Ultrasonic treatment improves physical and oxidative stabilities of walnut protein isolate-based emulsion by changing protein structure

Abstract

In this study, the effect of ultrasonic, heating, and enzymatic treatments on the structural and emulsifying attributes of walnut protein isolate (WNPI) was assessed and compared. Ultrasonic, heating, and enzymatic treatments all effectively improved WNPI's emulsifying capacity and stability, especially ultrasonication. Ultrasonication (400 W, 30 min) obviously made WNPI's structure looser and more unfolded, in turn led to 1.57-fold and 2.67-fold increase in EAI and ESI of WNPI, respectively. Besides, the exposure of WNPI′ hydrophobic groups induced by sonication, reflected in a 3.64-fold increase in WNPI's surface hydrophobicity, may also contributes to the increase of WNPI's emulsifying properties. Furthermore, the emulsion stabilized by UT-WNPI exhibited the best lipid and protein oxidative stabilities during storage at 55 °C for 7 days. Compared to emulsion stabilized by the untreated WNPI, hydroperoxide, TBARS, and protein carbonyl contents in the emulsion stabilized by UT-WNPI decreased by 46.92%, 40.74%, and 33.89%, respectively. Our results clarify ultrasonication is the best for improving the emulsifying properties of walnut protein, thereby broadening walnut protein's application in the emulsified food.