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 Pu-erh, produced in Yunnan Province, and the Anhua dark tea produced in Anhua County of Hunan 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.
The fermentation is carried out primarily by molds. Aspergillus niger was implicated as the main microbial organism in the Pu-erh 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 fermented teas are made in China, but several varieties are produced in Japan. In Shan State, Myanmar, lahpet is a form of fermented tea that is eaten, and similar pickled teas are also eaten in northern Thailand and southern Yunnan.
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 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, in areas on the borders of China and Tibet.
Fermented teas can be divided according to how they are produced. Piled teas, such as the Chinese post-fermented teas, and the Toyama kurocha 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 miang from Thailand and awabancha from Japan. A third category, including the Japanese Goishicha and Ishizuchi-kurocha, is fermented with the piled and pickling methods successively.
Fermented tea originates in China, where it is commonly known as hei cha (黑茶) or dark tea. 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:Yunnan: Pu-erh cha (雲南普洱茶, either "raw" Sheng Pu erh 生普洱 or "ripened" Shu Pu erh 熟普洱)
Hunan: Fu Zhuan cha (湖南茯磚茶 (黑茶), the famous Hua Zhuan 花砖 “flower brick”)
Guangxi: Liu Bao cha (廣西六堡茶, often sold as 松黑茶, loose dark tea)
Anhui: Liu An cha (安徽六安籃茶, Lu An basket tea )
Sichuan: Lu Bian cha (四川路边茶, Sichuan border tea)
Hubei: Qing Zhuan cha (湖北青砖茶, Hubei green brick tea)
Shapes include:Bamboo leaf logs
Cakes, or "bing cha" (餅茶)
Bricks, or "zhuan cha" (磚茶)
Loose, in baskets
Bird nests, or "tuo cha" (沱茶), usually Pu-erh
Squares, or "fang cha" (方茶)
Several distinct varieties of fermented tea are produced in Japan. Toyama prefecture's Kurocha is Japan's only piled tea, similar to the Chinese post-fermented teas. Toyama Kurocha 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. Awabancha (阿波番茶), produced in Tokushima prefecture, and Batabatacha, like the Toyama Kurocha associated with Asahi, Toyama, are made from bancha, or second flush tea leaves, with bacterial fermentation. Batabatacha has been found to contain significant amounts of Vitamin B12. Goishicha (碁石茶) from Otoyo, Kochi and Ishizuchi-Kurocha 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.
South Korea produces a tea called ddok cha (떡차), which is usually allowed to post-ferment after sale.
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 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.
The Shan people of Myanmar also consume a pickled tea known as lahpet. 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 by the Chinese as suancha (酸茶), is first packed into bamboo tubes, then buried and allowed to ferment before eating.
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 most Pu-erh. 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 Pu-erh teas are ripened while loose, then compressed. Fermented teas can be aged for many years to improve their flavor, again comparable to wines. Raw Pu-erh tea can be aged up to 50 years in some cases without diminishing in quality, and ripened Pu-erh can be aged up to 10 or 15 years. Experts and aficionados disagree about the optimal age.
Tea with earthy flavour is not the right quality
Some people have skeptical views on pu-erh tea. Normally, they dislike pu-erh mainly because of the moldy flavour which is caused by the poor quality of the tea. The earthy flavour in pu-erh tea does not only give an unpleasant drinking feeling, but also causes health risks. This defect that occurs in pu-erh is mainly due to the improper fermentation conducted under the wrong conditions, and therefore the incorrect type of mold or bacteria was cultivated. Some ripe pu-erh tea produces flavour like dried squid or like smelly feet. That odour present due to the contamination of bacteria which is caused by the improper fermentation process or drying process. Logically, any smell that we don’t appreciate is what our body does not require. For health reasons, it is not advisable to drink earthy or foul smelling pu-erh. To identify good quality ripe pu-erh, it has a flavour that is similar to dried Chinese dates or other types of dried fruit flavours. The variety of flavours mainly depends on the fermentation technique, and sometimes, the specific type of cultivar also contributes to certain flavours.
Many Tibetans and Central Asians use Pu-erh or other fermented teas as a caloric and micronutrient food, boiled with yak butter, sugar and salt to make yak butter tea.
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 "jin hua" (金花) because of the bright yellow color.