Fermentation is a biochemical process in which microorganisms such as *bacteria, **yeast, and ***fungi convert organic compounds - primarily carbohydrates - into simpler substances. This metabolic process occurs in anaerobic (oxygen-limited) conditions and is widely used in food preservation, medicine, and biotechnology. Historically, fermentation has played a significant role in human civilization, enabling the production of essential foodstuffs and medicinal preparations.

Fermentation is driven by microbial metabolism, breaking down complex molecules to produce energy. The primary types of fermentation include: Lactic Acid Fermentation - Carried out by lactic acid bacteria (e.g., Lactobacillus), this process converts sugars into lactic acid, enhancing food preservation and probiotic properties (e.g., yogurt, sauerkraut, kimchi). Alcoholic Fermentation - Yeasts such as Saccharomyces cerevisiae convert sugars into ethanol and carbon dioxide, which is fundamental in beer, wine, and bread production. Acetic Acid Fermentation - Acetobacter species oxidize ethanol into acetic acid, producing vinegar. Butyric Acid Fermentation - Certain anaerobic bacteria (e.g., Clostridium) produce butyric acid, playing a role in some traditional dairy products.

Fermentation increases vitamin and enzyme content, making foods more nutritious. Beneficial bacteria support gut health, digestion, and immune function. Nutrients become more easily absorbed due to the breakdown of complex compounds. Probiotic-rich fermented products help balance intestinal flora and support digestion. Many fermented foods contain bioactive compounds that enhance immune function. Traditional practices include fermenting medicinal herbs to increase their potency and bioavailability. Specific bacteria, yeast, or fungi determine the type of fermentation. Optimal conditions such as temperature and pH vary for different fermentation processes. Some processes are strictly anaerobic, while others require oxygen. The presence of sugars and other fermentable compounds influences the rate and outcome of fermentation.

In simple words!

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Fermentation of tea leaves is an ancient and sophisticated biochemical process that enhances tea's flavor, nutritional profile, and health benefits. Fermented tea, known as Hei Cha (black tea in Chinese terminology, distinct from Western black tea), includes varieties such as Pu-erh tea, Liu Bao tea, and Fu Zhuan tea. The fermentation process involves microbial activity and oxidation, distinguishing it from merely oxidized teas such as black and oolong teas.

Fermented tea undergoes microbial transformation, where microorganisms break down complex organic compounds into bioactive metabolites. The key scientific principles include: Microbial Action - Beneficial fungi (Aspergillus spp.), bacteria (Lactobacillus, Streptomyces), and yeasts contribute to fermentation. Biochemical Changes - The breakdown of polyphenols, catechins, and flavonoids results in unique flavors and bioactive compounds. Aging Process - Some fermented teas, especially Pu-erh, continue to develop complexity over time through post-fermentation microbial activity.

In simple words!

Fermentation is a natural transformation that unfolds within the tea leaf, guided by the hands of nature or human craftsmanship, from the moment it is harvested until it is brewed. Throughout this journey, the (dry) tea leaf holds onto about 3-5% moisture, allowing subtle yet profound chemical changes to take place. Though imperceptible to the eye, these transformations gradually reveal themselves, shaping the tea’s flavor, aroma, and character over time.

The key difference in fermentation nature and level among green, white, yellow, wulong, red, and fermented teas (Hei Cha/Pu-erh) lies in the extent of oxidation and the presence of microbial fermentation.


Yellow tea undergoes a mild, non-microbial fermentation during its unique Men Huang "wrapped yellowing" process. After slight oxidation, the tea is wrapped in damp cloths, allowing slow enzymatic reactions that mellow the flavor. The process reduces grassy notes, making it smoother and sweeter than green tea. No active bacteria or fungi are involved—this is a controlled, enzyme-driven process. Fermented tea (Hei Cha, Pu'Er tea) undergoes true microbial fermentation, where bacteria and fungi break down tea compounds over time. The process involves moisture and microbial activity, which enhances aging and produces probiotics. The tea is often piled and fermented through a “wet-piling” (Wo Dui) process, especially in Shu Pu'Er. This fermentation alters the chemical composition, creating earthy, woody, and complex flavors.

The term "fermentation" is often used in the tea world to describe both true microbial fermentation and simple oxidation, leading to confusion and debate. While commonly accepted in tea terminology, this usage is not entirely accurate from a scientific perspective. Oxidation and fermentation are two crucial chemical processes that significantly influence taste, texture, and nutritional value. Though often confused, these processes are fundamentally different in their mechanisms, effects, and applications.

Oxidation is a chemical reaction that occurs when a substance interacts with oxygen, leading to changes in its composition. This process is particularly common in the production of tea, wine, and certain food products. In simple terms, oxidation happens when oxygen molecules react with organic compounds, often breaking them down or altering their chemical structure. For example, in tea processing, oxidation occurs when tea leaves are exposed to air after being rolled or bruised, leading to the darkening of leaves and the development of complex flavors. Similarly, oxidation in apples causes browning when they are cut and exposed to air.

Oxidation is the enzymatic reaction where polyphenols in tea leaves react with oxygen, leading to the formation of theaflavins and thearubigins, which contribute to the color and flavor of black and wulong teas.

Catechins + O₂ + Polyphenol Oxidase → Theaflavins + Thearubigins + H₂O₂

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Fermentation is a biological process in which microorganisms such as bacteria or yeast convert organic compounds, typically sugars, into alcohol, acids, or gases. Unlike oxidation, fermentation does not necessarily require oxygen and often takes place in controlled, anaerobic environments. Fermentation occurs due to the metabolic activity of microorganisms. For instance, in tea production, microbial fermentation plays a role in aging certain teas, such as pu'er, where beneficial bacteria and fungi alter the flavor and texture over time.

Fermentation in tea often refers to microbial fermentation, such as in Pu-erh tea, where microbes break down complex compounds into simpler ones, altering the flavor profile.

Polyphenols + Microbes → Simpler Phenolic Compounds + Aromatic Compounds


Key Differences Between Oxidation and Fermentation


Oxidation is primarily a chemical reaction involving oxygen exposure, while fermentation relies on microbial activity to transform organic compounds. Understanding these differences is essential for food scientists, tea enthusiasts, and culinary experts who work with these processes to create unique flavors and preserve food effectively.

HEI CHA (DARK TEA)
Hei Cha is a broad category of post-fermented teas from China, commonly produced in the provinces of Hunan, Hubei, Guangxi, Anhui, Sichuan, and Yunnan. Each province has its nuances in the production and storage of fermented tea, which makes it quite different from other representatives of the same category.

Hei Cha was historically traded along the Tea Horse Road, where it became a staple for Tibetan, Mongolian, and Central Asian cultures. It is used in Traditional Chinese Medicine (TCM) for digestion, detoxification, and maintaining balance in the body. Fermented teas have gained global recognition for their health benefits and rich flavors, inspiring innovations in tea culture.

The microbial fermentation process enhances probiotic content, promoting digestion. Catechins and flavonoids are converted into more bioavailable antioxidants, which may improve cardiovascular health. Studies suggest that Pu-erh tea consumption aids in fat breakdown and cholesterol regulation. Fermented teas contain bioactive compounds that may help reduce inflammation and support immune function.

PU'ER TEA
Originating from Yunnan, Pu'Er comes in two forms:
Sheng (Raw) Pu'Er is naturally aged, undergoing a slow microbial fermentation process over years or decades.
Shou (Ripe) Pu'Er is artificially fermented using a wet-piling process (wo dui), accelerating microbial activity.

These categories of tea require special conditions for subsequent post-fermentation, which is often ignored in most sources telling about this specific feature of Pu'Er tea. For more detailed information about the professional approach of Pu'Er tea, please read our Storing and Aging Tea article.

FAMOUS FERMENTED TEAS

LU'AN HEI CHA - ANHUA FU ZHUAN - LIU BAO - HUBEI HEI CHA - YA'AN BRICK TEA - PU'ER TEA

Lu'An Hei Cha is a unique fermented tea from Lu'An, Anhui Province, China. As a member of the Hei Cha (dark tea) family, Lu'An Hei Cha undergoes a microbial fermentation process that enhances its flavor, aroma, and health benefits. With a rich history dating back to the Ming and Qing Dynasties, Lu'An Hei Cha has been valued as a traditional beverage and for its medicinal properties in Chinese culture. Lu'An Hei Cha is known for its smooth, mellow taste and distinct earthy notes. Dark, twisted leaves with a slightly glossy surface. A rich, aged fragrance with hints of wood, dried fruits, and medicinal herbs. A smooth and slightly sweet flavor with low bitterness and a lingering aftertaste. Deep reddish-brown, indicative of microbial transformation.

The fermentation process enhances Lu'An Hei Cha's bioactive compounds, contributing to various health benefits. Promotes gut health and supports digestion due to probiotic microorganisms. Contains bioactive compounds that may help regulate blood lipids. Fermented polyphenols exhibit strong antioxidative effects, protecting against oxidative stress. Traditionally used to aid weight management and improve energy levels.

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Production Process

Harvesting - Fixation - Rolling - Pile Fermentation - Drying - Aging

The production of Lu'An Hei Cha follows a meticulous method that contributes to its distinctive qualities. Mature tea leaves, typically from large-leaf tea varieties, are selected. Leaves are pan-fried or steamed to halt enzymatic oxidation while preserving essential compounds. This step is called Sha Qing 'Fixation'. The leaves are rolled to break down cell structures, facilitating microbial fermentation. The tea undergoes controlled fermentation under humid conditions, allowing microorganisms to transform polyphenols and amino acids. This step is called Wo Dui "Pile Fermentation". The fermented leaves are sun-dried or air-dried, after which they are aged to develop deeper flavors and complexity.

Check out our special CHING YUEN CHA, 2011 and JIA CANG LIU AN, 2009 teas from Rongkou, Qimen, Anhui, China.

Hunan Hei Cha is a variety of fermented dark tea from Hunan Province, China. As a member of the Hei Cha (dark tea) category, it undergoes microbial fermentation, developing a smooth, mellow taste and a rich array of bioactive compounds. Hunan Hei Cha has been historically significant in Chinese trade, particularly along the ancient Tea Horse Road, where it was a staple for ethnic minorities and long-distance travelers. Dark, tightly compressed bricks or loose, twisted leaves. Aged, earthy, with woody and floral undertones. Mellow, smooth, slightly sweet, with minimal astringency and a lingering aftertaste. Reddish-brown to deep amber, indicative of prolonged microbial activity.

Hunan Hei Cha offers a variety of potential health benefits due to its fermentation process. Supports gut health and digestion through beneficial microorganisms. Compounds in fermented tea may help reduce LDL cholesterol levels. Rich in polyphenols and flavonoids that combat oxidative stress. Traditionally consumed to aid metabolism and fat digestion.

Varieties of Hunan Hei Cha

Hunan Province produces several well-known Hei Cha subtypes. Fu Zhuan Cha (Fu Brick Tea). Distinctive for the presence of Eurotium cristatum, a beneficial golden mold that enhances probiotic properties. Hua Zhuan Cha (Flower Brick Tea): Noted for its strong, aged aroma and compact brick form. Hei Zhuan Cha (Black Brick Tea): A traditional compressed dark tea with a rich, earthy profile. Qian Liang Cha (Thousand Liang Tea): A large, tightly compressed cylindrical tea aged for long periods, often for decades.

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Production Process

Harvesting - Fixation - Rolling - Pile Fermentation - Compression- Aging

The fermentation and aging processes define the unique characteristics of Hunan Hei Cha. Mature, coarse leaves from large-leaf tea cultivars are selected. The leaves are briefly pan-fried or steamed to stop oxidation and preserve key compounds. The leaves are twisted and bruised to release juices and facilitate microbial activity. The tea is heaped in controlled humid conditions, allowing beneficial microorganisms to ferment the leaves. Some Hunan Hei Cha varieties, such as Fu Zhuan Cha, are pressed into bricks and aged, further developing their flavor and probiotic content.

Check out our special ANHUA FU ZHUAN, 2017 and BAI SHA XI FU ZHUAN CHA, 2014 from Anhua, Hunan, China.

Liu Bao tea is a traditional fermented dark tea from Guangxi Province, China. As a Hei Cha (dark tea) family member, Liu Bao undergoes a microbial fermentation process that enhances its flavor, aroma, and health benefits. With a history dating back to the Qing Dynasty, this tea has been highly valued for its medicinal properties. It was historically exported to Southeast Asia, particularly to Malaysia, where it remains popular among the overseas Chinese community. Liu Bao tea is known for its deep, earthy character and smooth taste. Dark, tightly twisted leaves, sometimes compressed into baskets or bricks. It has a rich, aged fragrance with camphor notes, Chinese herbs, wood, and dried fruits. Mellow and smooth, with an earthy, slightly sweet taste and a lingering aftertaste. Deep reddish-brown, indicative of prolonged microbial activity and aging.

Historically, it was transported along the Tea Horse Road and became especially popular in Malaysia, where it was widely consumed by Chinese immigrants working in tin mines. Today, aged Liu Bao tea is highly sought after by collectors, and it continues to be enjoyed in China and worldwide.

Liu Bao tea is highly regarded for its health-promoting properties, many of which stem from its fermentation process. The microbial content supports gut health and promotes digestion. Certain compounds in fermented tea may help regulate blood lipids. Fermented polyphenols contribute to anti-aging and immune system support. Traditionally consumed to aid weight management and enhance circulation.

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Production Process

Harvesting - Fixation - Rolling - Pile Fermentation - Drying - Aging

The unique characteristics of Liu Bao tea arise from its specific fermentation and aging methods. Mature, robust leaves from large-leaf tea cultivars are selected. The leaves are pan-fried or steamed to halt oxidation while preserving essential compounds. The leaves are twisted and bruised to release juices and initiate microbial activity. The tea is heaped under controlled humid conditions, allowing beneficial microorganisms to ferment the leaves. After fermentation, the tea is dried and stored for aging, lasting several months to decades, further enriching its complex flavor profile.

Check out our special LAO LIU BAO, 1981, from Guanxi, China, which has been traditionally aged in Hong Kong for several decades.

Hubei Hei Cha is a traditional fermented dark tea from Hubei Province, China. As a member of the Hei Cha (dark tea) family, Hubei Hei Cha undergoes a microbial fermentation process that enhances its flavor, aroma, and health benefits. With a history dating back to the Qing Dynasty, this tea has been highly valued for its medicinal properties and was historically traded across China and beyond, particularly in regions where fermented teas were prized for their aging potential and digestive benefits. Hubei Hei Cha is known for its deep, earthy character and smooth taste. Dark, tightly twisted leaves, sometimes compressed into bricks or cakes. A rich, aged fragrance with notes of wood, dried fruits, and a slight smokiness. Mellow and smooth, with an earthy, slightly sweet taste and a lingering aftertaste. Deep reddish-brown, indicative of prolonged microbial activity and aging.

Hubei Hei Cha is highly regarded for its health-promoting properties, many of which stem from its fermentation process. The microbial content supports gut health and promotes digestion. Certain compounds in fermented tea may help regulate blood lipids. Fermented polyphenols contribute to anti-aging and immune system support. Traditionally consumed to aid weight management and enhance circulation.

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Production Process

Harvesting - Fixation - Rolling - Pile Fermentation - Drying - Aging

The unique characteristics of Hubei Hei Cha arise from its specific fermentation and aging methods. Mature, robust leaves from large-leaf tea cultivars are selected. The leaves are pan-fried or steamed to halt oxidation while preserving essential compounds. The leaves are twisted and bruised to release juices and initiate microbial activity. The tea is heaped under controlled humid conditions, allowing beneficial microorganisms to ferment the leaves. After fermentation, the tea is dried and stored for aging, which can last from several months to decades, further enriching its complex flavor profile.

Check out our special HUBEI HEI CHA, 2004 from Chang Sheng Chan tea factory, Yichang, Hubei, China

Ya’an, a city in China’s Sichuan province, is famous as the birthplace of Tibetan brick tea, a staple in Tibetan and Mongolian diets for centuries. This compressed dark tea has played a crucial role in trade, culture, and daily life, particularly in high-altitude regions. The combination of Ya’an’s unique geographical conditions and centuries-old processing techniques has made this tea a prized commodity.

The production of Ya’an brick tea dates back over a thousand years, deeply intertwined with the Ancient Tea Horse Road. This trade route, similar in importance to the Silk Road, facilitated the exchange of tea from Sichuan to Tibet and beyond in return for horses and other valuable goods. During the Tang and Song dynasties, the demand for Tibetan tea grew exponentially, making Ya’an a center for tea production. By the Qing dynasty, the imperial court had established strict controls over Tibetan tea production, solidifying its status as a vital economic and political tool.

Ya’an is nestled in a subtropical monsoon climate zone, with abundant rainfall and moderate temperatures, making it an ideal location for tea cultivation. The region’s high humidity, misty mountains, and fertile soil contribute to the rich and robust flavors of its tea. The Min River, flowing through Ya’an, has also historically served as a key transportation route for tea distribution. The proximity to the Tibetan Plateau made Ya’an the perfect starting point for tea caravans traveling toward Tibet.

Ya’an brick tea remains a cultural symbol in Tibet, Mongolia, and other nomadic communities, where it serves as a key source of nutrients, especially in harsh climates. Today, Ya’an continues to be a leading producer of this historic tea, balancing ancient traditions with modern production techniques. The resurgence of interest in fermented teas worldwide has also boosted its recognition beyond Asia, bringing this historic beverage to a new generation of tea enthusiasts.

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Production Process

Harvesting - Withering and Rolling - Pile Fermentation - Compression and Aging

Ya’an brick tea is made from coarse, mature tea leaves. Older, larger tea leaves are harvested, which contain a higher tannin content ideal for fermentation. The leaves are left to wilt and then rolled to break down cell walls, aiding in oxidation. Unlike green or white tea, brick tea undergoes a post-fermentation process, similar to Pu-erh tea, which enhances its deep, earthy flavors. The tea is compressed into bricks or cakes and aged for months or even years, further developing its rich taste and unique microbial profile. The final product has a dark, strong, and slightly smoky flavor, often prepared by boiling it with yak butter and salt to create the traditional Tibetan butter tea.

Pu'Er tea is a famous fermented tea from Yunnan Province, China, known for its aging potential and rich, complex flavors. It comes in two main varieties: Sheng (Raw) Pu'Er and Shu (Ripe) Pu'Er. While both types originate from the same tea leaves, their processing methods and aging processes create distinct differences in taste, appearance, and aroma.

Young SHENG PU'ER has a fresh, astringent, and floral taste with bright citrus or vegetal notes. As it ages, it develops deeper, mellow, and earthy characteristics with a smooth mouthfeel. The aroma evolves from grassy and floral to woody and honeyed over time. Young leaves are greenish with silvery buds; aged leaves darken over time. The brewed tea liquor ranges from pale yellow to deep amber, depending on the aging stage. Naturally ferments over decades, developing a richer, more complex profile. Requires proper storage in a well-ventilated, humidity-controlled environment.

SHU PU'ER has a dark, rich, and smooth taste with earthy, nutty, and sometimes chocolaty notes. Lacks the sharp astringency of young Sheng Pu-erh due to its controlled fermentation process. The aroma is deep and reminiscent of damp forest, dried fruits, or Chinese herbs. Leaves are dark brown to black due to the fermentation process. The liquor is deep reddish-brown to almost black, showing a strong, full-bodied infusion. Already fully fermented, so it doesn’t change as drastically over time. Can improve with short-term aging, but long-term storage offers minimal transformation.

Both teas promote gut health, with Shu Pu-erh being particularly beneficial for digestion due to its microbial content. Pu'Er is often linked to weight management by aiding fat breakdown. Sheng Pu'Er retains more catechins, while Shu Pu'Er has more fermented polyphenols. Both teas may help regulate cholesterol levels and improve circulation.

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SHENG PU'ER - Production Process

Harvesting - Withering - Fixation - Rolling - Sun-Drying - Compression (Optional) - Aging

Fresh tea leaves are picked from Camellia sinensis var. assamica trees. The leaves are spread out to naturally wither and reduce moisture content. The leaves are pan-fired to halt oxidation, preserving the green tea characteristics. The leaves are twisted and bruised to release juices and enhance flavor development. The tea is dried under the sun to remove remaining moisture. The dried leaves can be left loose or compressed into cakes or bricks. Sheng Pu'Er slowly ferments over time, developing a richer, more complex flavor.

SHU PU'ER - Production process

Harvesting - Withering - Fixation - Rolling - Sun-Drying - Pile Fermentation - Drying and Sorting - Compression (Optional)

Fresh tea leaves are picked from Camellia sinensis var. assamica trees. The leaves are spread out to naturally wither and reduce moisture content. The leaves are pan-fired to halt oxidation, preserving the green tea characteristics. The leaves are twisted and bruised to release juices and enhance flavor development. The tea is dried under the sun to remove remaining moisture. After the sun-drying the leaves are piled, sprayed with water, and covered to encourage microbial fermentation under controlled heat and humidity for 45-60 days. After fermentation, the leaves are dried and sorted. Like Sheng Pu-erh, Shu Pu'Er can be left loose or pressed into cakes and bricks.

PU'ER TEA is a unique and distinct tea world with its own philosophy and characteristics. You can read more about each of these categories in our separate article dedicated to PU'ER TEA.

The secret of Sheng (Raw) Pu’er tea processing that allows it to continue aging, transforming, and fermenting over the years lies in several key factors like leaf material, minimal processing, natural microbial fermentation, controlled oxidation over time, storage environment and compression.

Sheng Pu’er is often made from large-leaf Camellia sinensis var. assamica, which has higher levels of polyphenols and pectin that support long-term aging. Older tea trees ("gushu") produce leaves with stronger microbial resilience, aiding in better transformation over decades. Sheng Pu’er is processed using sun-drying instead of high-heat roasting or steaming. This preserves the natural yeasts, bacteria, and fungi that enable post-fermentation over time. Unlike other teas, its enzymes remain active, allowing oxidation and fermentation to continue for decades. The tea contains living microbial cultures (such as Aspergillus, Penicillium, and lactic acid bacteria) that continue to break down tea compounds. As it ages, microbes slowly ferment the tea, developing complex flavors and reducing bitterness. Since Sheng Pu’er is not heavily processed, natural oxidation occurs over years, deepening flavors and reducing astringency. Tea polyphenols and catechins gradually convert into theabrownins and theaflavins, which smooth out bitterness and create depth. Humidity, temperature, and airflow impact how the tea ferments over time. Warmer, humid environments (e.g., Hong Kong, Guangzhou) speed up aging, while cooler, drier climates (e.g., Kunming, Taiwan) slow the process. Proper storage enhances microbial activity, allowing tea to evolve into richer, more complex flavors. Sheng Pu’er is typically compressed into cakes, bricks, or tuocha ("next"). This slows oxidation and microbial activity, allowing even fermentation and preserving flavors over time.

As passionate connoisseurs of fermented tea and true explorers of tea culture, we pay special attention to this category and invite all of you to discover other rare varieties of fermented teas produced in different countries around the world. We proudly invite you to explore our collection of fermented teas, where you can find rare selections from Vietnam, Korea, Japan, Taiwan, Africa, and even Russia.

FERMENTED TEA by AN SHIM TEA

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