Koji is a mold that is at the heart of many different Japanese ferments. It is used to make miso, sake, amazake, rice vinegar, soy sauce and mirin. Koji is grown on rice or barley, which are then used as the starter culture for further fermentation.
Making homemade koji is quite straight forward. The most difficult part of making homemade koji is finding the koji spores (koji-kin). You may be able to find koji-kin in your local Japanese grocery store, or you can buy it online. Just make sure that it’s koji-kin, not koji rice.
Once you have koji spores in your freezer, making koji rice or barley is fairly simple.
- 2 cups rice (white, or polished brown)
- ¼ tsp koji-kin culture
- Rinse your rice until the water runs clear (to remove all the starch).
- Soak your rice covered in water for 8-12 hours.
- Steam (not boil) the rice until it's softened, but still sticky (see notes for advice).
- Cool rice to room temperature.
- Thoroughly mix in the culture.
- Spread it out in a baking dish (to maintain the right amount of moisture). Cover with a damp cloth and maintain at 90 F (30 C) for 48 hours.
- Stir every 12 hours to break up the clumps and evenly distribute moisture. It is finished after 48 hours, when white mold fibers start to develop. (Don't let it go longer than that or it will spore).
- Store in the freezer until you are ready to use it.
- See below for incubation options and to learn how to save your own koji-kin mold spores for future batches.
-Since this is a longer ferment it’s important to keep everything sterile.
-You can also used pearled barley, and follow the same procedure to make barely koji.
-For steaming, I recommend using a vegetable steamer or colander lined with a tea towel. Just boil the tea towel in the steamer to sterilize it before using it to steam your rice.
Options for Koji Incubation
- I have a Brød & Taylor Bread Proofer & Yogurt Maker which I use for all my heated ferments. If you plan on doing a lot of fermenting, then I recommend making the investment. I use it for yogurt, cheese, sourdough starter and tempeh.
- Using a dehydrator or slow cooker that can be set to 30C is also a great option.
- Alternatively, you can try to grow your koji in the warmest location possible in your house. Perhaps in the oven with the light on, near a radiator or a hot water heater. The only trick is to measure the temperature as you go along, because koji mold will start to heat as it ferments. The right temperature is important because if it’s too cold your mold won’t grow, and if it’s too hot you will kill the spores.
How to Save Koji Mold Spores
Making koji-kin is easy. All you need to do is allow the koji mold to continue growing until it spores, then collect those spores. You will know that the koji-kin has spored when your koji turns from white to green. The green colour is the mold spores. Once your koji is green, then remove the damp towel, and allow the koji to dry out.
When the koji-kin has dried out, store it in the freezer for future batches of koji. When culturing future batches of koji, you only want to use the green powdered mold, not the rice, so sift the mold off of the rice before use.