Natto — Fermented Soybean Tradition (納豆)

Japan — natto's origin is disputed between two theories: (1) it was accidentally fermented when cooked soybeans were stored in straw (Bacillus subtilis lives naturally in rice straw), probably in the Heian period; (2) it was introduced from China as an adaptation of a similar fermented soybean product. Mito (Ibaraki Prefecture) became the natto production centre in the Edo period; the region's humid climate suited fermentation, and the Mito domain actively promoted natto production.

Natto (納豆) is Japan's most polarising fermented food — whole soybeans inoculated with Bacillus subtilis natto and fermented at 40–45°C for 15–24 hours, producing sticky, string-forming, pungent fermented beans with a complex ammonia-backed flavour, intense umami, and a distinctive stringy viscosity when stirred. Natto is consumed daily by approximately 40% of the Japanese population — predominantly in the Kantō and Tōhoku regions, while Kansai (Osaka/Kyoto) remains broadly natto-resistant. Nutritionally, natto is exceptional: it contains vitamin K2 (nattokinase), complete amino acid protein, probiotics, and high isoflavone content. The traditional natto preparation involves breaking the beans with chopsticks, adding karashi and soy sauce, and stirring vigorously until the beans cohere in sticky strings — the longer it is stirred, the more developed the flavour.

• Bacillus/Rhizopus-fermented soybean products as primary protein sources — tempeh and natto are both whole-soybean fermentation products using different microorganisms (Bacillus subtilis for natto; Rhizopus for tempeh), producing completely different textures and flavour profiles from the same raw material → Tempeh (fermented soybean cake) Indonesian
• Cheonggukjang is functionally identical to natto — Bacillus subtilis-fermented whole soybeans with the characteristic stringiness and ammonia pungency. The Korean version is typically used in a hot soup (jjigae) rather than eaten raw; both share the same fermentation principle and similar nutritional profile → Cheonggukjang (fast-fermented soybean paste) Korean

Natto's flavour is layered and challenging: the initial aroma is intensely ammonia-forward — the B. subtilis fermentation produces ammonia from protein breakdown, giving natto its distinctive sharp, barn-like smell. Below the ammonia: a deep, meaty, fermented umami from the soybean's broken-down proteins, and a slight bitterness from the bean's compounds. Once stirred with karashi and soy: the mustard cuts the ammonia, the soy adds salt and depth, and the combination becomes a rounded, intensely savoury, complex flavour that natto eaters describe as deeply satisfying. On hot rice: the warmth softens the ammonia edge slightly and the starch absorbs the liquid, creating a cohesive experience.