In the second part of Dr. Koizumi’s commentary, we turn to koji fungus (also known as koji mold)—the key player in making koji. While commonly referred to as a single group, koji fungus actually includes several varieties, such as yellow, black, and white strains, each with its own unique characteristics used in making miso, soy sauce, sake, honkaku shochu（single-distillation shochu）, awamori, and more. Dr. Koizumi also shares the origin of the Chinese character for “koji” as it’s used in Japan.
⇒Read Part 1: “Fermentation Benefits Us, While Rotting Does Not.”
Expert Commentator: Takeo Koizumi, Professor Emeritus, Tokyo University of Agriculture
Interview & text: Chieko Fujita / Photography: Koichi Mitsui / Composition: Contentsbrain / English Translation: LIBER
One area of particular interest to me within the world of fermentation is how strongly fermented foods may support the immune system. The health benefits of fermented foods have been widely discussed, and data has emerged suggesting they can help enhance human immunity. Take amazake, for example—a sweet fermented beverage made from rice and rice koji. It’s so rich in nutrients that it’s often called “a drinkable intravenous drip.” Even in the medical field, some doctors have begun to show interest in amazake, and there are people who want to study the connection between fermented foods and immune function.
Fermented foods outside Japan also offer notable health benefits. For example, kimchi, a fermented food from Korea, is said to have a very high immune-boosting effect. In Southeast Asia—countries like Vietnam, Laos, Cambodia, Thailand, and Myanmar—people regularly consume fermented foods that are known to support immunity. What’s particularly fascinating is the difference in fermentation molds by region. In Southeast Asia, Korea, and China, the dominant mold species is Rhizopus, commonly known as kumonosukabi⋆¹(literally “cobweb mold”). In contrast, koji fungus (Aspergillus) is found only in Japan.
⋆¹ Kumonosukabi: A mold of the genus Rhizopus, commonly found in China and Southeast Asia. It forms long hyphae over 1 cm in length, with black, spherical spores at their tips. In contrast, the yellow, black, white, and soy sauce-specific strains of koji fungus—all belonging to the genus Aspergillus—are unique to Japan.
Takeo Koizumi, Professor Emeritus at Tokyo University of Agriculture
In Japan, koji fungus has been officially designated as the national mold. Many countries have a national flower or a national bird, but Japan is the only country to have a national mold. There are three main types of koji fungus. The first is yellow koji fungus, used to make miso, soy sauce, mirin, sake, rice vinegar, amazake, and more. Its scientific name is Aspergillus oryzae. Oryzae comes from the Latin word for rice—it refers to mold that grows on rice.
The second type is black koji fungus, which is mainly used in the production of shochu and awamori. Black koji fungus naturally occurs only in Okinawa, which led the International Union of Microbiological Societies to name it Aspergillus luchuensis (“luchuensis” refers to Ryukyu, the former name of Okinawa). This mold is known for producing large amounts of citric acid, which helps prevent contamination from unwanted microbes during the fermentation process.
White koji fungus is a white mutation of black koji fungus. Its scientific name is Aspergillus luchuensis mut. kawachii. It is primarily used in the production of shochu. Like black koji fungus, it produces large amounts of citric acid, but it also has the added advantage of being less likely to stain clothing or equipment during production.
(Editor’s note: Sanwa Shurui uses this white koji fungus in the production of its barley-based honkaku shochu.)
Since around the 14th century, specialized producers have collected and commercialized koji fungus spores as a product known as tane-koji (koji starter).
Shown here in fine powder form, from left to right: white koji fungus, black koji fungus, and yellow koji fungus. (Photo courtesy of Bio'c Co., Ltd.)
Much of Japan’s food and sake culture owes its richness to the blessings of koji fungus. When you think about it that way, koji fungus is truly something to be thankful for. The origin of the word koji is also quite fascinating. In the Harima no Kuni Fudoki⋆²—a regional report compiled around the year 713—there’s a passage that says steamed rice was offered to the gods, and mold began to grow on it. This phenomenon was referred to as kabidachi (literally, “mold forming”). Over time, the word evolved: kabidachi → kamutachi → kamuchi → kauchi → and eventually became koji as we say today. In other words, the old Japanese word for koji was kabidachi, meaning “the forming of mold.”
⋆² Harima no Kuni Fudoki: One of the “Fudoki,” a series of reports compiled in the 8th century during Japan’s Nara period, documenting local culture, climate, and products for the emperor. Harima no Kuni refers to the area that is now part of Hyōgo Prefecture.
Let’s take a moment to look at the origins of the character 「麹」(koji). It’s made up of two parts: on the left is the character for “barley” (麦), and on the right is the character for “chrysanthemum” (菊). This character for koji is quite old and was brought to Japan from China.
On the other hand, there’s also the character「糀」(koji), which combines the character for “rice” (米) on the left with the character for “flower” (花) on the right. This is a kokuji—a character created in Japan. It’s as if flowers are blooming on the surface of the rice—quite fitting, don’t you think?
If we define what koji is, it refers to a state in which mold has propagated on grains. That’s why you’ll also find koji in places close to Japan with similar climates, such as Taiwan, China, Korea, and countries in Southeast Asia. However, only in Japan do we see the unique practice of using rice koji—rice with koji fungus cultivated on it—or barley koji, in which koji fungus is cultivated on barley, to brew alcoholic beverages in a way that has no equivalent abroad.
I served as chair of the initiative to have Japan’s traditional brewing techniques—which use koji fungus to produce shochu, sake, awamori, and mirin—recognized as a UNESCO Intangible Cultural Heritage. That effort bore fruit, and in December 2024, “Traditional Knowledge and Skills of Sake-making with Koji Mold in Japan” were officially inscribed on the UNESCO Intangible Cultural Heritage list. I hope this will continue to energize the sake and spirits industry moving forward.
⇒Read Part 1: “Fermentation Benefits Us, While Rotting Does Not.”
PROFILE: Takeo Koizumi
Born in 1943 into a sake-brewing family in Fukushima Prefecture, Takeo Koizumi is Professor Emeritus at Tokyo University of Agriculture and holds a doctorate in agriculture. He is a fermentation scholar, food culture expert, and author. His areas of specialization include food culture, fermentation science, and brewing studies. He has also served as a visiting professor at Kagoshima University, Fukushima University, Beppu University, Ishikawa Prefectural University, and Miyagi University, among others. Dr. Koizumi has authored more than 150 books on fermentation. He has held numerous roles, including Chair of the National Fermentation Town-Building Network Council and member of The Washoku Association of Japan. In December 2024, he served as the lead of the initiative that successfully registered “Traditional Skills and Knowledge of Sake-making with Koji Mold in Japan” as a UNESCO Intangible Cultural Heritage. Through a wide range of food-related activities, he continues to share the wonders of fermentation both in Japan and abroad.