Most nonenzymatic antioxidant activity (scavenging of free radicals, inhibition of
lipid peroxidation, etc.) is mediated by redox reactions. The antioxidant (AO) activity
of polyphenols (PPs), as ferric-reducing power, was determined for the first time
using a modified FRAP (ferric reducing/antioxidant power) assay. Reaction was followed
for 30 min, and both Fe(II) standards and samples were dissolved in the same solvent
to allow comparison. Selected representative PPs included flavonoids (quercetin, rutin,
and catechin), resveratrol, tannic acid, and phenolic acids (gallic, caffeic, and
ferulic). Carotenoids (beta-carotene and zeaxanthine), ascorbic acid, Trolox, and
BHA were included for comparison. Equivalent concentration 1 (EC(1)), as the concentration
of AO with a reducing effect equivalent to 1 mmol/L Fe(II), was used to compare AO
efficiency. PPs had lower EC(1) values, and therefore higher reducing power, than
ascorbic acid and Trolox. Tannic acid and quercetin had the highest AO capacity followed
by gallic and caffeic acids. Resveratrol showed the lowest reducing effect. Carotenoids
had no ferric reducing ability. Polyphenol's AO efficiency seemed to depend on the
extent of hydroxylation and conjugation.