Fermented tea

Fermented tea (also known as post-fermented tea, and literally translated as black tea from Chinese) is a class of tea that has undergone microbial fermentation, from several months to many years. The exposure of the tea leaves to humidity and oxygen during the process also causes endo-oxidation (derived from the tea-leaf enzymes themselves) and exo-oxidation (which is microbiallycatalysed). The tea leaves and the liquor made from them become darker with oxidation. Thus, the various kinds of fermented teas produced across China are also referred to as black or dark tea, not be confused with black tea in English, which is actually referred to as "red tea" (hong cha, ) in Chinese. The most famous fermented tea is pu'er produced in Yunnan province.

The fermentation of tea leaves alters their chemistry, affecting the organoleptic qualities of the tea made from them. Fermentation affects the smell of the tea and typically mellows its taste, reducing astringency and bitterness while improving mouthfeel and aftertaste. The microbes may also produce metabolites with health benefits. Additionally, substances like ethyl carbamate (urethane) may be produced.

The fermentation is carried out primarily by molds. Aspergillus niger was implicated as the main microbial organism in the pu'er process, but that species identification has been challenged by comprehensive PCR-DGGE analysis, which points to Aspergillus luchuensis as the primary agent of fermentation.

Most varieties of fermented teas are produced in China, its country of origin, with several varieties also produced in Korea and Japan. In Myanmar, lahpet is a form of fermented tea that is eaten as a vegetable, and similar pickled teas are also eaten or chewed in northern Thailand and southern Yunnan.