Interaction of Red Cabbage Extract with Exogenous Antioxidants

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.