Expression of Antiradical Power of Pigmented and Non-pigmented Rice (Oryza sativa L.) Varieties in Relation to their Colour Strengths Before and After Polishing

Abstract

Natural antioxidants have potential to counterbalance the effect of anti-oxidative stress and the deleterious effect on human health caused due to the synthetic antioxidants. Rice is one of the rich sources for antioxidants. Therefore to explore the total natural antioxidants in de-husked (un-polished rice), polished and cooked rice of forty which included 20 pigmented (red) and 20 non-pigmented (white) rice varieties were selected. These varieties were represented by new improved, traditional and exotic rice. Eighty percent Methanolic extract of these varieties were analyzed for total antioxidant activity which was expressed as anti-radical power (ARP) the reciprocal of EC50 value. The reducing power of 2,2-diphenyl-1-picrylhydrazyl (DPPH) was used to identify the efficient concentration (EC50) required to decrease the initial DPPH concentration by 50%. Brown rice and milled rice pericarp colour were measured by CR-20 spectrophotometric method to determine lightness, redness, and yellowness. Antiradical power of forty rice varieties was found to be highly diverse. Pigmented rice of un-polished, polished and cooked contained more antioxidants than that of non-pigmented rice. Mean ARP values for pigmented un-polished, non-pigmented un-polished, pigmented polished, non-pigmented polished, pigmented cooked and non-pigmented cooked rice were 20.7±1.77, 1.8±0.10, 10.3±0.92, 1.4±0.08, 1.4±0.07and 0.2±0.02 mL/mg, respectively. Due to presence of colour pigments in red rice varieties, they have shown comparatively higher ARP values. Two of the traditional and improved rice varieties namely Masuran and Nonabokra and Ld 368 and Ld 408 recorded the highest antioxidant contents with an ARP value more than 27 mL/mg. A drastic decline (average 35 and 88 %) of antioxidant contents were observed after milling and cooking irrespective of the presence of colour pigments in the outermost layer of the rice grain. Milling followed by cooking reduces the antioxidant content to minimum of 24 % and maximum of 96 %. However, on the average, pigmented cooked rice contain 6 times more antioxidants than non- pigmented rice. Colour values (lightness, redness and yellowness) of these varieties were significantly different among un-polished as well as polished rice. There was a significant decline in redness of pigmented varieties after polishing process. The mean redness values ranged from 3.5 to 15.2 and from 2.1 to 11.6 in un-polished and polished rice, respectively. Mean redness values of pigmented rice varieties varied between 9.0 and 14.3. Therefore, comparatively higher antioxidant capacity was observed with red pigmentation of rice grain which is reduced by polishing and further reduced by cooking.