Determination of the total phenol antioxidants content in the model mixtures using the Folin-Ciocalteu assay and the FRAP assay

Abstract

The total content of phenolic antioxidants (PhA) in tea, wine and other foodstuffs is usually determined without their separation. Active oxidizing agents, mainly Folin-Ciocalteu reagent (FC), are used as group reagents. The results obtained with the FC assay and recalculated to the standard substance vary significantly from the results obtained with the other spectrophotometric assays such as the FRAP assay. In order to elucidate the real reason of the above-mentioned discrepancies, the authors have analyzed the water solutions with the known PhA total content (10-5-10-4 mol/dm3). The correct results were obtained only for one-component solutions. For the model mixtures containing 2 or 3 components, the results of cΣ measurement with the standard FC procedure were distorted with significant systematic errors. When the universally used standard (gallic acid) has been applied, the error values (δс) were positive to the point of 50 % rel.; for the other standards the corresponding errors were much greater (by modulo). The main source of systematic errors during the PhA total content estimation was an intragroup selectivity of the signals. This effect could be related to the reaction stoichiometry features when FC reacts with the individual polyphenols. The subsidiary source of the systematic errors was the non-additivity of the light absorbance by the reaction products (reduced forms of the FC-reagent). For some mixtures, the deviations from the additivity were near 20% rel., this effect was detected for the first time. The substitution of the FC reagent with a less active oxidizing agent (FRAP reagent: Fe3+ + 2,2'-dipyridil) during cΣ determination depressed the intragroup selectivity of the signals and totally prevented the non-additivity of the light absorbance. These effects assured more correct estimates of the total PhA content (δс < 25 %). In addition, the FPAP assay is more sensitive and more specific method to determine the total polyphenols than the widely used FC assay. Therefore, authors recommend replacing the FC assay with the FRAP assay in the food industry to improve the quality control performance of tea, wines and other foodstuffs.