A model oxidation system to study oxidised phenolic compounds present in black tea

A model oxidation system was developed and optimised in order to provide a relevant tool to study oxidation products generated during the manufacture of black tea. The system utilised endogenous tea enzymes prepared by removing the phenolic compounds and caffeine from unprocessed tea leaves in such a way that the polyphenoloxidase (PPO) and peroxidase (POD) activities remained unaffected. This immobilised enzyme system was then used to oxidise the flavan-3-ols obtained from an ethyl acetate extract of green tea. The disappearance of the flavan-3-ols and the appearance of the different theaflavins were monitored by HPLC-UV. The theaflavin species detected were shown to be similar to those already described in black tea. The 1H NMR spectra of more oxidised compounds produced by the model system were compared with those observed in black tea. The 1H NMR spectra of the products generated by the model system and of the black tea extract were similar, suggesting that the washed leaf, heterogeneous catalysis system can successfully simulate the molecular changes that occur during tea production.