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 degrees 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 degrees C and 70 degrees C. As the extrusion temperature increased from 90 degrees C to 130 degrees 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 degrees 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 degrees C and 70 degrees C) extrusion pretreatment could promote the cross-linking degree of TGase-catalyzed WPI, and improve its emulsifying properties, WHC and surface hydrophobicity.