Antioxidant capacity of phenolic compounds in acidic medium: A pyrogallol red-based ORAC (oxygen radical absorbance capacity) assay

•Novel PGR-based ORAC assay in acidic media to assess antioxidant capacity presented.•The index is associated with the reactivity of phenols toward peroxyl radicals.•The method was applied to assess antioxidant capacity of beverages in acid medium.•The ORAC-like assay could be used to determine ascorbic acid in complex samples.

A novel ORAC (oxygen radical absorbance capacity) assay to assess antioxidant capacity of phenolic compounds in near-gastric conditions (pH 2.0) is presented. AAPH (2,2′-azo-bis(2-amidinopropane)dihydrochloride) was used as peroxyl radicals source, and fluorescein, pyranine and pyrogallol red were employed as target molecules. Only pyrogallol red (PGR) showed a behavior compatible with an ORAC assay under acidic conditions (ORAC-PGRa). Excepting Trolox and ascorbic acid, phenolic compounds protected PGR, giving kinetic profiles without the presence of an induction time. ORAC-PGRa values, which reflect the reactivity of the antioxidants toward peroxyl radicals, ranged from 0.2 (caffeic acid) to 29.1 (myricetin) gallic acid equivalents. The ORAC-PGRa method showed analytical parameters in agreement with other ORAC-like assays and was applied to wines, teas, commercial juices and herb infusions, peach juice being the sample with the highest ORAC-PGRa value (7.1 mM gallic acid equivalents). In addition, ascorbic acid concentration in complex mixtures can be determined from kinetic profiles.