Effect of microbial transglutaminase treatment on thermal stability and pH-solubility of heat-shocked whey protein isolate

Whey protein isolate (WPI) dispersions (5% protein, pH 7.0) were subjected to heat-shock at 70 degrees C for 1, 5 and 10 min. The heat-shocked WPI dispersions were treated with microbial transglutaminase (MTGase) enzyme, and thermal properties and pH-solubility of the treated proteins were investigated. Heat-shocking of WPI for 10 min at 70 degrees C increased the thermal denaturation temperature (T-d) of beta-lactoglobulin in WPI by about 1.5 degrees C. MTGase treatment (30 h, 37 degrees C) of the heat-shocked WPI significantly increased the T-d of beta-lactoglobulin by about 6.3-7.3 degrees C when compared with heat-shocked only WPI at pH 7.0. The T-d increased by about 13-15 degrees C following pH adjustment to 2.5; however, the T-d of heat-shocked WPI was not substantially different from heat-shocked and MTGase-treated WPI at pH 2.5. Both the heat-shocked and the heat-shocked-MTGase-treated WPI exhibited U-shaped pH-solubility profiles with minimum solubility at pH 4.0-5.0. However, the extent of precipitation of MTGase-treated WPI samples at pH 4.0-5.0 was much greater than all heat-shocked and native WPI samples. The study revealed that while MTGase cross-linking significantly enhanced the thermal stability of beta-lactoglobulin in heat-shocked WPI, it caused pronounced precipitation at pH 4.0-5.0 via decreasing the hydrophilic/hydrophobic ratio of the water-accessible protein surface. (C) 2012 Elsevier Ltd. All rights reserved.