Oxidation of caffeic acid in a wine-like medium: Production of dihydroxybenzaldehyde and its subsequent reactions with (+)-catechin

Dihydroxybenzaldehyde was generated from the iron-mediated oxidation of caffeic acid in a model white wine containing the non-flavonoid, caffeic acid. Higher concentrations of this aldehyde were obtained with increased iron concentration. The reaction of dihydroxybenzaldehyde with (+)-catechin, a flavanol compound, resulted in the production of both colourless and yellow/red-coloured compounds. Based on LC'MS studies, the colourless compounds were identified as isomeric forms of 3,4-dihydroxybenzenemethine-bridged (+)-catechin dimers. The coloured compounds were found to be a result of dihydroxybenzaldehyde reacting with either one or two (+)-catechin units. Compared to the reaction between (+)-catechin and glyoxylic acid, the reaction of dihydroxybenzaldehyde with (+)-catechin had a considerably slower rate of (+)-catechin degradation and also less colour formation. These results suggest that dihydroxybenzaldehyde has the ability to form coloured compounds in a reaction mechanism that is relevant to the enhanced colour development apparent during the oxidation of white or red wine.