Natto Production Bacillus Subtilis Fermentation

Overview

Natto (納豆) is produced through the fermentation of whole soybeans by Bacillus subtilis natto — a naturally occurring bacterium traditionally transmitted from straw (wara) wrapping, now commercially propagated as pure starter cultures. The production process: soybeans are soaked overnight, steamed until fully soft, inoculated with B. subtilis natto spores, and fermented at 40–42°C for 18–24 hours in humidity-controlled chambers. During this period, the bacteria produce nattokinase enzyme, poly-glutamic acid (responsible for the characteristic stringy, viscous threads between beans), vitamin K2, and a distinctive ammoniacal-earthy aroma. The finished natto has a sticky, stringy surface with a complex bitter-earthy flavour and the characteristic fermented soybean aroma that divides Japanese opinion. Regional variations are substantial: Mito in Ibaraki Prefecture is the acknowledged capital of natto culture, producing a full spectrum from hikiwari (crushed, more spreadable) to large-bean premium natto from specific heirloom soybean varieties. Premium 'naturally fermented' wara-natto (straw-wrapped in rice straw) is the artisan form using traditional wild fermentation. Natto eating technique matters: the conventional preparation involves 50–100 vigorous stirring motions to maximise the viscous threads before adding seasoning and raw egg yolk.

Flavour Context

Complex bitter-earthy-ammoniacal with strong umami; acquired taste profile; threads add viscous sticky texture; raw egg yolk and soy seasoning tames and rounds

Key Principles

{"Fermentation temperature critical: 40–42°C for 18–24 hours; below 38°C slows to failure","Stirring before eating: 50–100 strokes increases thread formation and mellows ammoniacal notes","Wara (straw) naturally contains B. subtilis natto — traditional fermenting vessel before commercial cultures","Hikiwari vs whole bean: crushed natto has stronger flavour, more digestible; whole bean has texture contrast","Poly-glutamic acid threads are the viscosity indicator — more threads correlates with more active fermentation"}

Pro Tips

{"For natto-disliking household members: hikiwari mixed thoroughly into sauce or dressing minimises the texture resistance","Premium natto tasting: try without tare packet first — the soybean quality and fermentation character stand alone","Natto pasta (natto spaghetti): hot pasta + natto + soy + mentaiko + nori is a classic Japanese home recipe","Natto toast with cheese: surprisingly compatible — natto's umami bridges well with melted cheese"}

Common Mistakes

{"Eating natto cold from the refrigerator — bring to room temperature for 20 minutes to maximise aroma and thread formation","Insufficient stirring — under-stirred natto has stronger ammoniacal notes and less integrated flavour","Over-heating natto — temperatures above 70°C deactivate nattokinase enzyme and destroy live culture character","Adding seasoning before stirring — mix unflavoured first for maximum thread development"}

Source

The Art of Fermentation — Sandor Katz; Japanese Farm Food — Nancy Singleton Hachisu

Cross-Cuisine Parallels

• Both are legume fermentations using Bacillus subtilis producing ammoniacal aroma, sticky texture, and concentrated umami — parallel evolutionary food solution → Dawadawa locust bean fermentation (Parkia biglobosa) African
• Both are whole soybean fermentations producing health-functional bacterial or fungal cultures, though tempeh produces dry cake form vs natto's sticky strands → Tempeh Rhizopus oligosporus fermentation Indonesian