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!
Acquiring Koji kin (spores)
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.)
There are generally two ways you will find tane koji. The first is pre-mixed with rice flour (for most people making it at the home scale). The other, which are pure spores, are more for commercial-scale koji making. The latter is much less expensive than the former, however, it is very difficult to work directly with pure spores. They are generally lighter than air, and unless you have a micro-gram scale, you may not be able to measure the right amount at the home scale. One gram of pure spores makes 11 pounds (5 Kg) of koji! The package will advise you to mix pure spores with rice flour to make it easier to handle. This is good advice,
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
Rice or Barley Koji
- Steaming baskets or trays
- Cedar or unfinished wooden tray or baking dish at least 13"x 9" with a 2 inch lip
- 2 clean dish towels
- 400 g (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
Soak & Prepare Grains
- Place the grains 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 grains.
- Steam grains around 45 minutes, or until they are al dente. You may need to add more water halfway through to ensure there's enough steam. Don't let your pot run dry!
- While the grains are steaming, prepare incubation tray bed. 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 finished steaming, carefully pour grains into the towel in the tray in a 1 inch (2.5cm) layer, and let cool down to 90°F/32°C.
Inoculate and Culture
- Once grains have cooled, gently and evenly sprinkle 1/3 of the tane koji over the bed. Stir the grains well. Repeat with another 1/3, stirring well, and finally the remaining 1/3. 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 needed to cover the bed, you can also use the towel you used to steam them. Just ensure that you wring out the steaming towel first to remove excess water.
- Add tray(s) to your incubating chamber. Ideal temperature range during the first 24 hours is 86° to 91°F/30° to 33°C, with 75% humidity. If you have a temperature probe, ensure it is clean, then stick it in the middle center of the grains bed.
- After 24 hours, you should see signs of the mycelium beginning to form all over the grains, and it should be fairly fragrant.
- Break up any clumps or mats to encourage more mycelium to form, then create several lengthwise furrows in the bed, which will provide more surface area for those oxygen-loving microbes to form mycelium.
- Check again at 36 hours, ensuring the temperature doesn't climb too high. The mass of koji will begin generating its own heat, so you may even be able to lower or turn off the heat source, so it doesn't overheat.
- 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 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
Here’s an interesting video about how Miso Master, one of the top U.S. miso brands, makes their koji at commercial scale. Koji and miso discussion starts at 32:30
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