Solubility and emulsifying properties of phosphorylated walnut protein isolate extracted by sodium trimetaphosphate

Abstract

Low solubility and poor emulsifying properties of walnut protein isolate (WPI) greatly influences its applications in the food industry. In the current work, WPI was extracted and phosphorylated from defatted walnut flour by sodium trimetaphosphate (STMP) at various pH values (7.0, 9.0, and 11.0). The measured phosphorus content of STMP-extracted WPI was approximately 7-10-fold higher than that of the alkaline-extracted WPI (A-WPI). The results of X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) further confirmed the successful phosphorylation of extracted WPI. Conformational studies using FTIR and circular dichroism (CD) indicated that phosphorylated WPI (P-WPI) contained higher α-helical and lower β-sheet contents than the A-WPI. Phosphorylation of WPIs altered the surface hydrophobicity and surface charges of the proteins, resulting in a significant improvement in the solubility of P-WPI. In addition, the emulsifying properties of P-WPI were significantly enhanced, which was exhibited by a 2.5-4-fold increase in the emulsifying activity index (EAI) and a maximum 6-fold increase in the emulsifying stability index (ESI). The emulsions stabilized by P-WPI manifested relatively smaller sizes and larger surface charges. This study demonstrated that phosphorylation modification can effectively modulate the surface hydrophobicity and charges of WPI to greatly improve its solubility and emulsifying ability.