Closeup of Sake Production: Koji Making

Greetings Sake Lovers, welcome to another KURAND Magazine article that introduces you to the world of sake.

So far in this series taking you through sake production process, KURAND Magazine has looked at how the brewer prepares the rice, polishing, washing, soaking and steaming it. A batch of that steamed rice is now taken to a special room turned into something called koji. Koji making is essentially the malting stage in the process, just like that required to make beer.

Sake Making in a Nutshell

A very basic diagram of the sake brewing flow can be found in the chart above. This is just a very basic outline. The process varies depending on the variety of rice and the target style of sake.

Making Koji

The Japanese word for making koji is seigiku. In the 1,2,3s of sake making, seigiku is number one. It is no exaggeration to say that good sake begins in the koji room. In beer making, the barley is malted by germinating the barley bud, thus turning it into a plant. As the plant grows it unlocks its energy stores by releasing enzymes which break down the starch and convert it into glucose. Rice can do the same thing. However, due to the fact that the germ — found in the outer part of the rice grain — has been polished away, brewers cannot use the same technique to malt the rice. Instead, they use a mould called koji. Rather confusingly, perhaps, koji is the name of the mould, but it is also the name of the malted rice. The brewer starts by cooling the steamed rice down from 40 degrees to around 32 degrees. 40 degrees is too hot for making koji and dangerous for the brewers to handle. When the temperature is right, the rice is transferred to a special room which is built with a special insulated design to retain humidity and heat.

Koji Making Process

The process can be broken down into 6 stages: tanekoji / tokomomi, kirikaeshi, mori, naka-shigoto, shimai-shigoto, and dekoji.

Tanekoji & Tokomomi

The brewer lays out the steamed rice onto a large table called a toko or bed and sprinkles spores of the mould onto it using a shaker. Feeding on the moisture in the rice grain, the mould starts to grow instantly, spreading its small feeding tubes into the grain and secreting enzymes that cut the starch chains up like a pair of scissors. Once the koji has been applied, the brewer mixes the rice to ensure all the rice is uniformly covered by the mould. The rice is brought together in the middle of the table to form a large mound. It is then covered with a cloth, thermometers are inserted and it is left to sit for 10-12 hours.

Kirikaeshi (Breaking up)

During its time under the cloth, the rice starts to stick together and solidifies.
The rice has to be broken up to prevent parts of the rice from getting too hot. If the rice gets too hot the koji will die, releasing unwanted aromas and flavors.

Mori (mounding)

Up until this stage, the aim of the process has been controlling the moisture levels of the entire batch of rice, but from this point onwards attention switches to controlling the moisture of each individual grain of rice. More or less precision is required depending on the target style of sake. The rice is moved to smaller containers to reduce the surface area thus retaining enough moisture for further growth. Extra precision is achieved by using smaller containers, the smallest of which is called a buta. Each buta can hold around 1.5-2.5kg of rice. Managing the buta is hard work and requires a lot of skill, so it is normally reserved for the best daiginjos. Some breweries do not use the buta even for daiginjo sake. One of the most time-consuming parts of this stage is rearranging the buta which are stacked on top of one another. They have to be continually rearranged because as heat rises the top row is always warmer than the rest. There are now machines that help with this task.

Naka Shigoto (middle work)

After being left to sit for a further 7-9 hours, the rice is spread out in order to encourage evaporation of moisture and bring the temperature down. The brewer also draws patterns in the rice to create furrows that acts as vents in the rice. This stage is very important because the period after mori is when the rice is at its most hottest and there is a danger that the excess heat will kill the mould.

Shimai-Shigoto (final work)

6-7 hours after naka-shigoto has been completed, the temperature of the rice is raised again. The rice is spread out in the same way as in the naka-shigoto stage. Shimai-shigoto is all about drying the rice while maintaining a steady mould growth. The pattern of mould growth is called the haze (ha-ze). For most sake the brewer will make so-haze koji. So-haze is where the mould completely covers the outside of the rice grain, but does not penetrate so deep into it. However, the more mould there is the more enzymes are produced in the fermentation and the faster starch is converted into sugar providing nutrients for the yeast. However, sometimes the brewer doesn’t want to provide such a nutrient rich environment for the yeast, such as when they are making ginjo sake.

As the aim of ginjo production is actually to starve the yeast of nutrients, instead of growing the mould on the outside of the rice grain, the brewer needs the mould to grow on the inside, ideally penetrating right into the white heart itself. This pattern is called tsuki-haze. To achieve this pattern, the brewer dries the outside of the rice grain so that the mould is encouraged to burrow deep into the core of the rice where there is plenty of moisture. It’s a task which sounds a lot easier than it actually is and brewers train for years to perfect the skills required to create this style of koji.

Note: you may remember in the last article we mentioned another type of haze, baka-haze. When the mould does not properly cover the grain or penetrate inside, it is called nuru-haze. There are actually many more types of haze, but these are the most important.


Dekoji (taking out)

Focus switches to completely cooling down the rice and stopping the growth completely. The rice is normally moved to a cooler room.

Like Looking After a Small Child

Generally, seigiku takes roughly 48-50 hours. That’s two full days of keeping a watchful eye on the koji. So yes, as you might have already guessed, brewers often work through the night. The only other task that requires this much care and attention and sleepless nights would have to be child rearing. In fact, many brewers will half jokingly (or perhaps not joking at all) make similar comparisons. Okay, it might be a stretch to claim that they are similar unless you have done both. But nevertheless, without good koji you cannot brew good sake, and that is why it is the number one more important stage in brewing.

Working in the koji-muro that is around 30 degrees is much harder work than most people imagine. Quite a bit of stamina is required to be able to stir the heavy warm rice.

Set to over 30 degrees and close to 80% humidity, the koji room is a tough environment to work in for long periods. As well as skill, you need to be healthy and have good stamina.

When asked for the secret behind the deliciousness of their sake, nearly all brewers will first talk about the koji.

So there you have it. This article has only scratched the surface of how koji is made. If you wish to learn more, check out our past special series of koji making with an entire article dedicated to each stage of the process. If you have enjoyed this article and feel ready to embark on a voyage of discovery, why not head on over to Japan, to Tokyo, and visit KURAND where you can taste over 100 different types of sake without time limits at your own leisure. Each sake comes with a story of how it was made and no two sake are made 100% the same way. We look forward to welcoming you very soon! In this next article in this series we will explain how the yeast starter is made.

Rice Preparation Stages: Recap