The term oxidative processing or “fermentation” refers to the enzymatic
browning reactions induced by oxidative enzymes such as
polyphenol oxidases (PPO) in the cells of tea leaves. The different
types of tea (white, yellow, green, oolong, black and pu-erh) basically
differ on the extent of the oxidative processing leading to the transformation
of flavan-3-ols in green tea (minimally processed) to theaflavins
and thearubigins in oolong (partial oxidation) and black tea (complete
oxidation). Therefore, this process is not only the most important
procedure for forming special flavor and tea pigments, such as
theaflavins and thearubigins, but also it influences the contents of the
major polyphenols in teas (Fig. 2).
The processing of pu-erh tea is significantly different when compared
to the other types of tea since it involves microbial fermentation
by Aspergillus niger. Basically, the withered leaves are under humid
conditions for a few weeks, during which the tea polyphenols are more
fully oxidized by the action of microorganisms and environmental oxygen
than black tea