I want to make natto, and recipes say that it should be fermented at ~38°C/100°F. My oven doesn't have a bread proving or defrost setting, and previous experiments with just the oven light on haven't reached that temperature. I keep my sourdough starter and kombucha at room temperature (~21°C/67°F) and it works well / I like the result. Will that work for natto?

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    You may find the oven light can maintain roughly that temperature after warming it up by turning it on properly for a few minutes. I can do that for proving dough in winter. I would expect the light to take many hours to heat the oven to equilibrium. (Not an answer, but maybe a solution)
    – Chris H
    Commented Oct 18, 2018 at 12:27
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    My grandmother made natto by making her kids sleep with it under their feet at night, like a hot water bottle. That's old school Japanese. Japanese futons are rather thick and you sink into them so it traps body heat rather well. If you could attach it to your body, or place it under your arm so that it doesn't get away from you during the night, it should work sufficiently. Commented Oct 18, 2018 at 20:02
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    I've just found out that some people use yoghurt makers or other insulated boxes. I might try that. I've also got a batch going at room temperature. It's starting to smell good so I guess I'll find out if it works soon!
    – z0r
    Commented Oct 20, 2018 at 2:12

2 Answers 2


From my research, the problem with fermenting nattō at lower temperatures is that it doesn’t inhibit the growth of other bacteria which can either inhibit the growth of the target bacteria, bactilis subtilis, or potentially introduce and incubate pathogenic bacteria which can harm your health. Natto bacteria are very comfortable with high temperatures and you should aim for between 38 and 42 degrees for a full 20 hours.

Try looking at Natto Dad - he has a blog and YouTube videos which were very helpful to me. My second batch of nattō is currently fermenting.

  • Thanks! After experimenting a bit, I agree. I ended up buying a cheap electronic yoghurt incubator with a natto setting.
    – z0r
    Commented Apr 18, 2020 at 1:51

You could investigate the possibility of exchanging the oven light with a higher wattage, f.x. from 15 w to 25 w. Just might make the difference.

  • Does a 10W difference provide enough heat energy for fermentation? Would it not depend on the volume of the oven and length of time? This might be a good solution to the problem but it needs more detail to show how it actually solves the problem posed in the original question.
    – gnicko
    Commented Aug 25, 2022 at 13:36
  • If you have a new question, please ask it by clicking the Ask Question button. Include a link to this question if it helps provide context. - From Review
    – gnicko
    Commented Aug 25, 2022 at 13:37

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