Koji is a cereal grain (like rice or barley) that has been inoculated with the mold Aspergillus oryzae. As this mold (whose family members include molds that form on bread and cheese) consumes the starches in the rice, it creates powerful enzymes. These enzymes then have a dramatic effect on the flavor of foods that koji is added to. The most well-known role of koji is its part in making miso, a fermented bean paste.
If you’ve made it this far, chances are you have already made miso once or twice, and are now looking to go deeper by creating your own koji at home!
To make koji at home, you need the Aspergillus oryzae spores (called tane koji or koji-kin), and an incubating system that can maintain the right temperature and humidity levels for up to 48 hours. You can order tane koji online (GEM cultures is very reputable, and sells great starters from Japan.)
Koji Nursery (aka Incubator)
Time, temperature, humidity and terroir are all important factors to modulate in order to successfully make koji. Learn how to build a multi-purpose homemade incubator here. You can also fashion an incubator from an oven, like I do when making tempeh. I’ve even made koji completely outdoors using a haybox cooker.
Recipe & Technique
- Steaming baskets or trays
- Cedar or unfinished wooden tray, or baking dish at least 13"x 9" with a 2 inch lip
- two clean dish towels
- 400g (about 1 lb.) dry white rice (a long grain variety like jasmine) or pearled barley
- 5 grams koji starter (koji mixed with rice) OR 1 gram pure koji spores
- Place the grain in 3 times the volume of clean filtered water.
- Soak at least 6 hours, or overnight.
- If using rice, drain well, then rinse several times until the rinse water is clear.
- Prepare your steamer. Add steamer baskets or tray to a large pot. Add enough water to the bottom so it is not touching the bottom of the steamer tray. Add dish towels to basket and cover with lid.
- Set pot on medium-high heat, and steam the towels for 20 minutes.
- With tongs, carefully remove one of the dish towels and set aside. Drape the other one inside the steamer basket.
- Add rinsed and drained grains to steamer. Spread evenly so steam can penetrate the entire mass of grain.
- Steam at least 45 minutes, or until the grains are al dente. You may need to add more water to ensure there's enough steam. Don't let it run dry!
- While grains are steaming, prepare tray. Drape the steamed, cooled dish towel in the tray. There should be enough extra towel so you can completely wrap the bed of grains on all sides.
- Once properly steamed, carefully pour rice into a bed in the tray in a 1 inch (2.5cm) layer, and let cool down to 90°F/32°C.
- Once cooler, gently and evenly sprinkle ⅓ of the tane koji over the bed. Stir the grains well. Repeat with another ⅓, stirring well, and finally the remaining ⅓. Stir for a total of 3-4 minutes to thoroughly incorporate the koji spores.
- If you are using the pure spores, you can use a flour sifter to gently disperse the spores. They will seem lighter than air, so observe carefully as the spores fall onto the rice bed.
- Cover the grain bed with the towel. If you need to, you can also use the towel you used to steam them. Just ensure that you first wring the steaming towel out first to remove excess water.
- Add to your incubating chamber. Ideal temperature range during the first 24 hours is 86° to 91°F/30° to 33°C, with 75% humidity.
- After 24 hours, you should start to see signs of the mycelium forming all over the garins, and it should be fairly fragrant.
- Break up any clumps or mats, then create several lengthwise furrows in the bed to create more surface area for those oxygen-loving microbes.
- Check again at 36 hours, ensuring the temperature doesn't climb too high.
- Koji should be finished between 40 and 48 hours. If it begins to turn splotchy colors like dark olive, yellow, it has begun to sporulate (possibly because it was too warm). Make a decision as to whether enough mycelium has formed if this occurs.
- Remove tray from incubator, then allow to cool down. Break up any clumps to get to as close to individual grains as possible.
- Use koji immediately, or refrigerate in a tightly sealed container for up to 3 days in the refrigerator.
Propagating your own Koji spores
When you’re first learning how koji is made, you may miss that sweet spot, when the koji has mostly myceliated (built up its body, white in color), but before it sporulates. Sporulation is simply the koji mold completing its life cycle. During this phase, spores are created, to spawn the next generation.
You can take advantage of this fact, and purposefully let a portion of a batch of koji go to spore. Then you capture the spores so you have tane koji for the next batch.
To sporulate koji, break up any clumps, and spread out into a single layer on the bed. Cover with a damp towel, and let it culture another 36 hours or so (in addition to the original 48 hours). When koji sporulates, it will be colored (dark green, blue, or grey). When it has fully sporulated, you will notice fine clouds of spores when you jostle the tray.
At this point, it’s time to dry the grains for storage. Remove the damp cloth, and wipe out any residual moisture from the chamber. Increase the airflow in the chamber (i.e. by opening the chamber door a bit more), and let sit another 48 hours. Then pack into an airtight container and store in a cool, dark place or in the freezer.
If at first you don’t succeed, keep at it! It takes a good bit of practice to successfully make koji at home.
Two of my favorite books on the subject:
- This amazing new book called Koji Alchemy* by Shih and Umansky dives deep into the world of koji. Its authors show you how to utilize this ingredient and its powerful enzymatic action to enhance lots of dishes, from chocolate chip cookies to dry-aged steak!
- The Noma Guide to Fermentation by Redzepi and Zilber gives a great overview and technique on making your own koji, as well as some non-traditional makes of miso
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