Validation of a quantitative assay for the total content of lipophilic and hydrophilic antioxidants in foods

One of the main methods used to assess total antioxidant content in foods is the Ferric Reducing Ability of Plasma (FRAP) assay. The FRAP assay has previously not been extensively validated. In the present study, 39 pure compounds, such as different polyphenols, ascorbic acid, tocopherols, tocotrienols and carotenoids dissolved in water/methanol, water/2-propanol and 2-propanol, were tested. Our results demonstrate that the FRAP assay can quantitate satisfactorily most hydrophilic and lipophilic compounds with antioxidant properties. The reaction was followed for 60 min. The most extensive reaction occurred within the first 4 min for most compounds and foods. The lipophilic tocopherols and tocotrienols were easily quantitated and reached endpoint within 4 min while carotenoids were somewhat more demanding due to low solubility and slower reaction kinetics. We conclude that the FRAP assay, with 4 min reaction time, is suitable for high-throughput screening of total antioxidant content of edible items.