Exploration of interaction mechanisms and functional properties of coffee flavonoids and β-casein via multispectroscopy and molecular dynamics simulation

Abstract

Milk coffee, a commonly consumed beverage, is prepared by mixing milk and coffee, which results in various interactions among their components. This study was aimed at investigating the interaction mechanisms of four coffee flavonoids (namely apigenin [AG], luteolin [LUT], quercetin [QC], and epigallocatechin gallate [EGCG]) and milk-derived β-casein (β-CN). Notably, LUT and QC bound to β-CN mostly via hydrogen bonds and van der Waals forces, whereas AG and EGCG primarily bound to β-CN via hydrophobic interactions. The trend of energy transfer efficiencies of the β-CN-flavonoid complex was as follows: β-CN-QC (50.20 %) > β-CN-LUT (43.68 %) > β-CN-EGCG (40.82 %) > β-CN-AG (35.34 %). Additionally, the dynamic behavior of the β-CN-flavonoid interaction and structural alterations in the protein were validated by molecular dynamics (MD) simulations. Multispectroscopy results revealed flavonoid-mediated alterations in the secondary structure of β-CN, including increased amounts of random coil structure and a decrease in the proportion of α-helix, which resulted in a more open and loose protein structure. The noncovalent interactions between flavonoids and β-CN lead to decreased protein surface hydrophobicity, increased solubility, improved emulsifying activity, albeit with decreased emulsifying stability, and improved foaming ability and foam stability. Furthermore, the complex exhibited a superior antioxidant activity to that of either protein or flavonoid alone, suggesting the synergistic action of the compounds on the antioxidant activity. Altogether, our results of this study offer a theoretical foundation for comprehending the interaction mechanisms among coffee polyphenols and milk proteins for the creation of a functional beverage such as milk coffee.