Enzymatic Hydrolysis of Heat-induced Aggregates of Whey Protein Isolate

Abstract

The effects of heat induced denaturation and subsequent aggregation of Whey Protein Isolate (WPI) solutions on the rate of enzymatic hydrolysis was investigated. Denaturation of whey proteins was monitored by reversed-phase and size exclusion HPLC and observed by native- and SDS-PAGE. Treated and un-treated WPI solutions (100 g L-1 protein) were hydrolysed to a target degree of hydrolysis (DH) of 5 % with Corolase® PP. Aggregate formation was monitored using light microscopy, with size distribution determined by particle size. Viscosity and surface hydrophobicity exhibited large increases with heat-treatment and the major protein components in WPI showed differences in their rates of aggregation. Results revealed an increased rate of hydrolysis of protein solutions, which were subjected to a pre-hydrolysis heattreatment. Light and Confocal Laser Scanning Microscopy (CLSM) images illustrated the optical clarification of the solution, weakening of the gel network and disintegration of aggregates indicative of hydrolysis. Comparison of samples where there was a heat-treatment prior to hydrolysis and a control non-treated hydrolysis reaction, revealed significant differences in the time to reach 5 %DH (P < 0.001). The heat-treatments ≥ 75 ºC for 5 min produced significantly (P < 0.001) more rapid reactions than the other 5 heat-treatments and the control un-treated reaction. The viscosity, surface hydrophobicity, and insolubility of the heat-treated WPI solutions subsequently declined upon their hydrolysis. The extensive aggregation in some heattreated solutions was postulated to relate to the congruent increased rate of hydrolysis. This study demonstrated that prior thermal treatment of ≥ 75 ºC for 5 min can accelerate the enzymatic hydrolysis reaction of WPI with Corolase® PP.