For the second edition of making natto, I traded my homemade incubator (styrofoam cooler with a light bulb) for a slow cooker. You can read about that in the post How to Ferment Natto. This one didn’t go so well.
Natto needs a stable temperature between 100-113 F for optimal fermentation. Slow cookers, even on the warm setting, will get too hot with the lid closed. So I placed a dish towel over the slow cooker and set the control to WARM.
At first it was losing too much heat, so I added a second dish towel. This seemed to be working. It was keeping a perfect temperature of 106 F for the first 8 hours. Then I went to sleep. When I woke up and checked on the natto, the temperature had jumped to 131 F. I was able to get the temperature back down to a safe range for the last few hours, but the damage was done.
Natto that exceeds the optimal temperature range doesn’t have the slimy gooey texture that makes it natto. It tastes rather bland and boring. I’m thinking that the high temperatures either killed or greatly damaged the natto culture.
You can see that the natto started at 106 F and then spiked to 131 F. Not sure if the humidity plays a role in natto fermentation, but you can see it got to 99% under the dish towels. Not sure why it spiked. I’m guessing that the water bath evaporated and this made the inside of the slow cooker hotter. This is an old slow cooker I got at a yard sale for $5.
I suppose at this point, I could pimp out my slow cooker by adding some electronics to get an optimal temperature, but I have zero skills in electronics. Plus I already have that working incubator method. But I do like the elegance of using the slow cooker.