Method for Determining the Total Content of Phenolic Compounds in Plant Extracts with Folin–Denis Reagent and Folin–Ciocalteu Reagent: Modification and Comparison

The method for determining the total content of phenolic compounds in plant tissue extracts with the Folin–Denis reagent and the Folin–Ciocalteu reagent has been modified to establish the correspondence of results obtained when using these methods. The method with the Folin–Denis reagent has been adapted for determinations in microvolumes. For the method with the Folin–Ciocalteu reagent, concentration of the latter (0.4 N, a 5-fold dilution of the standard reagent) and composition of the reaction mixture were selected to obtain almost the same optical densities of the products of reduction of the Folin–Denis and Folin–Ciocalteu reagents by polyphenol-containing ethanol extracts from wheat, buckwheat and tea callus tissue. The absorption spectra of the products of reduction of these reagents by gallic acid, rutin, (–)-epicatechin, as well as ethanol extracts from wheat, buckwheat, and tea callus tissue, were in the same range (680–770 nm) and had similar characteristics. The calibration graphs of dependence of optical density of the solutions on the concentration of standard substances (gallic acid, (–)-epicatechin, rutin) constructed using the Folin–Denis and Folin–Ciocalteu reagents had a linear pattern within a concentration range of 10–100 μg/mL and nearly coincided. The results of determining the content of phenolic compounds in ethanol extracts of plants differing in the ability to accumulate them showed highly similar and statistically significant values when using these two reagents.