|Place of origin||Japan|
|Region or state||East Asia|
|Main ingredients||Fermented soybeans|
Nattō (納豆) is a traditional Japanese food made from soybeans that have been fermented with Bacillus subtilis var. natto. It is often served as a breakfast food. It is served with karashi mustard, soy or tare sauce, and sometimes Japanese bunching onion. Nattō is often considered an acquired taste because of its powerful smell, strong flavor, and sticky, slimy texture. Within Japan, nattō is most popular in the eastern regions, including Kantō, Tōhoku, and Hokkaido, and a 2009 survey revealed that 70.2% of Japanese people find the taste pleasant, and others who may not find the taste of the food pleasant still eat it for health benefits.
Sources differ about the earliest origin of nattō. One theory is that nattō was codeveloped in multiple locations in the distant past, since it is simple to make with ingredients and tools commonly available in Japan since ancient times.
One story about the origin of nattō attributes it to the samurai Minamoto no Yoshiie (1039–1106), who was on a campaign in northeastern Japan between 1086 AD and 1088 AD. One day, his troops were attacked while boiling soybeans for their horses. They hurriedly packed up the beans, and did not open the straw bags until a few days later, by which time the beans had fermented. The soldiers ate it anyway, and liked the taste, so they offered some to Yoshiie, who also liked the taste.
Another story involves Prince Shotoku (574–622), who is said to have wrapped the oddments of boiling soybeans in straw bags for his horse. As people happened to eat these fermented beans and found them delicious, this type of fermented stringy beans soon gained popularity in Japan because of its unique taste and strong flavor.
Before nattō, there was a similar dish of fermented black soybeans food in China called 豉 (chǐ) or douchi. These are salted, fermented-and-aged whole soybean seasonings or condiments invented in China and spread throughout East Asia as a merchandise. This type of food is usually made from fermented soybeans and with an ample amount of salt; however, the ingredients and producing method differ in Japan. Chinese use both black and yellow soybeans to produce douchi whereas Japanese nattō only uses yellow soybeans. The amount of salt used also makes difference between douchi and nattō on their taste and appearance.
The cultivation method of soybeans and rice was imported from China to Japan during the Yayoi period, and later on, the circulation of salt began to flourish in Japan. This provided an opportunity for the production of douchi to become popular in Japan. Because salt was expensive and valuable at the time, it has been inferred that nattō was invented by accident during the production of douchi.  
There was also a wooden slip excavated in Heijō-kyō, which had the Chinese character 豉 written on it. The excavation of the slip is considered an evidence to support the hypothesis that the invention of nattō was based on the Chinese douchi imported to Japan.
The Chinese character 豉 entered Japan in the 8th century. It was pronounced "kuki" until the eleventh century, when nattō became a new name for fermented soybeans.
Commercialization of nattō in the Edo period
A change in the production of nattō occurred in the Taishō period (1912–1926), when researchers discovered a way to produce a nattō starter culture containing Bacillus subtilis without the need for straw, thereby simplifying the commercial production of nattō and enabling more consistent results.
Appearance and consumption
Nattō has a distinctive odor, somewhat akin to a pungent aged cheese. Stirring nattō produces many sticky strings. The dish is eaten cold with rice, mixed with the included soy sauce or karashi mustard. Other ingredients such as long onion or kimchi are often added.
Nattō is frequently eaten as nattō gohan (nattō on rice). Nattō is occasionally used in other foods, such as nattō sushi (nattōmaki), nattō toast, in miso soup, tamagoyaki, salad, as an ingredient in okonomiyaki, chahan, or even with spaghetti. Sometimes soybeans are crushed before fermenting. This is called hikiwari nattō.
Many find the taste unpleasant and smelly while others relish it as a delicacy. Nattō is more popular in some areas of Japan than in others. Nattō is known to be popular in the eastern Kantō region, but less popular in Kansai. A 2009 Internet survey in Japan indicated 70% of respondents like nattō and 30% do not, but about half of those who dislike nattō eat it for its health benefits.
For those who dislike the smell and texture of natto, "dried natto" and "fried natto" were developed around 1990. The smell and stickiness are reduced, making it easier to eat for those who do not like conventional natto. Another type of fermented soybeans called "Mamenoka (豆乃香)" has also been developed by improving the soybean and natto bacillus varieties to make it less sticky.
Nattō is made from soybeans, typically nattō soybeans. Smaller beans are preferred, as the fermentation process will be able to reach the center of the bean more easily. The beans are washed and soaked in water for 12 to 20 hours to increase their size. Next, the soybeans are steamed for six hours, although a pressure cooker may be used to reduce the time. The beans are mixed with the bacterium Bacillus subtilis, known as nattō-kin in Japanese. From this point on, care must be taken to keep the ingredients away from impurities and other bacteria. The mixture is fermented at 40 °C (104 °F) for up to 24 hours. Afterward, the nattō is cooled, then aged in a refrigerator for up to one week to allow the development of stringiness.
In nattō-making facilities, these processing steps have to be done while avoiding incidents in which soybeans are touched by workers. Even though workers use B. subtilis natto as the starting culture, which can suppress some undesired bacterial growth, workers pay extra-close attention not to introduce skin flora on to soy beans.
To make nattō at home, a bacterial culture of B. subtilis is needed. B. subtilis natto is weak in lactic acid, so it is important to prevent lactic acid bacteria from breeding. Some B. subtilis natto varieties that are closer to odorless are usually less active, raising the possibility that minor germs will breed. Bacteriophages are dangerous to B. subtilis.
Historically, nattō was made by storing the steamed soybeans in rice straw, which naturally contains B. subtilis natto. The soybeans were packed in straw and left to ferment.
End product and nutrition
Mass-produced nattō is sold in small polystyrene containers. A typical package contains two, three, or occasionally four containers, each 40 to 50 g. One container typically complements a small bowl of rice.
Nattō has a different nutritional makeup from raw soybeans, with lower calorie content and enriched in branched chain fatty acids (BCFA), a prebiotic made by Bacillus subtilis. By mass, nattō is 55% water, 18% protein, 11% fats, 5% fiber, and 5% sugars. Nattō is a source of calcium, magnesium, protein, potassium, vitamins B6, B2, and E. A serving of nattō (100 g) provides 29% of the Daily value (DV) of vitamin K, 22% of the DV for vitamin C, 76% of the DV for manganese, 48% of the DV for iron, and 22% of the DV for dietary fiber. Nattō has softer dietary fiber without the high sodium content present in many other soy products, such as miso.
Nattō is an extremely rich source of the MK7 variant of vitamin K2, with one study finding mean concentrations of 998μg MK7 per 100g of natto, over 500 times greater concentration than any other food tested. 
Many countries around Asia also produce similar traditional soybean foods fermented with other bacteria and fungi, such as shuǐdòuchǐ (水豆豉) of China, cheonggukjang (청국장) of Korea, tempeh of Indonesia, thuanao (ถั่วเน่า) of Thailand, kinema of Nepal and the Himalayan regions of West Bengal and Sikkim, tungrymbai of Meghalaya, hawaijaar of Manipur, bekang um of Mizoram, akhuni of Nagaland, and piak of Arunachal Pradesh, India.
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