Variation of Phenolics, Tocols, Antioxidant Activities, and Soluble Sugar Compositions in Red and Black Rice (Oryza sativa L.) During Boiling

Abstract

This study systematically examined hydrothermal effects of antioxidant substances, such as total phenolic (TPC), flavonoid (TFC), and proanthocyanidin (TPAC) contents, cyanidin‐3‐O‐glucoside (C3G), peonidin‐3‐O‐glucoside (P3G), α‐, γ‐, and δ‐tocopherols, and α‐, γ‐, and δ‐tocotrienols, as well as antioxidant activities, color parameters, and soluble sugar compositions in red and black rice. It showed that color differences (ΔE) of black rice were higher than those of red rice caused by boiling. The processed red and black rice exhibited significantly (P < 0.05) lower TPC, TFC, TPAC, C3G, P3G, and antioxidant activities compared with the raw rice except bound TPC and bound antioxidant activity. Interestingly, soluble free p‐coumaric and ferulic acids had higher contents in cooked red rice, and soluble free protocatechuic, vanillic, and sinapic acids had higher contents in cooked black rice. Boiling caused significant decreases of soluble conjugated phenolic acids and significant increases of insoluble bound phenolic acids in both red and black rice. Increases of total free tocol, glucose, and fructose contents were observed in most red and black rice. To increase the contents of some soluble free and insoluble bound phenolic acids, free vitamin E, and monosaccharides in red and black rice, boiled rice might be a good choice.