Approach to Black Tea Pigments based on Bioorganic Chemistry of Catechin Oxidation

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Black tea accounts for almost 80 % of the world's tea production and is the most important source of polyphenol in the world. However, little has been known about the chemistry of black tea polyphenols due to their complexity. Since most of the black tea polyphenols are produced by enzymatic oxidation of green tea catechins, first, we developed new in large-scale model fermentation system using pure green tea catechins and various plant homogenates.In this research, structures of four novel oxidation products of theaflavin and two new dimers of epigallocatechin and its gallate have been elucidated. In addition, accumulation of unstable dimer quinones of epigallocatechin and its gallate during tea fermentation has been firstly demonstrated. When the fermented leaves were heated briefly, the dimer quinones were converted to theasinensins, which are another major polyphenol characteristic of black tea. Formation and degradation of theaflavins and epigallocatechin dimer quinones are major pathways in catechin oxidation during tea fermentation and understanding the chemical mechanism is important in clarifying black tea polyphenols.We also found that the dimer quinone of epigallocatechin gallate was generated on autoxidation in aqueous solution. In addition, a new oxidation product generated by oxidative cleavage of the catechin B-ring was isolated. The latter was also found in commercial black tea products. The result showed that enzymatic and non-enzymatic oxidation on catechins are closely related to each other.Furthermore, a new pigment named theafladione was isolated from commercial black tea, which is produced by oxidation of one of the precursor of theaflavin.In order to apply black tea pigment theaflavin as functional food ingredients, we devdoped a method to produce a theaflavin-rich tea extract by treatment of green tea with unripe loquat fruit. The extract showed strong inhibition activity against amylase, which related to increase of blood sugar level.