Effect of copper on oxidation of (+)-catechin in a model solution system

Summary Autoxidation of ( +)‐catechin in a model solution system, containing cupric ions, was monitored by high‐performance liquid chromatography with diode array detection. The rate of (+)‐catechin oxidation and the extent of browning increased with the copper concentration, showing an effect of copper on browning of flavanols. The progression of the reaction showed that the appearance of new compounds, which were initially absent in the mixture, was concomitant with (+)‐catechin decrease. Numerous reaction products, including both colourless compounds and pigments absorbing at 440, 460 and 550 nm, were detected. Among the colourless derivatives, four dimeric compounds, consisting of two catechin units linked by a carboxymethine bridge, were detected. Their formation involved the condensation of flavanols, mediated by glyoxylic acid, following a mechanism similar to that previously observed with iron. The yellowish compounds that were detected were identified as xanthylium salts and their corresponding esters, showing absorption maxima at 440 and 460 nm, respectively. Comparison of the results obtained with those reported with iron showed that copper ions play similar roles in enhancing the oxidation rate of (+)‐catechin and flavanols.