Extraction of anthocyanins and polyphenols from black rice (Oryza sativa L.) by modeling and assessing their reversibility and stability

This study was aimed the extraction of total flavonoids, anthocyanins and phenolics, as well as the antioxidant activity of black rice (Oryza sativa) and to study the stability in relation to pH, light and copigmentation. Variations in temperature (10-50 °C), time (20-80 min), and solid-solvent ratio (1:15-1:45) were studied using a Box-Behnken design. The regression models were significant (P < 0.001) and determination coefficients ⩾0.900. Extraction at 34.7 °C for 80 min using a solid:solvent ratio of 1:30 rendered an extract with 51.26 mg 100 g−1 of flavonoids, 116.58 mg 100 g−1 of anthocyanins, 520.17 mg 100 g−1 of phenolics and 46.50% inhibition of the DPPH radical. A decrease in the color intensity was observed when pH values were changed while anthocyanins were reversible in the process of protonation/deprotonation. The addition of glucose, phytic and gallic acids in the optimized extract exposed to light displayed an intermolecular copigmentation. The main anthocyanin identified in black rice was cyanidin-3-glucoside.