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
    Oct 18 '18 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. Oct 18 '18 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
    Oct 20 '18 at 2:12

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
    Apr 18 '20 at 1:51

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