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 straightforward. 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.
Options for Koji Incubation
Koji needs to culture at 90 F (30C) for 48 hours. Here are a few options for keeping your koji kin warm enough.
- 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, tempeh, everything!
- A dehydrator or slow cooker set to 30C is also a great option.
- Alternatively, you can try to grow your koji in the warmest location possible in your house. Try 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 up 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
If you’ve made koji rice or barley, then making koji-kin is easy. All you need to do is culture the koji mold until it spores.
- Allow the koji-kin to culture for more than 48 hours. You will know that the koji-kin has spored when your koji turns from white to greenish-grey. The green color is mold spores.
- Once the koji is green, remove the damp towel, and allow the koji rice to dry out in the incubator.
- After the rice has completely dried out, store it in the freezer.
- When using homemade koji kin to make koji rice, you only want to use the green powdered mold, not the rice. So sift the mold off of the rice with a fine mesh strainer before using it.
Homemade Koji Rice
Koji rice (and koji barley) is used to make miso, sake, amasake, rice vinegar, soy sauce, and mirin. Here’s how to use koji kin to make koji rice and koji barley. See the sections above for information on incubation options and how to save your own koji kin.
- Prep Time: 15 minutes
- Cook Time: 15 minutes
- Total Time: 30 minutes
- Yield: 4 cups 1x
- Method: Fermented
- Cuisine: Japanese
- Diet: Vegan
- 2 cups rice (white or polished brown)
- 1/4 tsp koji-kin culture (see notes)
- Rinse the rice until the water runs clear (to remove all the starch).
- Soak the rice in water for 8-12 hours.
- Steam (not boil) the rice until it has softened. See notes below for my setup.
- Cool the rice to room temperature.
- Thoroughly mix the koji-kin culture into the rice.
- Spread the rice out in a baking dish. Cover with a damp tea towel (to keep it moist but not wet) and maintain it at 90 F (30 C) for 48 hours. See the section above for incubation options.
- Stir the rice every 12 hours to break up the clumps and evenly distribute moisture. After 48 hours, white mold fibers should have started to develop. Stop incubating at this point or the mold will spore.
- Store koji rice in the freezer until you are ready to use it to make miso, sake, or other koji ferments.
- See the section above for details on how to save koji-kin mold spores for future batches of koji rice.
- Since this is a mold ferment it’s important to keep everything clean and sanitized.
- For koji barley, use pearled barley instead of rice and follow the same procedure. Note: I haven’t personally tried this as I’m gluten-free.
- For steaming, I recommend using a vegetable steamer or colander lined with a tea towel. Just boil the tea towel to sanitize it before using it to steam your rice.
- Make sure you buy your koji kin from a reputable source. I recently heard from a reader who bought koji kin that was contaminated with a poisonous mold. It kept sporing with grey mold after just 24 hours. So if the recipe doesn’t seem to be working, it could be because the koji kin is contaminated.
Keywords: koji kin, koji barley, vegan, gluten free, sake, miso, soy free, nut free