Effect of pyruvic acid and ascorbic acid on stability of 3-deoxyanthocyanidins

BACKGROUND: 3-Deoxyanthocyanins are a rare class of plant pigments primarily found in sorghum and known to possess distinct chemical and bioactive properties. They have potential as natural food colourants. This work investigates how 3-deoxyanthocyanin pigments react with pyruvic acid and their stability in the presence of ascorbic acid. RESULTS: Substitution at C-5 was the major determinant of the mechanism of 3-deoxyanthocyanidin complexation with pyruvic acid: hydroxyl substitution favoured cyclic condensation, whereas methoxyl substitution favoured oxidative ring contraction. Pure 3-deoxyanthocyanidins generally showed poor stability in the presence of 500 mg L-1 ascorbic acid at pH 2.0 and 3.2 but were stable at pH 5.0; pyruvic acid improved their stability at pH 2.0 and 3.2 and enhanced their colour intensity at pH 5.0. Crude sorghum pigment extract was very stable in the presence of ascorbic acid: 31% colour loss at pH 2.0; 1.9- and 1.3-fold increases in colour intensity at pH 3.2 and 5.0 respectively. In contrast, red cabbage pigment lost 30-85% of its colour in the presence of ascorbic acid under the conditions used in the study. CONCLUSION: Crude sorghum pigments are very stable in the presence of ascorbic acid and may be useful as natural food colourants. (C) 2008 Society of Chemical Industry.