Interactions of black and green tea polyphenols with whole milk

Interactions of black tea polyphenols (BTP) and green tea polyphenols (GTP) with whole milk were comparatively studied. Upon the ultracentrifugation of BTP- and GTP-milk systems (40% v/v milk), most BTP and GTP partitioned into milk protein fractions: whey proteins and casein micelles, with 35.1% and 73.0% of total catechins bound to the casein micelles accordingly. The affinities of catechins for casein micelles were differentiated by the structures of catechins in the GTP-milk system but the BTP-milk system, being enhanced by a gallate group in catechins and the cis-form and weakened by a pyrogallol group. Fourier transforms infrared spectroscopy (FTIR) analysis showed that TP binding altered the secondary structures of milk proteins by reducing inter β-sheet, random coil and the large loop and increasing α-helix, intra β-sheet and turn structures, and more intense hydrophobic interaction was observed in the BTP-milk system. UV-vis spectra indicated no obvious impacts on TP molecules at a low concentration of milk proteins. With the increasing addition of tea infusion, BTP exhibited a similar fluorescence quenching ability to GTP, but the variance in the ORAC values of BTP-milk system was different from that of the GTP-milk system, suggesting that fluorescence quenching may not fully represent the interactions between polyphenols and proteins.