Voltammetric Evaluation of Polyphenol–Protein Interactions and Their Influence on the Antioxidant Capacity of Tea

A voltammetric method for evaluating polyphenol–protein interactions was developed based on the oxidation of polyphenols at an electrode modified with multi-walled carbon nanotubes and electropolymerized quercetin. Tea polyphenols (catechin and epigallocatechin gallate) and milk proteins (casein, bovine serum albumin, and β-lactoglobulin) were considered as model systems. It was found that proteins bind the test polyphenols (from 10 to 60%) with increasing protein content (from 1 : 0.25 to 1 : 1.5, by volume) in the mixture by decreasing the fraction of free polyphenol. The developed approach was used to assess the effect of milk on the antioxidant properties of tea (green, white, oolong, and black teas), which were expressed through the antioxidant capacity (AOC) parameter. A statistically significant decrease in the AOC of tea by a factor of 1.8–23 in the presence of 20 vol % milk was demonstrated.