Interactions between grape anthocyanins and pyruvic acid, with effect of pH and acid concentration on anthocyanin composition and color in model solutions.

The formation of vitisin A, an anthocyanin formed naturally in small quantities in maturing port wines, was studied in model wine solutions at a range of pH values (2.0-4.5) and pyruvate concentrations [molar ratios of pyruvic acid to total anthocyanins (PA/TA) ranging from 12.20 to 172.40]. Additionally, the effect of vitisin A formation on the color changes of these model wines was evaluated. Vitisin A was formed through the interaction between malvidin 3-glucoside and pyruvic acid, and vitisin A in acylated forms, having the 6-position of the sugar acylated with acetic acid (3-acetylvitisin A) and p-coumaric acid (3-p-coumarylvitisin A), formed through the interaction between pyruvic acid and malvidin 3-acetylglucoside and malvidin 3-p-coumarylglucoside, respectively; their identities were confirmed by spectral analysis and FABMS. The maximum formation of these new anthocyanin derivatives was at pH 2. 7-3.0, at the higher pyruvic acid concentration (PA/TA of 172.40 units). The vitisins A caused changes in the color of the solution and expressed about 11 times (pH 3) to 14 times (pH 2) more color than the normal anthocyanins. On aging, the model solutions changed from a bluish red, attributable to the main anthocyanins present, to a slightly more orange red, attributable to the vitisin compounds. The aged models containing vitisins A were all much redder than the more red-brown color of the models aged without pyruvic acid.