Amazing Koji Mold World: 04
Koji, as necessary ingredients for Japanese seasonings

Written and illustrated by Misa Ono

Japan is a country that has a climate of high temperature and high humidity; therefore, molds often grow anywhere. There are good and bad molds, and koji is a healthy mold. Koji that is grows in steamed rice is called kome-koji, Koji that grows in steamed barley is called mugi-koji, and if it grows in boiled soy beans, it is called mame-koji. Often, the organic rice grow dark green mold spores which are soy bean size at harvest season in autumn. It is called ina-koji and people used to grow koji mold with it. They used to make koji with freshly picked rice and produce koji seasonings such as miso during winter. The winter is the most favorable time of year for making good seasonings because there are not many bacteria and koji can ferment slowly in the cold environment. Rice and koji have a very close relationship and both of them are necessary to the Japanese.


There is also a dietary culture of making koji by growing mold in China and South East Asia; however they use mainly fresh grain to breed mold. It seems like only Japanese people use steamed or boiled grain to make koji. Why is the steamed rice good for making koji? Why not fresh rice or thoroughly cooked rice?

As the Koji grows, its spores form a tunnel through the rice grain in search of starch. When the rice has lower water content, koji tries to extend its spores as long as it can; however, if the rice has higher water content, it doesn’t. Also, if the rice is uncooked and dry, it doesn’t extend its spores either. Steamed rice with adequate firmness and moisture is the environment where koji can do its best activity. Just like human beings, the optimal environment that stimulates hard work is necessary for koji as well.

As the Koji spores grow, they create enzymes that break down the rice grains’ starch into sugar. This growth inside and outside of the rice grains is what produces kome-koji.

Look at fresh kome-koji closely, you will see something fluffy growing around the kome-koji. Since it looks like a flower, the Japanese use the kanji, Chinese character, “糀”for koji, that means rice “米”and flower “花”. How romantic the Japanese are!


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