*3.4. Assay of the Antioxidant Activity*

The antioxidant activity of optimized beech bark extracts was evaluated by DPPH, FRAP and TEAC assays. The beech bark extracts exhibited scavenging activity against all radicals as shown in Table 5. The strongest antioxidant activity is recorded for BBE2, where the strongest amount of phenols was recorded. Many bark extracts have been evaluated for their antioxidant capacities, commonly associated to their content of phenolic compounds [28–30]. Beech bark contains some of biologically active components, such as catechin, epicatechin, syringic acid, vanillic acid etc., indicating that the antioxidant activity of the extracts can be at least partially ascribed to these bio-components (Table 4). Grzesik et al. [31] studied the antioxidant properties of five catechins, compared with other natural or synthetic compounds. They concluded that catechins showed the strongest ABTS scavenging capacity and the strongest stoichiometry of Fe3<sup>+</sup> reduction in the FRAP assay [31]. Besides the direct antioxidant properties of catechins, they may show synergistic interaction with endogenous antioxidants and act as indirect antioxidants as well [32,33].

**Table 5.** Antioxidant activity of optimized beech bark extracts.


TPC—total phenolic content, DPPH—2,2-diphenyl-1-picrylhydrazyl, TEAC—Trolox equivalent antioxidant capacity, FRAP—ferric reducing ability of plasma, BBE1- extracts obtained with water 100%, BBE2—extracts obtained with ethanol–water 50:50, BBE3—extracts obtained with ethanol–water 80:20 ± Standard deviation.
