Koji: The Friendly Mold Behind Japan's Fermented Foods

Learn how koji mold, Japan's national fungus, shapes miso, soy sauce, sake, mirin and amazake, and how travelers can taste its work across Japan.

Published October 17, 2026 at 8:15 PM JST | By Tokyo Paul

Quick answer: Koji is the mold Aspergillus oryzae grown on rice, barley or soybeans, and it is essential for making miso, soy sauce, sake, mirin and amazake. It was proposed as Japan's national fungus in 2006. Its enzymes turn starch into sugar and proteins into savory flavors.

The Quiet Helper in Almost Every Japanese Kitchen

Miso soup at breakfast, a splash of soy sauce on rice, a sweet cup of amazake at a shrine: all of these everyday pleasures owe something to the same tiny organism. It is called koji, and it is a mold that has been put to work by Japanese cooks and brewers for centuries. The word may sound unusual, but koji is a friendly ingredient, and in recent years food lovers around the world have become more curious about it. This guide explains what koji is, which foods rely on it, and how visitors can taste and recognize its flavors during a trip.

It helps to think of koji as a kind of natural kitchen assistant. It does not add its own strong flavor so much as transform other ingredients, turning starches into sugars and proteins into savory building blocks. That transformation is the secret behind much of what we call umami.

What Koji Is and Why It Matters

The word koji refers to the mold Aspergillus oryzae, and also to the grain or soybeans on which it has been grown. Typically rice, barley or soybeans are steamed, cooled and then seeded with the mold, which spreads through the grains and releases enzymes. Those enzymes break starch into sugar and proteins into amino acids, which is why koji is so valuable for fermentation. The mold is known for producing enzymes such as amylases and peptidases, including types that work well in the salty conditions of miso and soy sauce.

The mold is so important to Japanese food culture that a scientist proposed it be called the national fungus, or kokkin, and the Brewing Society of Japan approved the proposal at its annual meeting in 2006. It is a charming honor for an organism that most people never see, and it reflects how deeply koji is woven into the country's food and drink.

The Foods and Drinks Koji Creates

Miso is made by fermenting soybeans with salt and koji, sometimes with rice or barley added. White miso is mild and sweet, red miso is stronger and aged longer, and mixed miso blends the styles. Soy sauce relies on a koji made from soybeans and wheat, and the mold is also used in making rice vinegar. Sake and shochu use koji to convert the starches of rice, barley or sweet potato into sugars that yeast can ferment.

Mirin, the sweet cooking wine, is traditionally made from steamed glutinous rice, rice koji and a distilled spirit, and its sweetness arises naturally during fermentation. Amazake is a traditional sweet drink made from fermented rice, either by adding koji to cooled rice or by using sake lees. The koji version is low in alcohol or non-alcoholic, and many shrines and temples offer it, especially around New Year. This is a lovely, accessible way to taste koji directly.

Soy sauce shows how adaptable koji can be. Boiled soybeans and roasted wheat are typically mixed in roughly equal amounts and combined with the mold, and the resulting koji is brewed into the dark standard style that dominates the country. A paler type popular in Kansai is made with amazake, the sweet fermented rice liquid, which is how it manages to add flavor without darkening the ingredients in a dish. Tamari, made mainly in the Chubu region with little or no wheat, is another variation.

A Long History in Japan

Amazake appears in early records from the Kofun period, including a mention in the Nihon Shoki, and soy-based fermented foods came to Japan with Buddhist influences from the continent. Scholars debate whether China or Japan was first to use the mold in fermented foods, and some genetic studies suggest that Japanese brewers gradually domesticated a wild relative, producing a strain that does not make harmful toxins. What is clear is that generations of Japanese brewers carefully selected and nurtured koji strains.

That legacy lives on in the many family-run breweries and miso shops across the country. Regional preferences developed too: darker red miso is popular in the Kanto region around Tokyo, while lighter white miso is favored in Kansai cities such as Kyoto and Osaka. Even soy sauce has regional styles, with a paler, saltier type common in Kansai.

How to Taste Koji on Your Trip

Start with the basics. Order a bowl of miso soup at breakfast and compare it with another in a different city. Try a flight of local sake at a tasting bar, many of which are happy to guide beginners, or visit a brewery that offers tours and tastings. Shops selling miso often offer small samples, and department store food halls display regional varieties side by side.

In winter, look for amazake at shrines around New Year, when it is often served warm to visitors. Cafes and convenience stores sell bottled versions, which are a quick way to try it. You may also come across shio koji, a seasoning made from koji, salt and water that cooks use to marinate fish and meat or to season vegetables, and it is sold in supermarkets in small bottles. A spoonful on grilled chicken or roasted vegetables shows how koji can tenderize and deepen flavor.

For home cooks, shio koji and rice koji are easy places to begin. Follow the instructions on the package, because the best amount and marinating time depend on the product. Keep fermented products refrigerated as the label directs. Many koji foods are plant-based, but not all dishes are, so those with dietary needs should still check broths and seasonings.

People with allergies to soy, wheat, barley or rice should read labels carefully, since koji is grown on those grains. Amazake is described as low-alcohol or non-alcoholic, and recipes differ, so travelers who avoid alcohol entirely, including those with children, should ask staff how a particular drink was made.

Home cooks curious about koji can look for rice koji or shio koji at supermarkets and specialty shops. The packages usually include instructions, and the most important step is simply following them closely. A small jar of fermenting koji seasoning on the kitchen counter or in the refrigerator is a charming reminder that food can be shaped by patient, living processes.

Quick Takeaways

Koji is the mold Aspergillus oryzae, grown on rice, barley or soybeans, and it is the key to miso, soy sauce, sake, mirin and amazake. It earned the nickname national fungus of Japan in 2006 thanks to its central role in the cuisine. Its enzymes break starches into sugars and proteins into savory flavors, which is why fermented foods taste so rich.

The nicest way to appreciate koji is to taste its work: a bowl of miso soup, a splash of shoyu, a warm cup of amazake. Each is a small window into a food tradition that makes patient use of a very helpful mold.

Frequently Asked Questions

What is koji?

Koji is the name for Aspergillus oryzae and for the grain or soybeans on which it is grown. Its enzymes break starches into sugars and proteins into savory amino acids. Typically rice, barley or soybeans are steamed and then seeded with the mold.

Which Japanese foods use koji?

Miso, soy sauce, sake, shochu, mirin, rice vinegar and amazake all rely on koji. Shio koji, a seasoning made with koji and salt, is also popular for marinating. Many shops sell small bottles of it.

What is amazake?

Amazake is a traditional sweet drink made from fermented rice, either with koji or with sake lees. It is low in alcohol or non-alcoholic and is often served at shrines and temples around New Year. Recipes differ, so ask about a particular drink.

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More guides from Tokyo Paul: Shojin Ryori: The Vegetarian Cuisine of Japan's Temples, Vegetarian and Vegan Dining in Japan: Practical Tips and Kuri: Japan's Chestnut Season and Mont Blanc Cakes.