Formation of epitheaflavic acid and its transformation to thearubigins during tea fermentation

The reactions of (−) epicatechin (I) and (−) epicatechin gallate (II) with gallic acid in a model tea fermentation system were studied. The primary oxidation products formed from the oxidation of (I) or (II) with gallic acid in short reaction periods were bright red condensation products named epitheaflavic acid (IX) and 3-galloyl epitheaflavic acid (X). (IX) and (X) appear to be identical to an unknown 'complex Q' reported earlier to be a trace substance in black tea extracts. Both (I) and (II) were oxidized in the model tea fermentation system, but gallic acid was not oxidized by itself. Oxidation of (I) in this system did not produce any (IX) unless gallic acid was also present. Some deesterification of (II) took place in the model tea fermentation system with the result that oxidation of (II) by this system produced some (IX) and some (X) besides other oxidation products. (IX) was not reactive in the model tea fermentation system by itself, but (IX) was rapidly transformed to thearubigins (acidic brown pigments found in black tea) when (I) was also present. These results furnish more information regarding the mechanism by which the thearubigins of black tea are formed, and they point out again the central role of tea flavanol oxidation catalysed by tea catechol oxidase in tea fermentation.