Folin-Ciocalteu, FRAP, and DPPH^• Assays for Measuring Polyphenol Concentration in White Wine

Abstract

Three methods, the Folin-Ciocalteu (F-C), Ferric Ion Reducing Antioxidant Power (FRAP), and 2,2-diphenyl-1-picrylhydrazyl Radical (DPPH^•) assays were compared to determine polyphenol concentration in white wine and to compare the effects of SO₂. The aim was to determine which method gives the best indication of the concentration of polyphenols that are likely to be oxidized in wine. In the FRAP assay, Fe(III) is a stronger oxidant than in wine and sulfite has the greatest effect. The DPPH^• assay is less robust as results are greatly affected by basic and acidic solvent impurities, and thus the acidity of wine samples is sufficient to slow the rate of reaction relative to that of calibration standards. In the DPPH^• assay, augmentation produced by SO₂ developed slowly, indicating that quinones are not formed initially, unlike the FRAP assay. Though the F-C assay is least selective, giving the highest values, when SO₂ is removed, the three methods rank wines similarly with respect to polyphenol concentration. However, the FRAP assay is preferred, being more robust than the DPPH^• assay and giving a better indication of the concentration of potentially oxidizable polyphenols than the Folin-Ciocalteu method. Values obtained were in the range reported using cyclic voltammetry.