2.4. DPPH Radical Scavenging Assay

Free-radical scavenging activity of the grape juice samples and the polyphenol extracts was determined following the procedure described by Brand-Williams [48] with some modifications [49]. The oxidative compound DPPH was used as a substrate, and the IC₅₀ values were calculated by expressing the concentration (mg/L) of polyphenol (or extract) that scavenges the DPPH radical by 50%. The assays were performed in 96-well plates (Nunc Delta Surface) with 200 µL of DPPH 60 µM dissolved in methanol, with variable amounts of grape juice or polyphenol extracts (0–20 µL). The mixtures were incubated for 30 min at room temperature in the dark, and the reaction was followed by measurements of the absorbance at 562 nm in a TECAN Sunrise spectrophotometer (Zurich, Switzerland). Gallic acid was included in the assay as a control. The lowest IC₅₀ values indicate the highest antioxidant capacity of the sample.