Physicochemical Properties of Moderately Heat-Treated Rice Protein Within Alkaline Solution and Its Evaluation as a Spray-Drying Microencapsulation Wall Material

This study addresses the shortcoming of rice protein, which has limited its widespread use as a food ingredient due to its extremely low solubility in neutral aqueous solution. Herein, rice protein (RP) was dispersed in aqueous solutions with different alkali concentrations (0.075 M~0.125 M), and then heat-treated (80 °C, 1~4 h) to obtain a modified RP. The physicochemical properties of the modified RP in neutral aqueous solution and its performance as a microencapsulated wall material were then comprehensively analyzed. The results showed that the solubility of the RP at pH 7.0 could be increased to more than 56.3% by alkali solution combined with moderate heat treatment for 1 h. Further analysis revealed that the enhancement of the RP solubility performance was mainly due to the depolymerization of rice glutenin cluster aggregates, with the average size decreasing to 140~180 nm, which was also accompanied by an increase in net zeta potential. Structural analysis pointed to a significant decrease in the surface hydrophobicity and free sulfhydryl content of the RP after thermal treatment in alkaline solution, while degradation of glutenin subunits (especially for the results of alkaline treatment at higher concentrations) and an increase in random coil content occurred. These physicochemical properties and conformational transitions of the modified RP contributed to its excellent emulsification properties and microencapsulation ability (encapsulation efficiency > 97%). Nevertheless, the redispersing properties of microcapsules prepared with the modified RP as a wall material were significantly weaker than those of sodium caseinate. These findings provide new guidance and insights into the modulation of functional properties and applications of RP.