Oxidation of tea catechins: Chemical structures and reaction mechanism

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, in vitro model fermentation experiments using purified catechins are very useful, and recently structures of some novel oxidation products of theaflavins, black tea pigments, have been elucidated. In addition, accumulation of unstable dimer quinones of epigallocatechin and its gallate during tea fermentation has been demonstrated, and the dimer quinones are converted to theasinensins, another major polyphenol characteristic of black tea, on heating. 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.