Simple Incubator for Ferments


Having a good grasp on incubation is very helpful when making mold type ferments like tempeh and koji, or for reproducing your own spores for future use. Putting together a simple incubator doesn’t require much more than what you probably already have lying around the house.

Tempeh made in a small Igloo “playmate” cooler.

In some regions of the world your home environment, as is, could be naturally suited for making a specific product, from hams hung in the mounts in Spain to beans wrapped in banana leafs in Indonesia to make tempeh cakes.

Here in the north east, in Pennsylvania, especially as I write this in December, we have to create our own tiny microclimate.

The concept of incubating, or growing mold, is second nature to all of us as our forgetful human nature makes us great at it. Assuming we have a food source to provide nutrients, the two factors we’re looking for is moisture and heat. The temperature generally affects the rate of growth, while different temperatures can have nuance affect on the prevalent microbes. The primary concern is a vessel that holds humidity.

The first incubator I was ever introduced to was a plastic clamshell container with fresh strawberries by a seventh grade science teacher used to teach us about mold. The second was an old broken refrigerator outside of Sandor Katz’s house that he lugged inside and put a lightblub in for warmth, which was my first ever experience in making tempeh. So it can be that easy.

Here what we’ve been doing for years with minimal gadgetry. It works great and I’ve never felt the need to improve on it.

A Low-Fi Cooler Method

After trying out every incubating technique I could, (my fridge experience ended in a gnarly fly infestation) I settled on using a regular good old Igloo cooler, which come in enough sizes for anyone to customize their experience. I’ve accumulated a bunch of shapes and sizes over the years, so either whatever you have or whatever you prefer. I suppose you’ll want to match the size of your cooler with the size of your ambition.

After a quick cleaning I’ll make a pot of very hot water and dump it in the cooler and close the lid as I prepare whatever ferment I’m about to incubate. What we’re doing here is essentially preheating the incubation environment so it’s warmed up when we start incubating. Once it’s time to load in I’ll dump out the resting water.

Without any fancy technology we’ll be dealing with some fluctuations in humidity and temperature but that’s okay. If you think about it the natural environments these molds were originally cultivated in had its own fluctuations. If the humidity dips then the mold culture may dry out but probably not to any consequence as long as you’re keeping an eye on it. Temperature drop will slow it down but same thing, no big deal.

At the point once the cooler is loaded up with inoculated goods boil some water into a mason jar or two. If the space is tight I’ll wrap some kitchen towels around it to avoid cooking any spores. I put it in without a lid at first and this does great for kicking off the humidity, while lifting the temperature.

Rice inoculated with koji about to leave on its tropical vacation!

Typically I’ll reboil the water every morning and every evening if it seems necessary. Sometimes it’s not and this will depend on the holding ability of the cooler your using, the season, and the weather.

Basically the environment we’re aiming for, without any fancy measurement tools, is warm and gooey, tropical-like. Feel it with your hand and ask yourself if mold would be happy in there. That’s the ticket.

Though I’ve never had any major problems be cautious of too much humidity, as it can make for a sloppy incubating medium. Lifting the lid and taking a peak once of twice a day could be advantageous. If there’s a lot of condensation building on the interior it may be due to air out for a few minutes. I like to leave the cooler lid ajar for a moment or two to allow for some airflow while incubating.

One optional technological modification

Certainly there are more precise ways to accomplish incubating too. My favorite tool for temperature regulation is a handy temperature controller. It’s probably the fanciest piece of technology I allow myself to use for ferments and it’s something I fully endorse for its versatility. I picked up a simple lamp from the thrift store and use its light as a heat source. When attached to the temperature controller it will turn on when the temperature goes below a preset number and turn off a few ticks above. With this helpful tool I really don’t have to do any maintenance during incubation. Here’s a nice affordable one.