Interactions between alpha-lactalbumin and beta-lactoglobulin in the early stages of heat denaturation

Interactions between whey proteins in mixed systems (10% w/w total protein) containing alpha-lactalbumin (alpha-la) and beta-lactoglobulin (beta-lg) heated at 75 degrees C for different times were studied using gel filtration chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. On its own, alpha-la did not form aggregates, and beta-lg formed large aggregates with no evidence for intermediates. However, the two proteins interacted to form soluble aggregates, as well as larger particles, by means of both disulfide bonds and hydrophobic interactions. Two main soluble products were identified: high molecular weight components with molecular masses in excess of 300 000 Da and intermediate aggregates with molecular weights in the region of 100 000 Da. The aggregation process was substantially affected, both quantitatively and qualitatively, by the relative proportions of alpha-la and beta-lg: when the weight fraction of alpha-la in the mixture was <0.3, the smaller aggregates were not induced, whereas they were formed at higher weight fractions and were the only product at a weight fraction of alpha-la of 0.95. The composition of the aggregates changed during heating and was different for each mixture, but it always approximated the beta-lg/alpha-la ratio of the initial mixture after about 8 min of heating. The different interactions observed at the different ratios of proteins may explain the enhancement of heat-induced gelation of beta-lg by alpha-la.