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Several varieties of douchi and douchi products
Douchi (Chinese: 豆豉, Pinyin: dòuchǐ, Jyutping: dau6si6) is a type of fermented and salted soybean. In English it is known simply as "black bean", and is also called fermented black soybeans, Chinese fermented black beans (Chinese: 黑豆豆豉, Pinyin: hēidòu dòuchǐ, Jyutping: hak1dau6dau6si6), salted black beans, salty black beans, or just "black beans". They are a flavoring most popular in the cuisine of China, where they are most widely used for making black bean sauce. 
Douchi is made by fermenting and salting soybeans. The process turns the beans black, soft, and mostly dry. The flavor is sharp, pungent, and spicy in smell, with a taste that is salty and somewhat bitter and sweet.
Fermented black soybeans are the oldest known food made from soybeans. In 165 BCE they were placed clearly marked in Han Tomb No. 1 at Mawangdui in South Central China. The tomb was sealed in about 165 BCE and was first opened in 1972. The high ranking lady to whom the undisturbed tomb belonged was probably the wife of the first Marquis of Tai.
In 90 BCE, in the Shiji (Simplified: 史记, Traditional: 史記, Pinyin: Shǐjì, Jyutping: Si2gei3), or the Records of the Grand Historian by Sima Qian, Chapter 69 refers to 1,000 earthenware vessels of mold-fermented cereal grains and salty fermented soybeans (shi). They were now an important commodity in China. When the prince of Huainan (legendary inventor of tofu) was exiled for inciting rebellion (in 173 BCE) against his brother, the Han Emperor Wendi, he and his retinue were, nevertheless, provided with such necessities of life as "firewood, rice, salt, shi (fermented black soybeans) and cooking utensils. Note that the date 173 BCE is before Han Tomb No. 1 at Mawangdui was sealed!
A free digital book titled "History of Fermented Black Soybeans (165 B.C. to 2011)," by Shurtleff and Aoyagi, has been published on the Web.
It is used as an ingredient for mapo tofu. Douchi is also used to flavor fish or stir-fried vegetables (particularly bitter melon and leaf vegetables). Unlike some other fermented soybean-based foods such as natto or tempeh, douchi is used only as a seasoning, and is not meant to be consumed in large quantities, being typically much more salty.
Small packets of douchi are available wherever Chinese foods are sold.
Around the world
In Vietnam, this sauce is called tàu xì or đậu xị and is made from the black soybean.
Black bean paste
In Chinese cuisine, a cooking sauce called black bean paste, black bean sauce or black bean garlic sauce (蒜蓉豆豉酱) is made from douchi, as well as garlic and soy sauce, a typical combination used for seasoning a dish. This paste is commercially available in glass jars from companies such as Lee Kum Kee, although most Chinese restaurant chefs prefer to use actual douchi to prepare such sauces rather than using commercially available black bean paste.
- Shurtleff, W.; Aoyagi, A. 2011. "History of Fermented Black Soybeans (165 B.C. to 2011)." Lafayette, California: Soyinfo Center. 398 pp. (754 references)
- BBC - Food - Blackbean sauce recipes