{"schema":"askedwell-answer-v1","url":"https://askedwell.com/pages/what-temperature-for/natto-incubate","question":"What temperature is needed for incubating natto?","short_answer":"Natto incubates at 100–104°F (38–40°C) for roughly 22–24 hours. Bacillus subtilis natto needs both steady warmth and air — a sealed container fails even at the correct temperature.","long_answer":"**Natto is a bacterial ferment, and the requirements are unusually strict**\n\nMost home ferments tolerate a wide temperature band. Natto does not. *Bacillus subtilis* var. *natto* has a narrow optimum, needs oxygen throughout, and produces its characteristic stringiness only when both conditions hold for the better part of a day. Miss either and the result is cooked soybeans rather than natto.\n\n**The two requirements, in order of how often they are missed**\n\n1. **Oxygen.** This is the requirement most often broken, and it surprises people coming from vegetable fermenting, where excluding air is the whole discipline. *B. subtilis* is aerobic. A sealed jar starves it. Traditional practice wraps beans in rice straw; modern practice uses a shallow layer in a covered but not airtight container — foil with holes pierced through, or a lid resting loose. Depth matters too: a thick bed suffocates its own centre, so a layer of roughly 1–2cm is the usual recommendation.\n\n2. **Temperature.** The published band is 100–104°F (38–40°C). This is warmer than yoghurt and much warmer than a lacto-ferment. Below roughly 95°F the culture grows sluggishly and competing organisms have room. Above roughly 113°F growth is impaired. Because the target band is only a few degrees wide, most home setups use a yoghurt maker, a dehydrator, an oven with the light on, or a proofing box — and a thermometer, because appliance dials in this range are frequently inaccurate by more than the width of the band itself.\n\n**Bean preparation matters as much as incubation**\n\nSoybeans are soaked until fully hydrated — commonly 12 hours or more, longer in cold water — then cooked until a bean crushes easily between finger and thumb. Under-cooked beans do not ferment properly. The beans should be hot when inoculated and drained well: pooled water at the base of the container creates an anaerobic zone that behaves like a sealed jar.\n\n**The timeline**\n\n- **Hours 0–8.** Little visible change. The culture is establishing.\n- **Hours 8–16.** A white film develops across the bean surfaces; a distinctive ammoniacal, savoury aroma begins.\n- **Hours 18–24.** Stringiness develops. Lifting a bean should draw visible threads. This is the finish signal.\n- **After incubation.** Refrigerate for roughly 24 hours before eating. This ageing step is widely described as necessary for the flavour and texture to settle, and it is routinely skipped by first-timers who judge the batch too early.\n\n**Why the aroma is not a fault**\n\nNatto smells strongly ammoniacal and is meant to. Unlike vegetable ferments, where an ammonia note signals bacterial overgrowth and failure, in natto it is the expected product of protein breakdown. The failure signals here are different: no stringiness at all after 24 hours, visible coloured mould, or a putrid rather than pungent smell.\n\n**Common failure modes**\n\n- **Sealed container** — the single most frequent cause. Aerobic organism, no air, no natto.\n- **Temperature drift** — appliance runs 10°F off and nothing happens for a day.\n- **Bed too deep** — the surface ferments and the centre does not.\n- **Undercooked beans** — insufficient available substrate.\n- **Judging too early** — stringiness continues developing through the refrigerated rest.","ranges":[{"condition":"Standard incubation","duration":"100–104°F (38–40°C) for 22–24 hours","note":"Then refrigerate ~24h before eating"},{"condition":"Below 95°F (35°C)","duration":"Sluggish or failed","note":"Competing organisms gain ground"},{"condition":"Above 113°F (45°C)","duration":"Growth impaired","note":"Outside the working band for B. subtilis natto"},{"condition":"First visible white film","duration":"8–16 hours","note":"Ammoniacal savoury aroma begins here"}],"variables":[{"name":"Oxygen availability","effect":"B. subtilis is aerobic — a sealed container fails even at perfect temperature. Cover loosely or pierce the covering"},{"name":"Bed depth","effect":"Roughly 1–2cm; thicker beds suffocate their own centre while the surface ferments"},{"name":"Bean doneness","effect":"Beans must crush easily between finger and thumb; undercooked beans ferment poorly"},{"name":"Drainage","effect":"Pooled water at the base creates an anaerobic zone that behaves like a sealed container"}],"sources":[{"label":"William Shurtleff & Akiko Aoyagi (Soyinfo Center)","note":"Extensive documentation of traditional and modern natto production including incubation conditions"},{"label":"Sandor Katz, \"The Art of Fermentation\" (2012)","note":"Home-scale natto method: warmth, air access, and the post-incubation refrigerated rest"},{"label":"Rich Shih & Jeremy Umansky, \"Koji Alchemy\" (2020)","note":"Contextualises Bacillus ferments alongside Aspergillus work, including equipment for holding narrow temperature bands"}],"faq":[{"question":"Why is my natto not stringy?","answer":"The usual causes are a sealed container (the organism is aerobic and needs air), temperature outside 100–104°F, or judging too early. Stringiness continues developing during the refrigerated rest after incubation."},{"question":"Can I use a yoghurt maker?","answer":"Yes, provided it holds 100–104°F and the container is not sealed. Verify with a thermometer rather than trusting the dial — appliance error in this range is often larger than the width of the target band."},{"question":"Natto smells like ammonia. Is that normal?","answer":"Yes. Unlike vegetable ferments, where ammonia signals failure, in natto it is the expected product of protein breakdown. The real failure signals are no stringiness after 24 hours, coloured mould, or a putrid rather than pungent smell."},{"question":"Do I really need to refrigerate it before eating?","answer":"It is the standard step. Roughly 24 hours of cold rest is widely described as necessary for flavour and texture to settle, and skipping it is a common first-timer mistake."}],"keywords":["natto temperature","natto incubation","bacillus subtilis natto","how to make natto","natto not stringy"],"category":"fermentation","date_published":"2026-08-11","date_modified":"2026-08-11","license":"CC-BY-4.0","attribution":"https://askedwell.com"}