Mechanism of the solubility increase of rice protein-ovalbumin co-assembly and its great potential as an emulsifier in high internal phase emulsion

The functionality (e.g., emulsifying ability) of rice protein (RP) is restricted largely by its low solubility. The formation of a co-assembly with RP and an exogenous protein through pH-cycle is one promising method, but the mechanism behind this has not been revealed. This study utilized ovalbumin (OVA) to improve the solubility of RP and investigated the effect of OVA on the physicochemical characteristics of RP, the interactions between RP-OVA co-assembly and water, the RP-OVA interactions, the key subunits related to the solubility improvement of RP, and the emulsifying capacity of the co-assembly. The formation of RP-OVA co-assembly considerably improved the solubility of RP (up to 73.12%), and the multi-level structure and surface property of RP were altered by OVA. The mechanism for the solubility increase was the hydrophobic interactions between OVA and RP, resulting in the exposure of some hydrophilic amino acids to protein surface and further generating a low surface hydrophobicity that allow more water molecules to permeate. Eight subunits in RP were critical for its solubility increase. The RP-OVA co-assembly prepared in this study showed an emulsifying ability even better than whey protein isolate (a commercial emulsifier) in high internal phase emulsion. This study may guide the solubility improvement of RP and widen the application of RP in the food industry as an emulsifier.