Effects of roasting conditions on anthocyanin, total phenolic content, and antioxidant capacity in pigmented and non-pigmented rice varieties

Roasting has been suggested to enhance the nutritional quality of many foods. The present work determined how roasting temperature (100, 150, and 200°C) and duration (10 and 20 min) affect anthocyanin, total phenolic contents, and antioxidant capacity in pigmented and non-pigmented rice. The concentration of anthocyanin in pigmented rice increased by 15% when roasted at 100°C for 20 min, but it dramatically decreased at higher temperature for longer time. The anthocyanin analysis profile showed that cyanidin-3-glucoside and peonidin-3-glucoside were found as the major compounds of pigmented rice, in which the first compound was about 14 folds higher than the latter, in both non-roasted and roasted rice. The concentration of cyanidin-3-glucoside slightly increased in the roasting rice but not for peonidin-3-glucoside. The total phenol concentration and antioxidant capacity in pigmented rice were not affected by the roasting treatments. In non-pigmented rice, total phenol concentrations and DPPH activity increased with increasing roasting temperature and duration. There were significant correlations between anthocyanin and total phenol concentrations in pigmented rice, and between total phenol concentration and antioxidant capacity determined by DPPH activity in non-pigmented rice. The optimum roasting temperature and time for maximising anthocyanin and total phenol was 100°C for 20 min for pigmented rice, and 200°C for 20 min for non-pigmented rice, as these treatments yielded the highest total phenol concentration and DPPH activity. The present work reveals that anthocyanin and total phenol concentrations of rice grains can be changed by roasting, but the direction and magnitude of the changes depend on the temperature and duration of the treatment as well as the rice variety.