Analysis of β-lactoglobulin-epigallocatechin gallate interactions: the antioxidant capacity and effects of polyphenols under different heating conditions in polyphenolic-protein interactions

The interaction and antioxidant capacity between epigallocatechin gallate (EGCG) and beta-lactoglobulin (beta-Lg) under thermal treatments at 25-121 degrees C were investigated in this study. Fluorescence spectroscopy analysis showed that EGCG complexed with beta-Lg mainly via non-covalent interactions and the binding affinity of EGCG to beta-Lg was enhanced with heat treatment. EGCG showed a strong binding affinity to beta-Lg after 85 degrees C heat treatment was applied, with a K-a of 30.69 (+/- 0.87) x 10(5) M-1 (pH 6.8, 298 K). Circular dichroism (CD) results showed that heat treatment did not result in greatly affected changes in the beta-Lg secondary structure induced by beta-Lg-EGCG interactions. MALDI-TOF/TOF-MS analysis showed that beta-Lg-EGCG covalent conjugates initially formed after heat was applied at 60 degrees C, and their proportions increased under heat treatment ranging from 85 to 121 degrees C. The amino group of a lysine residue was further confirmed as the covalent binding site of EGCG to beta-Lg. The beta-Lg-EGCG interaction showed little effect on the antioxidant capacity (ABTS and ferric reducing antioxidant power (FRAP) values) of EGCG after heat treatment at 25-60 degrees C, but did induce an obvious reduction at temperatures above 85 degrees C. This study will provide the foundation for the use of EGCG in processing dairy products (such as milk tea beverages) with desirable nutrition and physiological functions.