Role of polyphenol oxidase and peroxidase in the generation of black tea theaflavins

It has been reported earlier that when macerated tea leaf is fermented at l ower pH, the resultant black tea contains increased levels of theaflavin, a n important quality marker in black tea. In an attempt to investigate the b iochemistry and chemistry underlying this observation, in vitro oxidation e xperiments using polyphenol oxidase (PPO) from fresh tea leaves, horseradis h peroxidase (POD), and tea catechins, precursors for theaflavins, were car ried out. In vitro oxidation experiments using crude tea PPO resulted in hi gher content of theaflavins at pH 4.5 in comparison with pH 5.5, the normal pH of the macerated tea leaf. When purified PPO was used in the in vitro s ystem, surprisingly a reversal of this trend was observed, with more theafl avins being formed at the higher pH. A combination of pure tea PPO and POD led to an observation similar to that with the crude enzyme preparation, su ggesting a possible role for POD in,the formation or degradation of theafla vin. POD was observed to oxidize theaflavins in the presence of H2O2, leadi ng to the formation of thearubigin, another black tea pigment. This paper d emonstrates that tea PPO, while oxidizing catechins, generates H2O2. The am ount 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 appears to be due to lower turnover of formed theaflavi ns into thearubigins.