To assess the quality of natural antioxidants used as food additives, it is important to establish method for evaluating antioxidant activity. In this study, the DPPH and ABTS radical scavenging methods were evaluated. The antioxidant activities of 21 reference antioxidants, including flavonoids, polyphenols, vitamins, amino acids, and peptides, were determined by these methods. In both assays, a catechol and a pyrogallol moiety contributed to antioxidant activity. In addition, a 3-hydroxyl group in the C-ring of the flavonoids and the gallic acid ester moiety of the catechins were also important for activity. The antioxidant activities determined by DPPH and ABTS were similar with the exception of catechol compounds, for which values were～1.3 μmol TE/μmol higher in the DPPH assay than in the ABTS assay. We conclude that DPPH reflects reactivation of the catechol moiety of the antioxidants. A correlation study of DPPH and ABTS with the FRAP method showed that the DPPH method reflected ferric-reducing ability more precisely than did the ABTS assay. We conclude that the DPPH assay would be suitable as a standard method for evaluating natural antioxidants as food additives.
