Tea that has undergone microbial fermentation
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, which is actually referred to as "red tea" ("hong cha", 红茶) in Chinese. 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.
Other 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:
Shapes include:
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 B12, 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.
Korea.
 (; lit. "cake tea"), also called (; ; lit. "cake tea"), was the most commonly produced and consumed type of tea in pre-modern Korea. Pressed tea made into the shape of , the coins with holes, was called (; lit. "money tea"), (; ; lit. "money tea"), or (; ; lit. "green moss coin"). "Borim-cha" (; ) or (; ), named after its birthplace, the Borim temple in Jangheung, South Jeolla Province, is a popular variety.
Edible pickled tea.
Though the early history of tea is unclear, it has been established that for centuries people have chewed tea leaves. Few peoples today continue to consume tea by chewing or eating. 
In Northern Thailand, a pickled tea product called miang (เมี่ยง) is chewed as a stimulant. Steamed tea leaves are kept pressed into sealed bamboo baskets until the anaerobic fermentation produces a compact cake with the desired flavor. The fermentation takes four to seven days for young leaves and about a year for mature leaves. Miang is related to the Thai and Lao street snack miang kham.
Pickled tea known as lahpet is widely consumed in Burmese cuisine, and plays an important role in Burmese ritual culture. After fermentation, the tea is eaten as a vegetable.
A similar pickled tea is eaten by the Blang people of Xishuangbanna in Yunnan, China, on the border with Myanmar and Laos. The tea, known locally as miam and in Chinese as (), is first packed into bamboo tubes, then buried and allowed to ferment before eating.
Production.
During fermentation, the yeast in the mixture converts sugar to alcohol. But then, bacteria take over and convert that alcohol into acetic acid (vinegar) which is what gives kombucha its tangy taste. However, some residual alcohol can be left behind. In commercially produced kombucha, the amount is very low, typically less than 0.5% alcohol by volume - which means it can be labelled as non-alcoholic. But homemade kombucha, or kombucha that's fermented for a longer time, can have slightly higher traces of alcohol.
Many fermented teas do not arrive on the market ready for consumption. Instead, they may start as green teas or partially oxidized oolong-like teas, which are then allowed to slowly oxidize and undergo microbial fermentation over many years (comparable to wines that are sold to be aged in a cellar). Alternatively, fermented teas can be created quickly through a ripening process spanning several months, as with Shu Pu'er. This ripening is done through a controlled process similar to composting, where the moisture and temperature of the tea are carefully monitored. The product is "finished" fermented tea.
Fermented teas are commonly sold as compressed tea of various shapes, including bricks, discs, bowls, or mushrooms. Ripened teas are ripened while loose, then compressed. Fermented teas can be aged for many years to improve their flavor, again comparable to wines. Raw tea can be aged up to 50 years in some cases without diminishing in quality, and ripened can be aged up to 10 or 15 years. Experts and aficionados disagree about the optimal age.
Many Tibetans and Central Asians use or other fermented teas as a caloric and micronutrient food, boiled with yak butter, sugar and salt to make yak butter tea.
Ageing and storage.
Post-fermented tea usually gets more valuable with age. Dark tea is often aged in bamboo baskets, bamboo-leaf coverings, or in its original packaging.
Many varieties of dark tea are purposely aged in humid environments to promote the growth of certain fungi, often called "golden flowers" or () because of the bright yellow color.
References.
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