Abstract
The polyphenolic pigments of black tea include the theaflavins (TF), the flavonol glycosides (FG) and a group of uncharacterised brown pigments referred to as thearubigins (TR). The TR are shown to be a complex, heterogeneous group of compounds, the majority of which are unresolvable by reverse phase HPLC. Fractionation procedures have been modified, developed, combined and applied to black tea liquor, to improve the separation of black tea components and to allow fractions of less complexity be obtained. A degree of resolution has been achieved for TR fractions, unresolvable with reverse phase HPLC, using size exclusion HPLC. Calibration of the column suggested there may be little material above 2,000 daltons associated with the TR. Two heterogeneous TR fractions have been subjected to Porter's autoxidative assay for proanthocyanidins. The resulting solutions were analysed by reverse phase HPLC and shown to contain proanthocyanidins (9.4 and 16.1 %) and galloyl ester (7.1 and 7.4 %), some 73 and 84 % remains uncharacterised. A novel group of yellow pigments has been isolated for which the name theacitrins (TC) has been proposed. Data are presented that suggest the TC are possibly derived from the TF by oxidative opening of the dihydroxybenzene ring associated with the benztropolone structure. The contribution of the phenolic pigments to tea cream was investigated using reversed phase HPLC and found to be ? 86 % TR, 12 % TF and 2 % FG. Evidence is also presented for a synergistic interaction between TF and TR during cream formation. A method has been developed, using caffeine precipitation, for the isolation of TR material that is free from TF and FG Tea pigments have been stored in aqueous solutions to evaluate their potential for use as commercial colourants. The TF and TC were found to possess poor stability. The FG exhibited greater stability but possess low tinctoral power and poor water solubility. Many of the stability trials exhibited a darkening of colour associated with an increase in unresolvable TR material. Storage of an aqueous solution of a TF enriched fraction at 37 °C resulted in the production of unresolvable (TR-like) material. This material was found to possess molecular weights in excess of 2,000, indicating polymerisation. Evidence is presented to suggest the involvement of galloyl groups in the formation of non-hydrolysable bonds within this polymeric material.