Understanding interactions between antioxidants is crucial since in biological and food matrices, we are dealing with complex multi-component antioxidant systems. This study aimed to quantitatively characterize interactions of antioxidants in anthocyanin-rich aqueous red cabbage extract with several natural (ascorbic acid, gallic acid, and glutathione) and synthetic (Trolox and TEMPOL) antioxidants and to determine their synergistic or antagonistic nature in the ORAC assay. Four parameters derivable from the ORAC assay (extent of fluorescence protection, lag time, fluorescence half-life t_1/2, and maximum rate of fluorescence decay) were analyzed in terms of the integrated interaction coefficient (IIC), reflecting the dependence of the analyzed values on the concentration of antioxidants and the sample interaction coefficient (SIC) derived from interaction at a single set of concentrations. IIC analysis revealed synergistic interactions of Trolox with the red cabbage extract on the basis of fluorescence protection, lag time, and t_1/2. Interactions of TEMPOL with the extract were antagonistic as assessed based on all parameters but the lag time. A correlation between the anodic peak and the lag time and t_1/2 values was observed for the antioxidants studied. The interactions between antioxidants in complex mixtures are important as they affect the measured total antioxidant activity, which, depending on the nature of the interactions, may be lower or higher than the sum of activities of individual components.

Interactions between antioxidants are of interest, mainly for understanding their action in complex biological and food systems. This study aimed to evaluate interactions between the anthocyanin-rich aqueous red cabbage extract and several natural (ascorbic acid, gallic acid, and glutathione) and synthetic (Trolox and TEMPOL) antioxidants as a function of reaction time, concentration, and extract/antioxidant ratio in the ABTS^• decolorization and FRAP antioxidant activity assays. The measured interaction type showed dependence on assay type, reaction time, and the extract/antioxidant ratio, but no general dependence on the concentrations of the extract and the antioxidants over a 6-fold concentration range. In the ABTS^• decolorization assay, the interactions between the red cabbage extract and exogenous antioxidants were additive (Trolox), weakly antagonistic (ascorbic acid, gallic acid, and glutathione), and definitely antagonistic for TEMPOL. In the FRAP assay, the interactions were additive (ascorbic acid and Trolox), weakly antagonistic (gallic acid and TEMPOL), and definitely synergistic for glutathione. These results suggest the need for a series of assays covering a range of conditions to demonstrate a deviation from additivity in the interactions between antioxidants. The synergy of the interaction of glutathione with the extract in the FRAP assay indicates the possibility of a higher reactivity of this compound in Fe³⁺ reduction in complex systems than in an assay of the pure compound.

The generation of hydrogen peroxide has been documented in various plant-based beverages, such as coffee, tea, herbal infusions and wine, as well in energy drinks containing ascorbate and in plant-based food. There are no data in the literature on the presence and generation of hydrogen peroxide in beer and strong alcoholic beverages containing plant material. This study aimed to examine whether beer and selected strong alcoholic beverages (brandy, whisky and fruit liqueurs) contain hydrogen peroxide. The presence of hydrogen peroxide was found in freshly opened brandy, whisky, liqueurs and most diluted beers; subsequent incubation in an air atmosphere led to the generation of hydrogen peroxide. The presence of the electron paramagnetic resonance (EPR) signal of the semiquinone radical and the generation of the superoxide radical demonstrated in selected alcoholic beverages by the superoxide dismutase-inhibitable reduction of Nitrotetrazolium Blue and oxidation of dihydroethidium are in agreement with the two-step mechanism of generation of hydrogen peroxide by the autoxidation of phenolics. These results broaden the list of beverages containing and producing hydrogen peroxide.