A theoretical and experimental study: the influence of different standards on the determination of total phenol content in the Folin–Ciocalteu assay

Abstract

With the aim of choosing an appropriate standard for determining total phenol content (TPC) in food extracts, a theoretical study was done to demonstrate the electronic properties of nine phenolic compounds. Besides, TPC of three different tea extracts was determined by the Folin–Ciocalteu (F–C) assay with nine phenolic compounds as the standards. The frontier molecular orbitals (FMO), molecular electrostatic potential (MEP) and ionization potential (IP) of these standards were calculated with density functional theory. Results indicated the active sites of the nine standards by FMO and MEP. Moreover, the IP value of epigallocatechin gallate was about 15% lower than that of epigallocatechin, indicating that the 3-galloy group at C ring rendered a higher reactivity in the F–C assay. TPC of green tea measured by epicatechin was about 19% lower than that of gallic acid, suggesting that epicatechin was not an appropriate standard for tea extracts. It is deduced that gallic acid is a comparably good standard among commercial standards (relative standards). However, it is recommended that researchers should not choose a universal standard for all food extracts considering the heterogeneity and diversity of polyphenols in food extracts.