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I've been making sourdough and kombucha for a while now, and the results are great. I've never taken any particular care with the utensils. I wash my hands before handling the SCOBY, and I use fresh tap water for it and for the sourdough, but that's about it. No boiled water, no sterilised jars and bowls. So far so good.

Now I'm thinking of making nattō. All of the recipes I've seen say several times to sterilise all the utensils. I'm skeptical of this, since nattō has been made for almost 1000 years and I doubt people would have been able to achieve the level of cleanliness asked for by these recipes. Indeed it used to be made in bundles of straw, which should have had plenty of bacteria in it other than nattō-kin.

So, what is the real risk of food poisoning when making nattō? Can anyone share their experience of making it without sterilising everything?

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Your question has two facets.

I’ll answer the first one. While brewing beer or wine as well, you’re advised to sterilize all the equipment and use a specific culture of bacteria. The reason for that is, if the “stuff” you’re trying to ferment gets some wild yeast/mold/bacteria, your end results won’t be what you desire. So you’re trying to minimize that risk by sterilizing it.

I believe people with relevant food-safety experince will better answer your second question on safety of fermentation. As I will leave it unanswered. Even though I also think in similar terms with you.

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The whole point is that nobody is able to give you a meaningful prediction model for food poisoning, or else food safety would be much easier. The models which exist and are used by the FDA to set up food safety rules are as complex as weather prediction models. "Used for 1000 years" is also completely unrelated to modern food safety, during most of these 1000 years people just accepted that entire villages get wiped by a typhus epidemic now and then.

It is impossible to prove a negative, but I find it unlikely that someone has done empirical studies on food poisoning resulting specifically from natto fermented under nonsterile conditions. That would be quite expensive, and funding bodies prefer to give money for research questions connected to more pressing problems. Also, a quick search on Google scholar didn't turn up anything even remotely related. In the absence of such empirically established numbers, it is impossible to give an answer which literally addresses your question.

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