This is a written companion to a video tutorial I made for my preferred method of culturing Koji Rice in an immersion circulator. The video contains a little bit of additional background information and troubleshooting, but this written guide should also be fully instructive on its own. This is by no means the only method for growing Koji at home, but it is my preference due to the level of temperature control.
Buy koji spores, they are easy enough to source online. There are multiple strains available, so buy one that closely matches the rice and final product you intend to make.
Soak your rice, preferably overnight, and if using a short or medium grain rice (such as glutinous rice or sushi rice) also thoroughly rinse the rice until there is no trace of exterior starch. The package of spores you buy should recommend the amount of rice to cook.
Steam your rice in small batches until slightly undercooked/al dente. The goal is fluffy and slightly chewy rice that doesn’t stick together. Even if you have a rice cooker that works well, it may not allow you the control to get the correct texture for koji.
Cool down the rice as each batch is completed. I recommend spreading it out onto trays as thinly as possible.
Once the rice has cooled down completely, you can sprinkle over your koji spores. If it’s easier you can also dilute the spores with rice flour so they are easier to distribute.
Now you can put the rice into one or two containers that will fit inside your sous vide basin. I use a standard hotel pan/steam table as my basin and a 6 inch deep half-size insert for my koji rice. If you are using this exact set up, I would not make more than about 2 kg of rice, because you don’t want the layer of rice to be too thick. In addition to standard sized inserts, any stainless steel pot/container that you can stably float inside your sous vide setup should work.
Ideal incubation temperatures for koji are between 28 and 32 Celsius. I was using a Joule immersion circulator and I found that my insert was fairly consistently 2°C below the water temperature. In which case I could have set my water temperature anywhere between 30-33°C to stay comfortably within ideal temperatures. If you are using a different immersion circulator, I recommend filling the containers you intend incubating the rice in with water and placing them inside an active circulator. You can then determine the approximate temperature difference between your circulator and containers floating in the water.
Once your containers of rice are secure, you want to cover them individually with foil and then cover your entire set up in foil to prevent evaporation.
You generally want to let the rice incubate for a total of 48 hours, but twice a day (ideally every 12 hours) you want to gently stir and mix the rice. This can be done either with a clean wooden spoon or gloved hands. If you’re making a large quantity of rice, it may be beneficial to spread the rice out onto some extra trays during the mixing.
After the full 48 hours you should have koji rice ready to use. As early as 24 hours into the incubation, you should notice a sweet pleasant smell and some noticeable fungal filaments in between the rice. The final fermented rice may have some white fuzz on it, which is completely normal. Even some traces of green mold aren’t overly concerning, Koji changes from white to green when it’s getting ready to emit spores.
Congratulations! You now have Koji Rice ready to be turned into a variety of products. You can make miso, shio Koji, sake, or more modern preparations such as koji assisted aging of meat, or various amino sauces and pastes. Specific procedures for all of those will come at a later date.