Interactions Between Anthocyanins, Phenolic Compounds, and Acetaldehyde and Their Significance in Red Wines

Interactions between pure anthocyanins [malvidin (Mv) 3-glucoside, Mv 3,5-diglucoside, and Mv 3-p-coumarylglucoside-5-glucoside], phenolic compounds [ (-)-epicatechin, (+)-catechin, (-)-epigallocatechin, procyanidin dimer B2, and procyanidin trimer C1], and acetaldehyde were studied in tartrate buffer, pH 3.5, at room temperature in air under darkness and light. Several reactions occurred simultaneously. The anthocyanins and phenolic compounds reacted very slowly, with eventual formation of yellow xanthylium salts (one presumed glycosylated, others sugar-free), confirming that condensation occurred via the anthocyanin 4-position. Unreacted procyanidins B2 and C1 were partly transformed to epicatechin and lower and higher polymeric procyanidins, the reactions being catalyzed by daylight. There was little reaction between the anthocyanins and acetaldehyde except with Mv 3-glucoside, which was slowly polymerized. However, the addition of acetaldehyde to mixtures of anthocyanins and phenolics caused rapid and spectacular color augmentation with shifts toward violet, with extent of the shift varying with the type of component. The orders of decreasing reactivity were Mv 3-glucoside > Mv 3-p-coumarylglucoside-5-glucoside > Mv 3,5-diglucoside, and trimer C1 = dimer B2 > epicatechin. Conversely the extent of color increase and its stability decreased in reverse order, with Mv diglucoside > acylated diglucoside > monoglucoside, because of precipitation with the latter. Color augmentation was due to the formation of highly colored new compounds, detectable by chromatography and believed to consist of anthocyanins and phenolics linked by CH₃CH bridges. A reaction scheme is proposed in which the initial reaction product of acetaldehyde and phenolic combines with the anthocyanin at position 8, followed by anhydrobase formation. A small amount of a compound resulting from interaction of Mv 3,5-diglucoside, (+)-catechin, and acetaldehyde was isolated and its properties examined. The significance of the findings are discussed in relation to color changes of red wines.