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

Three methods, the Folin-Ciocalteu (F-C), Ferric Ion Reducing Antioxidant Power (FRAP), and 2,2-diphenyl-1-picrylhydrazyl Radical (DPPH center dot) 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 center dot 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 center dot 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 center dot 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.