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.
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.
Natto preparation (eating): add karashi (Japanese yellow mustard) and soy sauce before stirring — the karashi modulates the ammonia character. Stir vigorously until sticky strings form and the beans become uniformly coated. Add spring onion and fresh grated ginger (optional). Eat immediately over hot rice. The bean size matters: small beans (ko-tsubu, 小粒) have a higher natto-to-rice flavour concentration; large beans (ōtsubu, 大粒) have a more substantial bite. Production: the soybeans must be cooked to precisely the right softness — firm enough to hold shape, soft enough for B. subtilis to penetrate — before inoculation.
The high-end natto category (artisan natto from specific soybean varieties) has developed significantly in recent years — natto from Mito (Ibaraki, traditionally Japan's natto capital) made from locally grown soybeans has a noticeably richer, more complex flavour than mass-produced natto. Natto has found its way into fine dining through the work of chefs who value fermentation — Yoshihiro Murata (Kikunoi) has served natto with caviar as a flavour tension preparation. The opposite flavour poles (natto's ammonia-funk + caviar's saline-oceanic richness) create a pairing that is challenging and memorable simultaneously.
Not stirring enough — understirred natto lacks the sticky, cohesive quality that integrates the flavour. Adding soy before stirring — the salt in soy inhibits the string development; add soy while stirring, not before. Eating natto cold — from the refrigerator, natto's flavour is muted and its stickiness is reduced; allow to sit at room temperature 10–15 minutes.
Japanese Farm Food — Nancy Singleton Hachisu; The Art of Fermentation — Sandor Ellix Katz