Dynamics of Antioxidant Capacity in Pigmented and Non-Pigmented Rice Varieties: Influence on Pericarp Colour Strength Before and After Milling and Cooking

Antioxidants help to mitigate oxidative stress, prevent non-communicable diseases, and promote health. Rice (Oryza sativa L), a staple food, contains antioxidants and comes in various varieties locally. This study assessed the antioxidant capacity of pigmented and non-pigmented rice varieties grown in Sri Lanka, focusing on the relationship between pericarp colour strength and antioxidant levels before and after milling and cooking. Forty rice varieties, including improved, traditional, and exotic types, were cultivated, harvested, processed, and sampled concurrently. The DPPH assay was used to assess the free radical scavenging capacity of rice at three processing stages: whole, milled, and cooked. The effective concentration (EC₅₀), defined as the concentration required to scavenge 50% of DPPH radicals, was calculated. The reciprocal of EC₅₀ was expressed as the antiradical power (ARP). Additionally, the total antioxidant capacity was measured using Ferric Reducing Antioxidant Power (FRAP) assay. Colour values for lightness, redness, and yellowness of whole, milled, and cooked rice were recorded with a CR-20 colorimeter. Both methods revealed significantly higher antioxidant capacity in pigmented whole rice (p ≤ 0.05), with traditional varieties such as Masuran and Nonabokra and improved varieties such as Ld 368 and Ld 408 demonstrating promising antioxidant levels. A significant decline in antioxidants was observed after milling and cooking (p ≤ 0.05), with a greater loss occurring from whole to milled rice than from milled to cooked rice. Cooked pigmented rice had six times more antioxidants than non-pigmented rice. Milling resulted in a substantial reduction in both antioxidant capacity and redness, while cooking caused a comparatively smaller loss (p ≤ 0.05). Overall, antioxidant activity was reduced by up to 96% as a result of milling and cooking. Although processing reduces redness and increases lightness in pigmented rice varieties, a deeper red colour does not necessarily indicate higher antioxidant levels. Positive correlations between antioxidant capacity and pericarp colour values suggest that lighter reddish-yellow rice had significantly greater antioxidant capacities.