Color properties of four cyanidin-pyruvic acid adducts

Four anthocyanin-pyruvic adducts were synthesized through the reaction of cyanidin 3-O-glucoside, cyanidin 3-O-rutinoside, cyanidin 3-O-sophoroside, and cyanidin 3-O-sambubioside with pyruvic acid, structurally characterized by liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR), and their chromatic properties were studied (pH and SO2 stability assays). Overall, these pigments were shown to have a higher resistance to discoloration toward pH variations and also in the presence of SO2, being that this resistance to discoloration was explained by a higher protection of the chromophore group against the water or bisulfite nucleophilic attack that gives rise to the colorless hemiacetal form. Only slight differences in the protection against the nucleophilic attack of water and bisulfite were found to occur between all of the cyanidin-pyruvic acid adducts studied. Indeed, anthocyanin-pyruvic acid adducts with glucose or sambubiose attached to the 3-O position of the flavylium moiety were shown to have smaller bleaching constants compared with similar pigments that possess a rutinosyl or sophorosyl moiety. The study of the pigments (A-D and cyanidin-3-O-glucoside) color parameters, namely, chroma (C), lightness (L), and the hue angle (h(a,b)), obtained from the CIELAB system, revealed that different patterns of sugars in the anthocyanin-pyruvic acid adduct moiety affected the referred three parameters of color. The loss of saturation (DeltaC < 0) and the increase of lightness (DeltaL > 0) presented by the cyanidin-pyruvic acid adduct solutions at acidic pH values (1.0 and 2.0) showed that they are much less colored than the cyanidin-3-O-glucoside. For higher pH values (5.0 and 7.0), the reverse trend was observed. This means that the cyanidin-pyruvic acid adducts A-D are much more colored than the anthocyanin at these pH values. The higher coloring capacity of these pigments at higher pH values may be an important feature, indicating a putative application of these compounds in food products.