Introduction: A common challenge in estimating the antioxidant activity (AA) of various compounds or the total antioxidant capacity (TAC) of complex materials such as food products is the differences between results obtained from different assays. Moreover, such assays do not provide information on the composition of antioxidants present in a sample. As the thermodynamic condition for a redox reaction is that the redox potential of the oxidant must be higher than that of a reductant (antioxidant), it is of interest to check whether it is possible to estimate the content of antioxidants in various ranges of redox potentials using a set of assays employing oxidants/indicators of different values of redox potentials.

Methods: In the study, the AA of eight antioxidants (Trolox, ascorbic acid, glutathione, gallic acid, allicin, and three nitroxides) and the TAC of an aqueous garlic extract were estimated using Fe(III)phenanthroline reduction, ORAC, FRAP, ABTS^• decolorization, CUPRAC, DPPH^• decolorization, ferricyanide reduction, and 2,6-dichlorophenolindophenol (DCIP) reduction assays (E_o’ of 1.14, about 1, 0.70, 0.68, 0.59, 0.53, 0.36, and 0.23 V, respectively). The antioxidant activities were related to Trolox and expressed in Trolox equivalents (TEs).

Results: The thermodynamic condition made some antioxidants unreactive with indicators of sufficiently low E_o’, but otherwise, no dependence between the AA and redox potentials of oxidants/indicators and antioxidants was observed. Some antioxidants exhibited low reactivity in certain assays (e.g., glutathione in assays based on Fe(III) reduction). The TAC of the garlic extract did not show any regular dependence on the redox potential of the oxidant/indicator, being the highest in the test of ABTS^• decolorization assay (1.06±0.16 mol TE/L) and the lowest for the Fe(III)phenanthroline reduction assay (0.11±0.02 mol TE/L).

Conclusions: These results indicate that kinetic factors play a primary role in determining the AA and TAC in various assays.