Novel oxidation products of cyanidin 3-O-glucoside with 2,2'-azobis-(2,4-dimethyl)valeronitrile and evaluation of anthocyanin content and its oxidation in black rice.

The radical oxidation mechanism of anthocyanin derivatives was investigated by the reaction of cyanidin 3-O-glucoside in the presence of radical initiator 2,2'-azobis-(2,4-dimethyl)valeronitrile (AMVN) in EtOH and aqueous CH3CN. Six different oxidation products were isolated, depending on the solvent employed. These products were identified using NMR spectroscopy and multistep derivatisation reactions. Of the products obtained, two novel oxidised anthocyanin derivatives were isolated from black rice under prolonged storage. A radical reaction mechanism is proposed on the basis of these reaction products. Quantification of oxidised anthocyanins in black rice is demonstrated as a method to verify freshness of the rice.