Effect of cultivars and milling degrees on free and bound phenolic profiles and antioxidant activity of black rice

Six black rice cultivars (Heukjinju, Sintoheugmi, Heukhyangchal 1, Bosukheukchal, Sinnongheukchal, and Josengheukchal) and varying milling degrees (step 0, 0%; step 1, 4.2%; and step 2, 10.5%, w/w) were used to evaluate the effects of cultivars and milling degrees of black rice (Oryza sativa L.) on the total phenolic contents (TPC), total flavonoid contents (TFC), antioxidant activity (2,2-diphenyl-1-picrylhydrazyl free radical assay), and phenolic composition in free and bound phenolic fractions. Unpolished (step 0) Sintoheugmi showed significantly higher TPC, TFC, antioxidant activity, phenolic acid levels, and anthocyanin levels than other unpolished cultivars (p < 0.05). As milling degree increased, TPC, TFC, antioxidant activity, phenolic acid levels, and anthocyanin levels decreased significantly (p < 0.05). TPC, TFC, and antioxidant activity were significantly higher in free phenolic fractions than bound phenolic fractions of black rice extracts, regardless of cultivars (p < 0.05). The major phenolic acid was ferulic acid, and the major anthocyanin found in free phenolic fractions in black rice samples was cyanidin-3-O-glucoside. The sum of individual phenolic acid levels ($255.2{\pm}0.0{\mu}g/g$) and the sum of anthocyanins levels ($831.4{\pm}0.3{\mu}g/g$) were significantly higher in Sintoheugmi black rice than in the other cultivars for step 0 (unpolished rice) (p < 0.05). For step 1 and step 2, Heukjinju black rice contained significantly higher sum of phenolic acid levels and sum of anthocyanin levels than the other cultivars (p < 0.05). For use as a better functional ingredient, it is, therefore, important to consider different milling degrees together with different black rice cultivars having the highest antioxidant component.