The manufacturing procedure for natto begins like that for miso; however, the organisms involved in the fermentation are different and the final product bears little resemblance to miso. Natto is made from whole, somewhat small-sized soybeans that are cleaned, soaked for twelve to twenty hours at ambient temperature, and steamed at 121°C for twenty to forty minutes. The thoroughly cooked and cooled beans are then inoculated with about 106 to 107 spores per Kg of Bacillus subtilis var. natto (formerly Bacillus natto), and the material is well mixed.The incubated beans are then divided into 100 g portions and placed into packages. According to traditional practices, bundled rice straw was used as the container (some manufactures still use straw); however, polyethylene bags are now common. In either case, the beans are moved into aerobic incubators at 40°C at 85% relative humidity for sixteen to twenty hours. Growth of B. subtilis var. natto occurs primarily at the surface and is accompanied by a change in color (from yellow to white) and synthesis of a highly viscous polysaccharide material. The latter can completely cover the entire bean surface, accounting for nearly 1% of the total dry weight of natto.After incubation, the natto is held at 2°C to 4°C to minimize further growth.
The final product is quite different from miso and shoyu. Its main flavor attribute is sweetness, due to the presence of the polysac-charide material, which is composed of a fructose-containing polymer. The polysaccharide also confers a definite sticky and stringy texture to the product, which, although characteristic of natto, nonetheless causes some consumers to consider other menu options.
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