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

红茶几乎占世界茶叶产量的80%，是世界上最重要的多酚来源。然而，由于其复杂性，人们对红茶多酚的化学成分知之甚少。由于大多数红茶多酚是由绿茶儿茶素的酶促氧化产生的，因此，首先，我们开发了一种新的大规模模型发酵系统，使用纯绿茶儿茶素和各种植物均质。本研究阐明了四种新的茶黄素氧化产物和两种新的表没食子儿茶素及其没食子酸酯二聚体的结构。此外，还首次证实了茶发酵过程中表没食子儿茶素及其没食子酸酯的不稳定二聚醌的积累。当发酵的茶叶被短暂加热时，二聚醌转化为茶素，这是红茶的另一个主要多酚特征。茶黄素和表没食子儿茶素二聚醌的形成和降解是茶发酵过程中儿茶素氧化的主要途径，了解其化学机制对澄清红茶多酚具有重要意义。我们还发现表没食子儿茶素没食子酸酯在水溶液中自氧化生成二聚醌。此外，还分离到了一种由儿茶素b环氧化裂解产生的新氧化产物。后者也存在于商业红茶产品中。结果表明，酶促氧化和非酶促氧化对儿茶素的作用密切相关。此外，从商品红茶中分离出一种新的色素，称为茶黄素酮，它是由茶黄素的前体之一氧化而成的。为了将红茶色素茶黄素作为功能性食品原料，研究了用生枇杷果处理绿茶生产富含茶黄素的茶提取物的方法。提取物对淀粉酶有较强的抑制作用，其抑制作用与血糖升高有关。

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.