Fermented tea (also known as post-fermented tea or dark tea) 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 microbially catalysed). 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 dark tea, not be confused with black tea. The most famous fermented tea is 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 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.
History
The early history of dark tea is unclear, but there are several legends and some credible theories.
For example, one legend holds that dark tea was first produced accidentally, on the Silk Road and Tea Road by tea caravans in the rainy season. When the tea was soaked in rain, the tea transporters abandoned it for fear of contamination. The next year, nearby villages suffered from dysentery, and decided to drink the abandoned mildewed tea in desperation. The legend concludes that the tea cured those suffering, and quickly became popular.
More historical accounts attribute the first production of dark tea to the Ming dynasty in the 15th and 16th centuries. It may have been first traded by tea merchants much earlier than the legends state, across the historical borders of Han and Tibetan cultural areas.
Varieties
Fermented teas can be divided according to how they are produced. Piled teas, such as the Chinese post-fermented teas, and the Toyama produced in Japan, are fermented with naturally occurring fungus under relatively dry conditions. Other fermented teas, called pickled teas, are fermented in a wet process with lactic acid bacteria. Pickled teas include from Thailand and from Japan. A third category, including the Japanese and Ishizuchi , is fermented with the piled and pickling methods successively.
China
Fermented tea originated in China, where it is commonly known as () or dark tea. is produced in many areas of China, mostly in the warmer southern provinces. It is commonly pressed into bricks or cakes for ageing.
The most famous and important producing areas and varieties include: Anhui: (, Anhui Lu'an basket tea) Guangxi: (, Guangxi Liubao tea, often sold as , loose dark tea) Hubei: (, Hubei green brick tea) Hunan: ( (), the famous “brick tea”) Jingyang, Shaanxi: ( (), the famous “brick tea”) Sichuan: (, Sichuan border tea) Tibet: (, Tibetan tea, often called Tibetan brick tea) Yunnan: (, either "raw" or "ripened" )
Shapes include: Bamboo leaf logs Cakes, or () Bricks, or () Loose, in baskets Bird nests, or (), usually Squares, or ()
Japan
Several distinct varieties of fermented tea are produced in Japan. Toyama prefecture's is Japan's only piled tea, similar to the Chinese post-fermented teas. Toyama is traditionally prepared by boiling in water, adding salt and stirring with a whisk as in a traditional tea ceremony. It is consumed on religious occasions or during meetings in the Asahi area of the prefecture. (), produced in Tokushima prefecture, and , like the Toyama associated with Asahi, Toyama, are made from , or second flush tea leaves, with bacterial fermentation. has been found to contain vitamin B, but in insignificant amounts for human diets. () from Ōtoyo, Kōchi and Ishizuchi grown at the foot of Mount Ishizuchi in Ehime prefecture are made by fermenting the tea in a two step process, first with aerobic fungi, then with anaerobic bacteria.