Effect of esterification condensation on the Folin–Ciocalteu method for the quantitative measurement of total phenols

The Folin–Ciocalteu method is widely applied for the determination of the total phenolic contents in natural products. This method is significantly affected by the addition of sodium carbonate. The currently applied Folin–Ciocalteu methods may have been modified without any validation in the quantitative standards and the order of processes. In this study, serial experiments were performed to investigate the effect of phenolic calibrations based on the classic Folin–Ciocalteu method. Esterification condensations were observed in the assays with prior basification for gallic acid and catechin used as quantitative standards. The phenolic contents obtained in the samples differed depending on when basification occurred compared with the gallic acid calibration. The bias of the classic Folin–Ciocalteu method derived from cross-linkage of molecules was first defined in this study. The performance of the Folin–Ciocalteu method is optimised and validated again.