What are we trying to do while making koji?

OK, so what exactly are we trying to do when making koji? Well, to examine this we need to consider the role koji plays in Sake Brewing. In sake brewing we use koji to provide a wide variety of products. These include products that provide flavor and aroma elements as well as enzymes which degrade proteins and starches into smaller component parts. For example proteins are disassembled into peptides and amino acids while starches are converted into smaller starches, dextrins and sugars.

Rice starts out with 7% to 8% protein, but the higher polished the rice the less protein will be left. However, even with lower levels of milling we do not focus on koji’s production of enzymes to break down this protein. Rice starch is our main focus and needs to be broken down as effectively as possible into sugars. Koji produces alpha, beta and gamma amylase. Depending on how we culture the koji, we can emphasize protein or starch degrading enzymes. High temperature cultivation, 98°F to 104°F, lead to the production of saccharification enzymes whereas lower temperature cultivation, from 98°F down to 68°F, emphasizes protein degrading enzymes. So to make good koji for sake brewing we must culture the koji at the higher temperature range.

Another important aspect of cultivation is to ensure that koji mycelia reach deep into the grain. When the mycelia work hard to bore into the grain more saccharification enzymes are produced. The primary factor that can prevent the mycelia from boring into the grain is when the grain is too moist and the fungi can get all the moisture it needs from the surface. Under these conditions the fungi will produce a small amount of enzymes which will go to work right away in the moist environment on the surface producing sugar. With the readily available sugar for the fungus there is no need for it to push into the grain or produce more enzymes. Koji produced under these conditions and with these results is called Nurihaze koji.

Tsukihaze koji or koji with the mycelia reaching deep into the grain is produced with rice that has about 38% moisture after steaming and has been cooled such that the surface of each grain is relatively dry. Too dry and the koji-kin (spores) will not stick properly but beyond that it should be dry. Then, cultivating with moist air for the first day enables the koji to get a good start. The lower the seimai-buai, milling rate, of the rice the less humid the air should be. For example, a brown rice would be safer in a more humid environment.  On the second day, moving to dryer conditions helps the koji mycelia to move into the grain.

A koji that is between Nurihaze and Tsukihaze koji is Sohaze koji which is more like Tsukihaze in that the entire surface of the grain is fully covered but unlike Tsukihaze the mycelia do not penetrate very far into the grain. For this reason, Sohaze koji has less saccharification power and is used in circumstances where there is less need to produce sugars; for example, the moto.

Of the three amylase enzymes produced alpha and gamma are the most prevalent but because of the low pH of the mash alpha amylase has a very low activity level. Alpha amylase is well suited for a pH of 5.5 but a sake mash will be closer to a pH of 3. Gama amylase on the other hand thrives at a pH of 3 and produces glucose. The very low temperature of the mash inhibits all of the enzymes which slows the rate of sugar production to a trickle. The slow rate of sugar being added to the mash helps the yeast stay healthy longer than in fermentation where there is a high concentration of sugar at the start. A high concentration of sugar increases the osmotic stress on the yeast cell walls. This increased osmotic pressure causes the yeast to shut down sooner than they might with lower levels. It is, this characteristic of the sake brewing process that allows sake to reach the high alcohol levels it does.

Rice Prep…

Rice preparation consists of washing, rinsing, soaking, draining, packing, steaming, cooling and finally inoculating and then incubating. Let’s cover each of these steps separately.

White rice, whether milled as table rice or milled to a higher degree specifically for brewing sake, has unwanted material on it that we need to remove. This material can simply be the rice flour (Nuka) from the milling process. However, if we are using table rice talc may be added to assist with milling and other materials are often added to enhance or enrich the final product. The later often include several of the following: Folic Acid, Niacin, Iron, Zinc, Selenium and Vitamins B-1, B-12 and E. Washing the rice in cold water removes these particulates. This will remove iron that contributes to bad taste development in sake and surface powders that can cause the steamed rice to be stickier than we would like.

Once thoroughly washed the rice should be rinsed in cold running water to rinse away the particulates that have been washed free. In some cases you may choose to do both the wash and rinse in a single step with cold running water. You know you are done when the water that starts with milky white run off turns clear.

Now that we have clean milled rice we want to raise its water content up to 25-35%. Commercial brewers are very specific about how much water they want the rice to absorb. In fact they have different amounts for their various styles and whether the rice will be used for making koji or not. In some cases brewers use a stop watch to make sure the rice does not soak for too long and take up too much water. Anyway, soaking the rice in cold water is the method used. The time needed to reach the desired water up-take level depends on the temperature of the water as well as the type and milling rate (seimai-buai) of the rice.

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 homebrew 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.

We will not need to be as particular as Commercial brewers about the amount of up-take but rather shoot for the general ball park. To hit this ball park using rice with a 60% seimai-buai soak the rice in cold water for one hour. If you are using rice milled as table rice, seimai-buai 90 to 93%, soak for two hours. While these times are close to best, the additional water up-take from soaking for much longer has only a small impact on the sake. For this reason, some homebrewers soak their rice over night to get an early start on steaming in the morning.

As you get better at making koji you may want to be more exacting in the water up-take level. To do this you can add an experiment for the particular type of rice you use. Wash and rinse some rice and divide it into 5 equal portions. Weigh and record the results. Place all portions into cold water to soak. At 30 minute increments remove one portion, thoroughly drain and weigh. The percentage water up-take is the final weight divided by the starting weight minus 1 with the result multiplied by 100. Plotting these five points against their soak time will give you a curve you can used to determine how long you should soak your rice for the up-take you desire.

%water up-take =  ( ((Final weight) / (Starting weight))  - 1 ) * 100

Getting back to the topic at hand, once the rice has soaked for the period we want it is time to start draining the rice. Leaving the rice sit in a colander, strainer or sieve for about an hour will do the trick. The drier the rice the less sticky it will be and the easier to remove all the lumps after steaming. Once nicely drained it is time to prepare for steaming the rice.

To steam the rice we want to ensure the steam has to rise up through our rice to get out of the pot. We control this with the proper packing of our drained rice into our steamer. We want to have an even layer of rice that covers the entire steamer base (or each level of the steamer we will use). Ensure the rice evenly covers the base out all the way to the walls of the steamer so no steam can find a quick path to escape without going through the rice bed. Laying down a layer of cheese cloth or butter muslin before packing the rice will ease clean-up. With the rice packed into the steamer we are ready to steam.

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. In general, steam the rice for 45 minutes. Be sure to check the water level in your steamer about half way through to make sure you have enough water. Running dry can destroy your steamer, smoke your rice or both.

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. The washing, rinsing and draining steps help here; by removing the outer starchy coating on the rice there is less sticky surface after steaming. This helps with cooling and surface drying and exposes more of the surface area for each grain for inoculation. 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 for the first 24 hours. 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. A heating pad or hot water bottles can also be used to warm the environment. 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. By this time you should remove the glass of water so the incubation environment becomes less humid.  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 110F you will want to perform these more regularly in order to better regulate the temperature.

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?

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