AAPH-oxidised whey protein isolate forms a weak interfacial layer and promotes lipid oxidation in oil-in-water emulsions

The objective of this work was to investigate the interfacial adsorption properties and lipid oxidation of oil-in-water (O/W) emulsions prepared by 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH) oxidised whey protein isolate (WPI) with different oxidation degrees. The results indicated that as the AAPH concentration increased, the degree of oxidation of WPI also increased significantly, as confirmed by the higher protein carbonyl and N'-formyl-L-kynurenine levels. AAPH interacted with WPI mainly through hydrogen bonding and hydrophobic interactions. Higher AAPH concentration changed the structure of WPI, which verified by the results of intrinsic tryptophan fluorescence, circular dichroism spectra and SDS-PAGE. Moreover, the ζ-potential and adsorbed proteins of O/W emulsions prepared by oxidised-WPI decreased with increasing degree of protein oxidation, but the droplet size of O/W emulsions increased. In addition, higher oxidation degrees of oxidised-WPI promoted lipid oxidation of O/W emulsions, reflected by an increase in lipid hydroperoxide and malondialdehyde levels. These findings indicated that oxidised WPI could lead to quality deterioration of O/W emulsions, especially for those with a higher degree of oxidation.