A method of producing natto includes inoculating a mutant strain of Bacillus subtilis var. natto onto steamed or boiled soybeans, and fermenting the soybeans inoculated with the mutant strain by maintaining a temperature of the soybeans at 40 degrees centigrade to 53 degrees centigrade The number of the mutant strain spores after fermenting the soybeans is 5x105 or less spores per gram of the fermented soybeans. A single colony of the mutant strain cultured in a selective solid medium supplemented with biotin at a gas phase temperature of 49 degrees centigrade for 48 hours has a region having a hue of yellow (Y), a value of 9 or more, and a chroma of 2 or less, as defined by the Munsell Color System, and a surface area of the region is 20 percent or less of the total surface area of the single colony.

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