Effects of alkali treatment on structural and functional properties of chickpea protein isolate and its interaction with gallic acid: To improve the physicochemical stability of water–in–oil emulsions

Abstract

To expand the application of chickpea protein isolate (CPI) in water–in–oil (W/O) emulsion systems and to address its limitations in inhibiting lipid oxidation in W/O emulsions, alkali treatment was used to enhance the functional properties of CPI and to promote its combination with gallic acid (GA). The results showed a slight increase in the antioxidant activity of CPI upon pH 11 treatment, which was attributed to the exposure of sulfhydryl groups (3.39 μmol/g). The interfacial activity of CPI treated with pH 11 was enhanced (e.g., interfacial tension of 1.72 mN/m and interfacial adsorption of 93.20%). In addition, the alkali treatment promoted the covalent binding of CPI and GA, especially under mild alkaline conditions (pH 9, the binding equivalent was 13.12 mg/g protein). The results of amino acid and intrinsic fluorescence spectrum indicated that the binding sites for CPI and GA might include Met, Glu, His, Lys, Arg and Trp residues. The combination of CPI and GA further changed the structural and functional properties of CPI. When alkali–treated CPI and CPI–GA combinations were added to emulsions, the increase in physical stability was attributed to a reduction in droplet size and an increase in ζ–potential. In terms of chemical stability, the higher interfacial activity and the higher GA binding equivalent of CPI contributed to the higher accumulation of GA at the droplet interface. This ultimately improved the chemical stability of the W/O emulsion (57.03% and 61.34% reduction in primary and secondary oxidation products respectively). This work provides guidance on the application of interfacial engineering to modify proteins with high interfacial and low antioxidant activity and to improve the physicochemical stability of W/O emulsions.