Oil-water interfacial and emulsifying properties of lupin protein and lupin protein-pectin mixtures at neutral and acidic pH conditions

Lupin proteins have high nutritional value and can be used to form foams, emulsions, and gels. However, their functionality becomes poor at acidic pH due to reduced protein solubility. Protein solubility and functionality at acidic pH can be improved by adding polysaccharides. In this study, we compared the oil-water interfacial and emulsifying properties of lupin protein isolates (LPI) and LPI-pectin mixtures at pH 7.0, pH 6.0, pH 4.0, and pH 3.5. Overall, LPI-pectin complexes at pH 4.0 and 3.5 showed better interfacial properties and emulsion stabilization than LPI at pH 4.0 and 3.5, while LPI-pectin mixtures at pH 7.0 and 6.0 displayed comparable interfacial and emulsion properties as LPI at 7.0 and 6.0. We observed that LPI at pH 4.0 and pH 3.5 adsorbed faster to the oil-water interface than LPI-pectin complexes at pH 4.0 and 3.5, due to smaller particle sizes of LPI, while LPI and LPI-pectin mixtures at pH 7.0 and 6.0 showed comparable adsorption rates to the oil-water interface. The LPI-pectin mixtures at pH 6.0 and 7.0 formed weak oil-water interfaces, with a stiffness comparable to LPI at these pH values. While at pH 3.5 and 4.0 LPI-pectin complexes formed stiffer oil-water interfaces than LPI. As a result, LPI-pectin complexes at pH 4.0 and 3.5 showed better emulsifying properties and emulsion stability against coalescence during high-shear mixing. The emulsion stabilized with LPI-pectin complex at pH 3.5 showed more extensive flocculation than the complex at pH 4.0, due to the reduced charges of the complex at pH 3.5 and depletion flocculation induced by non-adsorbed complexes. Our study reveals that the complexation of proteins with pectin at acidic pH could be a useful way to improve the oil-water interfacial and emulsifying properties of proteins at acidic pH, and could potentially be used in the food industry to develop plant protein-based emulsion products.