Use of Porter's reagents for the characterisation of thearubigins and other non‐proanthocyanidins

Abstract

Abstract A previously published method for the HPLC analysis of the anthocyanidin pigments produced by the autoxidative depolymerization of proanthocyanidins by heating in a solution of n ‐butanol/HCl (95/5 v/v) and iron(III) salts (Porter's reagents) has been refined. The pigments are detected by monitoring at 530 nm, but by monitoring the eluate also at 280 nm and 380 nm a wider variety of products resulting from the same autoxidation and/or hydrolysis/transesterification reactions can be detected. Such products have been shown to include aglycones (particularly flavonols from flavonol glycosides) and n ‐butyl gallate (from galloyl esters). The advantages of monitoring the wider range of transformation products are illustrated by applying the method to the characterisation of two heterogeneous thearubigin fractions isolated from black tea liquor. The caffeine‐precipitatable thearubigin fraction contained 181 g proanthocyanidins kg −1 (as procyanidin dimer equivalents), a trace (∼2 g kg −1 ) of flavonols (as aglycones), 74 g kg −1 galloyl ester and 5·6 g kg −1 elemental N (determined by microanalysis): caffeine was not detectable and some 74% of the fraction remains uncharacterised. The theafulvin fraction contained 105 g proanthocyanidins kg −1 (as procyanidin dimer equivalents), < 1 g kg −1 quercetin and 71 g kg −1 galloyl ester, but neither caffeine nor elemental N was detectable; some 82% of the fraction remains uncharacterised. The proanthocyanidins of both hump fractions contained not only prodelphinidins and procyanidins but also propelargonidins which have not previously been reported in tea.