Effect of water content on glass transition and protein aggregation of whey protein powders during short-term storage

The objectives of this study were to investigate the moisture-induced protein aggregation of whey protein powders and to elucidate the relationship of protein stability with respect to water content and glass transition. Three whey protein powder types were studied: whey protein isolate (WPI), whey protein hydrolysates (WPH), and beta-lactoglobulin (BLG). The water sorption isotherms were determined at 23 and 45 degrees C, and they fit the Guggenheim-Andersson-DeBoer (GAB) model well. Glass transition was determined by differential scanning calorimeter (DSC). The heat capacity changes of WPI and BLG during glass transition were small (0.1 to 0.2 Jg(-1) degrees C-1), and the glass transition temperature (Tg) could not be detected for all samples. An increase in water content in the range of 7 to 16% caused a decrease in Tg from 119 down to 75 degrees C for WPI, and a decrease from 93 to 47 degrees C for WPH. Protein aggregation after 2 weeks' storage was measured by the increase in insoluble aggregates and change in soluble protein fractions. For WPI and BLG, no protein aggregation was observed over the range of 0 to 85% RH, whereas for WPH, similar to 50% of proteins became insoluble after storage at 23 degrees C and 85% RH or at 45 degrees C and >= 73% RH, caused mainly by the formation of intermolecular disulfide bonds. This suggests that, at increased water content, a decrease in the Tg of whey protein powders results in a dramatic increase in the mobility of protein molecules, leading to protein aggregation in short-term storage.