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. And in the end the “art” of incubation 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 the jungle of Indonesia to make tempeh cakes.

Here in the north east, in Pennsylvania, especially as I write this in December, I have to create my own little microclimate.

The concept of incubating, or growing mold, should be second nature to all of us. Our forgetful human nature makes us great at it. Assuming we have a food source to provide nutrients, the two factors we’re aiming for is moisture and heat. And if you think about it, since we’re masters at growing mold in our refrigerators, moisture is the standout factor. The temperature most often will affect the rate of growth. So our primary concern is a vessel to hold 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 is the super easy I’ve been doing it for years I use with minimal gagetry. It works great for me 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.

I’ve accumilated a variety 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. Almost any cooler out there should be solid enough at holding temperature as well as retaining humidity. It’s also nice having lids that can lift easily so you can open them slightly for some airflow.

After cleaning out the cooler to keep down any microbial competition (which I’ve never had trouble with despite skipping this step on occasion) 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 constant changes. If the humidity dips then the mold culture may dry out but probably not to any consequence as long as we’re on top if it. Temperature drop will slow it down but same thing, no big deal.

At the point once the cooler is loaded up with inocculated goods I’ll 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 innocculated 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 envrionment 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. Sometimes I can feel my arm hairs curl and I know I’m good on humidity.

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 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 versitility. 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.