Characterization of soluble aggregates from whey protein isolate

The aggregation behavior of a commercially available whey protein isolate upon heat treatment at neutral pH and low ionic strength was determined using a combination of size exclusion chromatography, multi-angle laser light scattering, dynamic light scattering and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The objective of the work was to examine the effect of heating on the denaturation and subsequent aggregation of 10% (w/v) WPI solutions at neutral pH and low ionic strength. Previous work on heat-induced whey protein aggregation has focused in particular on β-lactoglobulin (β-lg), which is the main protein responsible for heat-induced aggregation; however, because of the presence of α-lactalbumin (α-lac) in WPI, differences in the aggregation behavior occur. Time and temperature affected the amount of residual native α-lac and β-lg, showing a decrease in native protein as aggregates formed. The aggregates differed in molecular weight and hydrodynamic diameter depending on temperature of heating. Electrophoresis of soluble aggregates, collected by preparative size exclusion chromatography, showed a ratio of about 2.0 β-lg/α-lac, regardless of temperature or time of heating. On the other hand, the composition of the residual native protein suggested higher α-lac reactivity at 65 °C than at 75 and 85 °C compared to β-lg, with generally faster rates of denaturation for both proteins with increasing temperature of heating.