Japanese fermented foods: Koji

Written by Kevin Kilcoyne (@kevinjkilcoyne)

With countless varieties of condiments, dishes, and pickles (known generally as tsukemono), Japan has an incredibly rich culinary history of fermented products. To name a few there are miso, shoyu (soy sauce), natto, umeboshi (pickled plum), umeshu (plum liqueur), sake, and katsuobushi (dried, fermented bonito fish). All are the products of centuries of experimentation and perfection in an environment rich in fermentation-friendly microbes and a climate that suits them perfectly.

As diverse as Japanese fermented foods might be, one microbe stands above the rest and that is the mold koji.

Also known as Aspergillus oryzae, koji is responsible for many of the ingredients that make up the foundation of Japanese cuisine. Professor, researcher, and author Makoto Kanauchi says that products like miso, mirin, soy sauce and sake, would not exist if it weren’t for the symbiosis between koji and the fermentation-conducive environment of Japan. Furthermore, he says that koji is so beloved by the Japanese people that it has been called the Japanese national fungi and is even celebrated yearly on October the 12th, National Fungus Day.

When speaking of koji, the term koji is used when referring to both the microbe itself as well as the koji-cultured grains used in fermentation. Grains like rice, barley, and soy beans are inoculated with the koji mold and then fermented to create the koji culture. They are then used to create the fermented products like soy sauce, miso and sake. In Japanese, however, the mold and the culture can be differentiated even further. The mold spores themselves are called koji-kin (麹菌) and although various grains can be used to create a mother culture, the most common is steamed rice, which once inoculated, becomes kome-koji (米麹/米粉) or rice koji.

Kome-koji is a key ingredient in the production of mirin, sake and miso. It is also the key ingredient in another traditional staple of Japanese cuisine called shio koji, (salt koji). Like so sauce and miso, shio koji is used to season dishes, whether as a marinade or a substitute for salt, and can be used to cure and preserve foods as well.

The difference with shio koji, however, is both in the way in which it is produced and the way it flavors the ingredients it is combined with. Shio koji is simply a mixture of kome koji, salt, and water. The mixture is then left to ferment. As it ages, it develops flavor as the microbes do their work, turning something that was once dominantly salty into something mildly salty with a faint sweetness.

As with many fermented foods, the products produced using koji are often more nutritious than the original un-fermented ingredients. Shio koji is no different. When rice is introduced to koji-kin and allowed to culture, it releases hundreds of enzymes and compounds within the rice that would otherwise be inaccessible or less bioavailable. In a way, the fermentation process acts as a pre-digestion stage, making foods more easily digestible and nutritive. This attribute extends to shio koji as well, which when used as a marinade, helps to break down proteins and tenderize meat and fish, bringing out the umami in the ingredients, while also breaking down starches to draw out their sweetness.

Rich in probiotics, ingredients like miso and shio-koji also help to keep our digestion systems healthy and our gut microbiomes in check. So perhaps in your next recipe, substitute some salt for shio koji and some sugar for amazake, for a healthy boost in your gut flora. You might just find a depth of flavor you had not experienced with the same ingredients before.

About the author: The spark that lit Kevin Kilcoyne’s interest in Japanese culture began in elementary school through a friendship with his then classmate Keisuke. Since then, that passion has evolved and bloomed to encompass more than just video games and manga, leading Kevin to live in Japan as a participant of the JET program. During his time in Japan, Kevin sought out as many foods as he could, the experiences and taste memories lingering long after they had gone. Now he is forging a path to link his passions for Japanese food, history, and visual culture and is planning for his return to live in Japan once again. For now, you can find Kevin on Instagram (@kevinjkilcoyne) where he posts his illustration work. Keep an eye out for more posts and updates as Kevin delves more deeply into his passions for writing and food.