The effect of pH on the formation of coloured compounds in model solutions containing anthocyanins, catechin and acetaldehyde

The qualitative and quantitative effect of pH on the formation of new coloured compounds from the interaction between malvidin 3-glucoside, (+) catechin and acetaldehyde was studied. Five different solutions were prepared at pH 2, 3, 3.7, 4 and 5, and the concentration of the reactants and newly formed compounds was monitored by HPLC. In all mixtures there was a decrease in the concentration of malvidin 3-glucoside and catechin, coupled with the formation of two principal new compounds thought to be dimers (labelled A and B), linked at different positions, and several minor compounds. The rate of loss of the reactants and the longevity and concentrations of A and B varied according to pH, with the greatest amounts and most rapid formation of these compounds occurring at pH 2. The catechin concentration still decreased when there was no malvidin 3-glucoside remaining. This observation indicated that further polymerisation reactions were occurring, involving catechin molecules only, or catechin and coloured polymers.