed for beta-lg denaturation. (C) 1998 Elsevier Science Ltd. All rights reserved.