The non-volatile, water-soluble constituents of black tea were studied by using reversed-phase HPLC with a photodiode-wray detector, using a linear gradient developed for this purpose. The data generated by this detector from one injection of a black tea liquor allowed information on the nature and purity of the constituents to be obtained.
A brown thearubigin fraction was isolated from a black tea liquor, using chromatography on a column of Solka-Floc cellulose. The paper chromatograms of the fraction resembled the chromatograms of thearubigin fractions produced in the past. The substance contained no protein or caffeine, and was free of flavonol glycosides. Reversed phase HPLC showed the fraction to be a mixture of polymers, the chromatograms showing a convex broad band. Infra-red spectroscopy showed the substance to be phenolic, and lac NMR showed it was a flavanol polymer with various linkages to the proanthocyanidin polymers. The NMR spectra in this paper are the first NMR spectra of polymeric thearubigins. The brown fraction was designated the theafulvin fraction.
Abstract:The chemical oxidation of polyphenols has been used to produce black tea pigments. (+)-Catechin, (-)-epicatechin, (-)-epicatechin gallate and (-)-epigallocatechin gallate were isolated from convenient sources, their identity being confirmed by mass spectrometry and HPLC. The oxidation of a mixture of (-)-epicatechin gallate and gallic acid gave an oxidation product of (-)-epicatechin gallate in addition to epitheaflavic acid 3-gallate. The catechins were oxidised chemically and the products analysed by HPLC, the chromatograms being compared with a black tea chromatogram. The reactions gave both resolved and unresolved pigments, and many of the resolved pigments had HPLC retention times close to black tea pigments. Each catechin behaved differently, the chromatograms of the oxidation products from each starting material being clearly distinguishable. Resolved pigments were obtained by the chemical oxidation of other phenolic compounds. This work provides a convenient method for studying the formation of resolved and unresolved black tea pigments.
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