Natto: changing soybeans through fermentation

Production stages

Industrial production generally involves washing, soaking and steaming soybeans, adding a starter containing heat-resistant natto-bacterial spores, and placing the beans in containers. The bacteria grow during controlled warm fermentation, which requires oxygen; the beans are then cooled for refrigerated maturation. Sticky material includes polyglutamic acid and levan, a polysaccharide, and forms conspicuous strands when the beans are stirred or lifted.

Air, loose filling and cooling

Steaming softens the beans, helping bacterial growth and enzyme action. Because the bacteria need oxygen, the cooked beans are filled loosely rather than pressed into a solid mass; ridges in some containers help air circulate. Cooling after fermentation suppresses bacterial activity before refrigerated maturation and transport.

Bean components and the sticky coating

Fermentation changes the beans chemically as well as coating them. Bacterial proteases break down some soybean proteins, releasing amino acids, including glutamic acid, that contribute savouriness. Free glutamic acid is distinct from polyglutamic acid, a long-chain component of the sticky material. Cooking softens the beans; fermentation changes their components; stirring redistributes and aerates the coating already present in the finished food.

Bean size and fermentation routes

Stringy natto can use beans of different sizes, kept whole or split. Temple natto instead begins with mould-grown soybean koji and undergoes a longer maturation with salt. Ibaraki’s soboro natto is another kind of variation: it combines stringy natto with dried radish and seasoning, rather than using the temple-natto fermentation process.

Mixing and serving

Stirring gathers the sticky material around the beans and incorporates air, changing the texture. Stirring more times does not increase nutrient levels or the amount of umami compounds. Serving natto over rice is common, but it also accompanies other foods. In Miyagi, natto mochi coats toasted rice cakes with natto seasoned with ingredients such as soy sauce and grated radish; households vary the combination.

Regional origin legends name figures such as Prince Shotoku or Minamoto no Yoshiie, but no single inventor is established. Saori Mitsuboshi’s 2019 study identifies a reference to stringy natto in Shojin Gyorui Monogatari, thought to date from the mid-Muromachi period. Chopped natto was used in soup before whole-bean servings became prominent between the late Bunka and Tenpo eras of the Edo period, changing how the fermented beans were prepared and served.

Research, markets and packaging

Meiji and Taisho microbial research and starter cultures changed control of production. Industrial production expanded in the mid-twentieth century. Supermarkets emerging in the late 1950s encouraged mechanised mass production, while cold-chain transport helped preserve quality. Mito’s regional history records natto sales at the railway station around 1889.

How pure culture changed production

In 1918, Jun Hanzawa at Hokkaido University developed a method using a pure culture of natto bacteria and promoted the supply of starter cultures. Direct inoculation offered greater control than relying on microbes attached to straw.