Antioxidants composition, antioxidant activities and storage stability of pigmented rice brans and their application as food ingredient in wheat bread

Abstract

One red rice and 3 black rice samples were identified and quantified their antioxidant compounds compared with normal rice in different milling fractions. Bran was the milling fraction containing major portion of rice antioxidants. On average, bran contained 7 and 62 times of total antioxidants content higher than unmillled and milled fraction, respectively. The antioxidants content in black rice was higher than in red and normal rice. The highest antioxidant content was found in black rice bran no.1. The major anthocyanins of pigmented rice were cyanidin-3-glucoside and peodinin-3-glucoside. Cyanidin-3-glucoside was the major anthocyanin found in all pigmented rice samples, the percentage of which was 90-95% (except black rice no.2; 58%). Most of phenolic acids in rice were bound phenolic. Six phenolic acids were detected in this study including gallic, protocatechuic, hydroxybenzoic, p-coumaric, ferulic, and sinapic acids. Black rice contained higher contents of protocatechuic and hydroxybenzoic acids than red and normal rice. The methanolic extracts of rice bran samples showed high antioxidant activities in all measurements which were the determination of DPPH radical scavenging activity, reducing power and lipid peroxidation inhibition. The antioxidant effects of pigmented rice are related to their phenolic pigments in the bran. Rice brans were studied for their potential as food used by adding into wheat bread. Brans from normal, red and black rice no.1 were selected to substitute with wheat flour at 5 and 10%, compared with traditional bread as a control. Addition of rice bran increased G’ (storage modulus) and G” (loss modulus) of bread doughs and also decreased the deformation of creep test. All wheat bread added rice bran (WRB) showed higher antioxidant activities than control. Rice antioxidants were retained in bread after baking especially γ-oryzanol which lost only 16-34% by baking. Cyanidin-3-glucoside was detected in black rice bran added breads and lost by 82-89% after baking. 5%WRB showed no adverse effect on baking qualities. However, addition of rice bran at 10% reduced specific volume and increased crumb firmness of final bread products. Stabilization of rice bran by using opened steam heating was able to deactivate lipase enzyme in rice bran and after 2 months of storage, stabilized rice bran contained <10% of free fatty acid. Stabilization destroyed rice bran antioxidants especially black rice bran which lost 80-100% of their anthocyanins. During storage at ambient temperature in cloth bags, α-tocopherol showed the highest degradation rate constant as compared to other antioxidants. Black rice bran retained high antiradical and antioxidant activities throughout the storage at ambient temperature.