In order to make koji for brewing sake we need to go through the same steps that we do to prepare rice for brewing sake. In fact, it is often the case that a portion of the rice goes directly into the brewing sake while another portion is used for koji. So, as always we begin by washing the rice. This is to remove all contaminates, including the powder (Nuka) that remains after milling and any material added by the miller, like iron. Removing the nuka lowers the stickiness we will have after steaming. And, as mentioned elsewhere, iron is very bad for sake as it darkens the sake and speeds a reaction of residual sugars with amino acids that harms the flavors and aromas.
Washing the rice is followed by steeping the rice until we reach the desired water uptake level, usually between 25% and 35%. If the uptake of water is too little the rice will not steam properly leaving a hard uncooked center that the koji mold will not penetrate. On the other hand having too much water uptake will cause the rice to be too mushy and sticky after steaming. This results in a base that is too easy for the koji mold to penetrate and this prevents or lowers the production of transformative enzymes we want. Most rice used for hombrew sake will need to steep between 30 minutes and two hours. As homebrewers we are more often than not more guilty of soaking too much than too little.
Following the steeping, we first need to thoroughly drain the rice. The drier the rice the less sticky it will be and the easier to remove all the lumps after steaming. Having drained the rice, we steam the rice for somewhere between 30 minutes and one hour. To tell when the rice has been steaming for long enough, examine a rice kernel, cut it in half, it should be uniformly gelatinized. If you bit into it, it should have a consistent resistance all the way through; firm but not hard.
Once rice steaming is complete; we remove the rice from the steamer and spread it out for cooling. A good place to do this is on a cookie sheet or similar pan with lots of bottom surface area. All the clumps should be broken up. This helps with cooling and exposes more of the surface area for each grain. After you have the clumps broken apart it is time to prepare the koji starter. Koji starter, koji-kin, Aspergillus Oryzae, all names for the same thing, are usually provided as a powder containing the spores that will inoculate the newly steamed rice. Only a small amount of this powder is needed. Because of the relatively small amount of powder, it can be difficult to spread it evenly over all the rice.
A trick I learned from the people at Gem Cultures is to take a small amount of flour and toast it in a dry pan until it is lightly toasted. We can then mix out lightly toasted flour with the koji-kin to double the amount of powder. This makes it much easier to evenly distribute the powder over the rice for a uniform inoculation. The toasting is, in effect, sanitizing the flour to make it safe for use. Without toasting there is a danger that bugs present in the flour could get a foot hold in the koji and cause the batch to go bad. Whether you add the flour or not is up to you since it is just to make it easier.
While the rice is still warm but less than 115F, maybe around 100F is good, sprinkle some of the koji-kin over the rice; about 1/3rd of the total powder. Mix the rice and koji-kin well and spread out again. Repeat by sprinkling the next 1/3rd of the powder and mixing well until all the koji-kin has been mixed very well with the rice. As much as possible we want to cover every grain of rice with a bit of koji-kin. Wrap the mixture in a tightly woven cloth that will breathe but not stick to the rice. At this point we are ready to place the inoculated rice into a warm and humid place where it can grow.
We want an environment between 90F and 95F that is pretty humid. An ideal way to create this environment is to use a picnic cooler. These are insulated, cheap and come in lots of sizes. The one I use even has a mechanism (some ICs) for heating and cooling. Pre-warm and humidify the environment (could start about the same time as steaming) so the rice balled up in cloth will go into a friendly place. An open glass of warm water is plenty to keep the environment humid.
Place a temperature probe in your rice and place in the pre-warmed cooler. A second temperature probe to monitor the environment temperature is ideal but not required. Check the temperature regularly to ensure the rice reaches and remains in the range of 90F to 95F. At about 12 hour intervals, open the bundle to check and stir the koji. It may have signs of the white fussy mold by the second check (24 hours) or not. After 36 hours the koji will be producing its own heat to the point that you will need to be more diligent about temperature control. Begin 2-3 hour interval checks where you open the bundle and stir the koji. If the koji is reaching temperatures above 100F you will want to perform these more regularly in order to keep the temperature from exceeding 110F for any significant period of time.
From 40 to 54 hours the koji should complete its process. While stirring the koji, you can begin to check it for completeness. Cut a grain of koji in half to see how far the koji mold has penetrated the rice kernel. It should cover about half the diameter of the kernel. Once complete the koji should be spread out and cooled to room temperature. Once at room temperature, koji can be bagged or placed in some container to be refrigerated or frozen depending on how soon you want to use it. If storing for a month or more, freezing is the way to go.
And there it is, are you ready to give it a try?
Update 11-14-10: I was at SakeOne last Tuesday and learned a little piece of new information that slipped my notice before. This was the first time I was there while they were actually incubating koji. They happened to be in the second half of the process. I went into the room; very close to 100F but dry! That seemed strange to me. When I asked Greg about this he said that the first half you want the room to be humid (does not need to be 90%) but not the second half. If the room is humid during the second half the koji will have plenty of moisture and will just increase the mass of fussy mold on the surface. However, if the room dries out the mold will have to go into the heart of the rice to find moisture. This will mean the mold will have to produce more enzymes to get to the moisture, which is just what we want; lots of enzymes. I see John Gauntner also makes this point in his course.
They set the room to 92F for the first half with lots of moisture. They then move the koji to a second room with low moisture and run a schedule slowly uping the temperature in the second half to about 99F. As the temperature goes to high or low they blow the corrected temp air through the batch to bring it into the desired range and aerate it.
Summary of Equipment used / needed:
- Wok and bamboo steamer
- Bowl for washing and steeping rice
- Flat bottom pan for cooling and mixing
- Cloth with tight weave for wrapping
- Cooler to function as an incubator
- Thermometer (I like those with a probe that can be in the rice while the monitor is outside and easily visible)
- Temperature control, one of:
- Controller with heater
- Jars or bags that can be filled with hot water to control the temperature
- Heating pad