Heian-period Japan (traditional legend traces to 11th century samurai Minamoto no Yoshiie); Ibaraki/Mito as commercial centre from Edo period; industrial production from 20th century
Nattō (納豆) — fermented soybeans produced through Bacillus subtilis var. natto fermentation — is one of Japan's most polarising foods and one of its most nutritionally significant. The sticky, stringy, pungent soybean product is a breakfast staple throughout Japan (particularly Eastern Japan and Tohoku), eaten mixed with mustard (karashi) and tare sauce over rice. Production involves cooking soybeans until soft, inoculating with B. subtilis natto bacteria, and fermenting at 40–45°C for 16–24 hours in small straw containers (warastutsu) or, in industrial production, polystyrene packs. The straw-fermented natto (waranatto) represents the traditional production method — Bacillus subtilis naturally inhabits rice straw, and the historical practice of wrapping cooked soybeans in straw bundles was the discovery mechanism for natto fermentation. The fermentation produces nattokinase — a potent fibrinolytic enzyme with demonstrated cardiovascular benefits — along with menaquinone (Vitamin K2), beneficial to bone density and arterial health. This nutritional profile has generated international interest in natto as a functional food. Natto's flavour profile — ammoniacal, deeply savoury, fermented — is an acquired taste; the stringiness (hikimono) is the signature textural element, produced by polyglutamic acid and fructan polymers generated during fermentation. The more the natto is stirred before eating, the more pronounced the stringiness — serious natto enthusiasts stir 50–100 times to achieve maximum hikimono development.
Strongly savoury, ammoniacal, yeasty, umami-rich; stringy texture with soft bean interior; the flavour is often described as blue cheese-like in its intensity — powerful, deeply fermented, acquired
{"Soybean selection matters: small to medium soybeans ferment more evenly and produce better hikimono than large beans; specific natto-variety soybeans (such as Suzuhime, スズヒメ) are bred for natto production","Inoculation requires pure B. subtilis natto culture — commercial natto spore powder allows precise dosing; using store-bought natto as inoculum is possible but less consistent","Temperature control is critical: 40–42°C is optimal; above 45°C inhibits bacterial activity; below 37°C slows fermentation and risks contamination by non-natto bacteria","Humidity must be high (90%+) during fermentation — covered containers or humid fermentation chambers prevent the bean surface from drying, which would inhibit bacterial colonisation","Fermentation time determines flavour intensity: 16–18 hours produces mild natto; 20–24 hours produces more pungent, strongly flavoured natto preferred in Eastern Japan","Post-fermentation refrigeration is essential: cold-stopping the fermentation fixes flavour; at room temperature, natto continues to ferment and develops increasingly strong ammonia off-notes"}
{"Traditional waranatto (straw-fermented natto) is considered superior by enthusiasts: the natural straw environment provides trace minerals and aromatic compounds from the straw itself; the texture is typically looser and the flavour more complex than industrial natto","Stirring natto 100+ times before eating dramatically increases hikimono (stringiness) — this is both a taste preference and a ritual; some tasters believe extensive stirring improves flavour integration","Combining natto with karashi mustard, tamari (a thicker, more umami-rich soy sauce), and finely cut negi (green onion) is the canonical serving — the karashi cuts through the ammonia notes while negi freshness lifts the whole","Natto is a functional ingredient in cooked dishes despite its raw breakfast reputation: natto soup (natto-jiru, a Tohoku tradition), natto tempura, and natto pasta all represent the ingredient's versatility beyond its raw form","Premium artisan natto from small producers in Mito, Ibaraki Prefecture (considered natto's spiritual home) uses heritage soybean varieties and traditional straw fermentation — the difference from industrial product is dramatic"}
{"Under-cooking soybeans — the beans must be tender enough to penetrate easily with a fingernail; under-cooked beans produce poor bacterial penetration and uneven fermentation","Contaminating the inoculation environment with non-natto bacteria — all equipment must be sanitised with boiling water or alcohol before contact with soybeans","Opening the fermentation container during the critical 12–16 hour window — temperature disruption and oxygen introduction can cause uneven surface colonisation","Storing finished natto at room temperature beyond a few hours — the fermentation continues, rapidly developing excess ammonia and over-fermented flavour","Using soybeans that have been stored too long and developed rancid fat — off-flavours are amplified through fermentation"}
The Art of Fermentation — Sandor Katz; Japanese Farm Food — Nancy Singleton Hachisu