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The Fridge, Your Inoculation Room

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tags icon bread fermentation

Sourdough likes 24 °C—the desired dough temperature. This temperature is very difficult to maintain in a normal Belgian kitchen. Bakers usually calculate the water temperature (l’eau de coulage) they require before adding it to the mixture, based on the ambient room and flour temperature. However, keeping the dough at that point isn’t easy: it cools down rapidly on my marble kitchen workspace. Another reason to work with 2 kg of flour: more mass cools down less rapidly.

A better solution than putting the rising dough in a “warm spot”—I hate cookbooks that give rough indications such as that one—is to constantly influence the room temperature within a closed environment: with a simple lamp and a thermostat.

A probe, a spot, and a fan, sitting in a tr-no, wait, a fridge?

That closed environment was easy enough to find, and is well-isolated. Professional bakers have large walk-in rooms where carts full of sleepy dough can be pushed into. Both the temperature and the humidity of these rooms is configurable. I proof my dough overnight in my “regular” fridge, at 6 °C. We don’t eat meat so lower fridge temperatures aren’t needed. This is called a retarded fermentation step: as the temperature drops, the yeasts become less active, giving the bacteria a chance to develop flavor (acetic and lactic acids). Bread baking takes up to 36 hours here, although many of those hours are just spent in waiting mode.

Converting a spare fridge into a machine that doesn’t cool but keeps the temperature just above your average room temperature insures easily reproducible bread instead of having to deal with fluctuating temperatures. In my case, I mounted a 40 W spotlight on a thermostat that can be configured once it’s activated. The black wire in the photo above is the temperature probe of that thermostat.

A universal thermostat adapter plug that can monitor from -40 °C to 99 °C can be bought online, such as this one on Conrad. As not to waste too much energy, and because the fridge is quite large for a single lamp, I made sure the heat circulates somewhat by adding a simple 12 V computer fan at the back:

The back of the simple contraption.

The thermostat with two plugs: fan and lamp.

In order to have the possibility to switch off the fan completely, two separate plugs are provided: one representing the fan and one representing the spotlight. By the way, the spot is in a super simple fixture that we had mounted in our previous house for which there was no longer any space.

It is best to let bread rise in a humid environment without too much draft, because otherwise the dough can become crusty. If you’re incubating other foods, such as tempeh (fermented soybeans), you do want a draft: hence the extra plug.

The only problem left to solve is getting the humidity level just right. Putting a pitcher of boiling water on the bottom always helps, especially in combination with the heat of the spot. In practice, since I like working with quite wet bread dough, I almost never bother.

It is very easy to turn an abandoned refrigerator into a trusty inoculation room with a minimum of material. By the way, I don’t recommend anyone to make a wooden box around the spotlight and the fan, as in the photo. We have done this to prevent burning, but the wood naturally molds at the bottom because of the moisture!

Inoculated soybeans.

26 hours later at 32 °C: soybeans turned into tempeh!

Homemade tempeh is also much tastier than the pre-packaged ones: it has a very strong aroma and it allows for endless variations (e.g. use other beans, let it ferment longer or shorter, …). Make as much or as little as you want. In my case this was 1.5 kg of beans for 3 EUR, excluding electricity costs. After 20 minutes, the incubation chamber is ready to go (at 32 °C, coming from room temperature, which was a mere 19 °C at that point). The Rhizopus oligosporus fungus itself generates a relatively large amount of heat after incubation for 14 hours: after 20 hours the thermostat consistently measured 34.5 °C, while I had set the maximum at 32 °C.

I’ve had much success preparing both tempeh and koji, which is Japanese fermented rice using another fungus called Aspergillus oryzae. Buying fresh koji from an Asian supermarket is nearly impossible in my area, so what better way to solve this than to simply make it yourself? The koji disappears in things like amazake, miso, and shio koji. That’s right, I make my own miso. From scratch. Thanks to this fridge hack.

And now you can too!

Originally published in Dutch on

I'm Wouter Groeneveld, a Brain Baker, and I love the smell of freshly baked thoughts (and bread) in the morning. I sometimes convince others to bake their brain (and bread) too.

If you found this article amusing and/or helpful, you can buy me a coffee - although I'm more of a tea fan myself. I also like to hear your feedback via Mastodon or e-mail. Thanks!