*2.6. Antioxidant Activity Assays*

The capacity to scavenge the DPPH, monitored according to the method described by Martins et al. [15,17] was performed by using a SPECTRO star Nano microplate reader (BMG Labtech, Offenburg, Germany). The results were expressed as Trolox equivalents (TE) per gram of dry extract (mg TE/g of dry extract).

The radical scavenging activity of the beech bark extracts against ABTS was measured according to Mocan et al. [18]. The results were expressed as milligrams of TE per gram of dry extract (mg TE/g dry extract).

In FRAP assay, the reduction of Fe3<sup>+</sup>-TPTZ to blue-colored Fe2<sup>+</sup>-TPTZ complex was monitored [19]. Briefly, the FRAP reagent was prepared by mixing ten volumes of acetate buffer (300 mM, pH 3.6), one volume of TPTZ solution (10 mM TPTZ in 40 mM HCl) and one volume of FeCl3 solution (20 mM FeCl3·6H2O in 40 mM HCl). The reaction mixture (25 μL sample and 175 μL FRAP reagent) was incubated for 30 min at room temperature (in the dark) and the absorbance was measured at 593 nm (SPECTROstar Nano Multi-Detection Microplate Reader with 96-well plates, BMG Labtech, Ortenberg, Germany). A TroloxTM calibration curve (0.01–0.10 mg/mL) was plotted, and the results were expressed as milligrams of TE per milligram of dry extract (mg TE/mg dry extract).
