Effects of different pH conditions on interfacial composition and protein-lipid co-oxidation of whey protein isolate-stabilised O/W emulsions

The physical stability, interfacial composition and protein-lipid co-oxidation of O/W emulsions stabilised by whey protein isolate under different pH conditions (3.0, 5.0, 7.0 and 9.0) with fixed storage times were assessed in the present study. The results revealed that different pH conditions had distinct effects on the physical stability of the emulsions, as verified by the droplet size, ζ-potential and interfacial composition. Acidic conditions had an unnoticeable impact on protein-lipid co-oxidation. Remarkable physical barrier-like protection of the lipid core provided by adsorbed proteins (AP) was observed in the O/W emulsions under neutral conditions. However, alkaline conditions (pH 9.0) could induce the most severe protein-lipid co-oxidation when the storage time was extended, as indicated by the highest levels of protein and lipid oxidation products (carbonyl, N′-formyl-l-kynurenine, lipid hydroperoxides, thiobarbituric acid-reactive substances and volatile lipid oxidation products), in addition to the obvious molecular changes in unadsorbed proteins (UAP) or AP. Free radicals and metal ions could more easily react with highly unfolded and negatively charged AP under strong alkaline conditions. Our findings clearly revealed that the co-oxidation phenomenon of O/W emulsion systems were closely related to the synergistic effects between the transformation of interfacial composition and aggregation of oil droplets that occurred under different pH conditions.