Ferric reducing ability of plasma (FRAP) assay
The FRAP assay is a method of measuring the ability of reductants (antioxidants) to reduce Fe3+–Fe2+. The formation of blue-coloured Fe2+-TPTZ complex (Fe2+ tripyridyltriazine) increases the absorbance at 593 nm. The method of Kubola and Sirithon (2008) was used with some modifications. The stock solutions included 300 mM acetate buffer, pH 3.6, 10 mM TPTZ solution in 40 mM HCl, and 20 mM FeCl3⋅6H2O solution. The fresh working solution was warmed at 37 °C before using. Rice extracts (300 μl) were allowed to react with 1.7 ml of the FRAP solution. The absorbance at 593 nm of the mixture was measured after 60 min of reaction. The results were calculated by standard curves prepared with known concentrations of FeSO4, and were expressed as μmol FeSO4/g fresh weight.