| Abstract
| - It has been reported earlier that when macerated tea leaf is fermented at lower pH, the resultantblack tea contains increased levels of theaflavin, an important quality marker in black tea. In anattempt to investigate the biochemistry and chemistry underlying this observation, in vitro oxidationexperiments using polyphenol oxidase (PPO) from fresh tea leaves, horseradish peroxidase (POD),and tea catechins, precursors for theaflavins, were carried out. In vitro oxidation experiments usingcrude tea PPO resulted in higher content of theaflavins at pH 4.5 in comparison with pH 5.5, thenormal pH of the macerated tea leaf. When purified PPO was used in the in vitro system, surprisinglya reversal of this trend was observed, with more theaflavins being formed at the higher pH. Acombination of pure tea PPO and POD led to an observation similar to that with the crude enzymepreparation, suggesting a possible role for POD in the formation or degradation of theaflavin. PODwas observed to oxidize theaflavins in the presence of H2O2, leading to the formation of thearubigin,another black tea pigment. This paper demonstrates that tea PPO, while oxidizing catechins,generates H2O2. The amount of H2O2 produced is greater at pH 5.5, the optimum pH for PPO activity,than at pH 4.5. Hence, an observed increase of theaflavins in black teas fermented at pH 4.5 appearsto be due to lower turnover of formed theaflavins into thearubigins. Keywords: Camellia sinensis; theaflavins; catechins; black tea; polyphenol oxidase; peroxidase
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