Molecular mechanism of the interactions between coffee polyphenols and milk proteins

The interaction between coffee polyphenols and milk proteins enhances the chemical stability of coffee polyphenols; however, the mechanism underlying this interaction remains elusive, especially at the amino acid level of the proteins. This study investigated the non-covalent interactions of coffee polyphenols (chlorogenic and caffeic acids) with various milk proteins (α-casein, β-casein, κ-casein, α-lactalbumin, and β-lactoglobulin). Fluorescence spectroscopy was used to examine the affinity of the coffee polyphenols for the milk proteins. The fluorescence intensity was found to be dependent on the proline residue content in the milk proteins. Coffee polyphenols were approximately twice as soluble in proline solution as in water, indicating thermodynamically favorable interactions with proline. Molecular dynamics simulations indicated that caffeic acid interacts with the proline side chains of peptides, which is attributable to hydrophobic interaction. The present findings provide mechanistic insights into the interactions between coffee polyphenols and milk proteins at the amino acid level, thereby contributing to a deeper understanding of the enhanced chemical stability of coffee polyphenols in the presence of milk proteins. This work also presents general cautions regarding the spectroscopy of polyphenols.