Modified cerium(IV)-based antioxidant capacity (CERAC) assay with selectivity over citric acid and simple sugars

A Ce(IV)-based reducing capacity (CERAC) assay was developed to measure the total antioxidant capacity (TAC) of foods, in which Ce(IV) would selectively oxidize antioxidant compounds but not citric acid and reducing sugars. The redox potential of the Ce(IV) oxidant was fine-tuned in 0.3 M H2SO4 + 0.7 M Na2SO4 aqueous medium for selective oxidation. In the classical Ce(IV)-based assay for which the name CERAC was proposed, the presence of citric acid (at 1.5 × 10−5 M) caused approximately 25% reduction in Ce(IV) (at 2.0 × 10−4 M) recovery, whereas in the present method, the presence of citric acid (at 1.0 × 10−4 M) caused negligible error in the TAC measurement of quercetin. The trolox equivalent antioxidant capacity (TEAC) values in the order of quercetin > rutin > gallic acid > catechin > caffeic acid ≥ ferulic acid > naringenin ≥ naringin > trolox ≥ ascorbic acid were established with the proposed method and were found to be compatible with those found with other antioxidant assays. It is noteworthy that naringin and rutin were also hydrolyzed in the acidic medium of the method so as to exert their full antioxidant capacity not measured by other TAC assays. The proposed TAC assay with Ce(IV) is simple, low-cost, rapid, and can be easily applied in modestly equipped conventional laboratories.