Formation of quercetin from taxifolin and quercetin 7-O-glucoside from taxifolin 7-O-glucoside by cooking sorghum grains: Possible mechanisms of their formation

Sorghum grains contained taxifolin (dihydroquercetin) and taxifolin 7-O-glucoside (T7G). Quercetin and its 7-O-glucoside (Q7G) were formed by heating sorghum grains in distilled water. The formation of the above components was also observed by heating an extract of the grains prepared using 0.1 M sodium phosphate with 0.15 M NaCl (pH 7.0). It was shown that quercetin and Q7G could be formed from taxifolin and T7G, respectively, which were isolated from sorghum grains, by heating around pH 7. The formation of quercetin was accompanied by the formation of two taxifolin isomers, namely, taxifolin 2,3-cis isomer and alphitonin. Two isomers were also formed from T7G accompanying the formation of Q7G, and the characteristics of the isomers were similar to the taxifolin isomers. Taking the mechanism of the isomer formation into account, it is discussed that quercetin and Q7G might be formed by the oxidation of chalcones, which were formed via quinone methides of taxifolin and T7G, respectively.