Low temperature extrusion promotes transglutaminase cross-linking of whey protein isolate and enhances its emulsifying properties and water holding capacity

Impact of extrusion pretreatment (50, 70, 90, 110 and 130 °C) on physicochemical, emulsifying properties and water holding capacity (WHC) of transglutaminase (TGase) cross-linked whey protein isolate (WPI) were investigated in this study. The results of molecular size distribution and SDS-PAGE proved that there were more macromolecular polymers formed in TGase cross-linked WPI after extrusion pretreatment (E-WPI-TGase) and the consumption of free amino groups for E-WPI-TGase was the largest at the extrusion temperature of 50 °C and 70 °C. As the extrusion temperature increased from 90 °C to 130 °C, particle size of WPI after extrusion pretreatment (E-WPI) was not changed significantly, compared with that of E-WPI-TGase (p > 0.05). In addition, the emulsifying properties, surface hydrophobicity and WHC of E-WPI-TGase were higher than those of WPI cross-linked by TGase (WPI-TGase) (p < 0.05). Significantly, the emulsifying activity, surface hydrophobicity and WHC of E-WPI-TGase at the extrusion temperature of 50 °C was increased by 63.78%, 18.58% and 96.63% compared with that of unextruded WPI-TGase, respectively. Therefore, these results indicated that low temperature (50 °C and 70 °C) extrusion pretreatment could promote the cross-linking degree of TGase-catalyzed WPI, and improve its emulsifying properties, WHC and surface hydrophobicity.