Last time we discussed milling, washing, and steaming rice. That was a precursor to this step. The next step is making koji. For the purpose of this post koji-kin refers to the spores and koji refers to the rice with the mold on it. Koji-kin (Aspergillus oryzae) is a fungus that gets spread on the rice and allowed to grow on the rice. This type of koji-kin is generally referred to as yellow koji. There are a few different types of koji but yellow is the most common for making sake. It is also used in making miso, amazake, and soy sauce.
The steamed rice is spread out on mats to allow the rice to cool down. Once it is cooled enough the rice is transferred to the brewery’s koji room. Once the rice is in the koji room it is spread out in a thin layer on the table. Then the koji-kin is applied to the rice and is allowed to grow on the rice. The koji growth on the rice is carefully monitored to ensure just the right amount is grown on the rice. The rice starts off in the hottest and most humid section of the koji room to allow for quick growth. It is then transferred towards the coolest and less humid part of the koji room to slowdown it growth.
Making the koji is a very important step in the brewing process. The koji-kin and koji perform a very crucial step. The koji-kin and koji have enzymes that break down the starch in the rice into sugar that is used for fermentation. This is important because starch cannot be fermented into alcohol. Don’t worry we will go over fermentation next time.