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Fermented tea

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Fermented tea, often called dark tea or post-fermented tea, is a unique class of tea. It is distinct from black tea, which is known as "red tea" (hong cha) in China. This tea undergoes a microbial fermentation process that can last from several months to many years. During this time, tea leaves are exposed to humidity and oxygen. This exposure triggers two types of chemical changes. The first is endo-oxidation, which comes from enzymes inside the tea leaves. The second is exo-oxidation, which is catalyzed by microbes.

Puer tea, Chinese green tea, Rostov-on-Don, Russia
Puer tea, Chinese green tea, Rostov-on-Don, Russia

These chemical changes transform the tea's sensory qualities. As oxidation progresses, both the leaves and the brewed liquor become much darker. The fermentation process also alters the tea's chemistry to improve its organoleptic qualities. This means the smell changes and the taste becomes more mellow. It typically reduces bitterness and astringency while improving the aftertaste and mouthfeel. Some microbes may even produce metabolites that offer health benefits. However, substances like ethyl carbamate (urethane) may also be produced during the process.

Microbes play the most important role in this transformation. Molds are the primary drivers of fermentation. Scientists once believed Aspergillus niger was the main organism involved. However, newer PCR-DGGE analysis suggests that Aspergillus luchuensis is actually the primary agent. There are three main ways to produce these teas. Piled teas, such as Toyama in Japan, use naturally occurring fungus in relatively dry conditions. Pickled teas use a wet process involving lactic acid bacteria. A third type, like Ishizuchi in Japan, uses both methods in succession.

JinHua "golden flower" from Liu Bao
JinHua "golden flower" from Liu Bao

The history of dark tea is a mix of legend and historical record. One legend says dark tea was an accident on the Silk Road and Tea Road. It claims tea caravans left tea soaked in the rain. The transporters feared contamination and abandoned the wet tea. Later, local villagers ate the mildewed tea to survive a sickness. The legend says the tea cured their dysentery. Other accounts suggest dark tea production began during the Ming dynasty in the 15th and 16th centuries. It may have been traded even earlier across Han and Tibetan borders.

China is the primary producer of fermented teas, especially in warmer southern provinces. Yunnan province is famous for Pu'er tea, which can be "raw" or "ripened." In Anhui, people make Lu'an basket tea. Guangxi produces Liubao tea, which is often sold as loose dark tea. To help with aging and storage, tea is often pressed into specific shapes. These include bricks, cakes, discs, or even bird nests. Some varieties are shaped into squares or mushrooms. In Korea, Tteokcha was a common tea. It was often pressed into shapes resembling coins with holes.

Tteokcha
Tteokcha

Tea can also be consumed as food rather than just a drink. In Myanmar, a fermented tea called lahpet is eaten as a vegetable. It is a major part of Burmese ritual culture. In Northern Thailand, people chew a pickled tea called miang as a stimulant. The leaves are kept in sealed bamboo baskets for fermentation. For young leaves, this takes four to seven days. For mature leaves, the process takes about a year. The Blang people in Yunnan eat a similar tea called miam, which is fermented in buried bamboo tubes.

Because of the way it ages, fermented tea can be very valuable. Raw tea can be aged for up to 50 years without losing quality. Ripened tea is often aged for 10 to 15 years. Many varieties are purposely aged in humid environments. This promotes the growth of certain fungi. These fungi create bright yellow spots called "golden flowers" (jin hua).

JinHua "golden flower" from Liu Bao
JinHua "golden flower" from Liu Bao

In Tibet and Central Asia, tea serves a vital nutritional purpose. People boil fermented tea with salt, sugar, and yak butter to create yak butter tea. This provides essential calories and micronutrients.

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File:Puer tea, Chinese green tea, Rostov-on-Don, Russia.jpg
Puer tea, Chinese green tea,...
File:Tteokcha.png
Tteokcha.png
File:JinHua "golden flower" from Liu Bao.jpg
JinHua "golden flower" from Liu Bao.jpg
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