The Mechanism of Color Changes in Aging Port Wine

Color density, total pigment color, total sulfur dioxide (SO₂), and total and free aldehyde were monitored during aging of port containing varying amounts of aldehyde and SO₂. The initial color increase in port is attributed to interactions of free aldehyde with anthocyanins and other phenolics. Free aldehyde is the residual or net concentration resulting from consumption of aldehyde in the reactions producing color, formation of aldehyde by coupled oxidation of ethanol, and liberation of free aldehyde from aldehyde-bisulfite adducts by oxidation of SO₂. In a port containing a large excess of SO₂, aldehyde content and color density (corrected for temporary bleaching by SO₂) remained unchanged, but total pigment color decreased, indicating aging by reactions not involving aldehyde. Color changes are interpreted in terms of two competitive reactions, viz aldehyde-induced reactions superimposed upon direct condensation of anthocyanins with other phenolics.