Impact of binding interaction modes between whey protein concentrate and quercetin on protein structural and functional characteristics

The utilization rate of proteins can be effectively improved through the interaction between proteins and polyphenols. In this study, non-covalent and covalent whey protein concentrate (WPC)-quercetin (Q) complexes were prepared. The binding principle between WPC and Q and the structural and functional properties alteration of WPC on binding with different concentrations of Q were evaluated. Fluorescence spectroscopy results demonstrated significant structural alterations of WPC characterised by fluorescence quenching and a red-shifted maximum wavelength of emission peak after the interaction between WPC and Q, and a significant decrease in beta-sheet and a remarkable increase in random curl content with the increase of Q concentration in fourier transform infrared spectroscopy (FTIR). Moreover, WPC and Q were more likely to form covalent complexes rather than non-covalent ones, which was featured with higher Q binding quantity and lower sulfhydryl group content at the same Q content. The binding sites number under covalent interaction mode was about 4.75 times that of the non-covalent counterparts. Combination of Q to WPC decreased surface hydrophobicity and improved foaming capability and emulsifying activity as well as antioxidant capability both for non-covalent and covalent interaction. However, non-covalent binding increased foam stability and emulsifying stability and decreased the protein solubility characterised by protein aggregation, while opposite trends were found for covalent interaction. The interfacial stability index variations were explained by the changes of zeta-potential of complexes with the increase of Q concentration. This study provides clarifies the principle behind the binding of WPC with Q and the possible application of WPC-Q complexes in food.