Oxidation (tea)

When tea leaves are rolled or broken, the plant's own enzymes such as polyphenol oxidase come into contact with the catechins. With oxygen, these form theaflavins and thearubigins, which give the liquor its coppery colour and create malty or fruity notes. Green tea is not oxidised, oolong partly and black tea fully.

In the trade this step is often called "fermentation". That is technically inaccurate, as no microorganisms are involved. True microbial fermentation or post-fermentation only occurs in dark teas such as pu-erh and other hei cha.