What Is Koji? 5 Common Koji Uses

Koji is a strain of Aspergillus oryzae, a fungus used for various culinary purposes, including the production of alcoholic beverages like sake or shōchū, or invaluable condiments like miso, mirin, and shōyu (soy sauce).

Like yeast starters in bread making, koji kickstarts the fermentation process when added to other base ingredients, like soybeans, rice, or wheat grains like barley. When applied to proteins, koji acts as a curing agent in the production of charcuterie. The enzymes present in koji molds (like amylase and glutamate) transform starches and proteins into amino acids and sugars. The resulting flavors define the realm of umami: sweet, savory, salty, with an undertone of pleasing funk.

Koji can also refer to other molds in the same bacterial family; for example, Aspergillus sojae, rather than Aspergillus oryzae, may also be used to make miso and shōyu.

Koji rice—steamed rice inoculated with koji mold and allowed to ferment for just over two days—acts as a starter for many fermented foods in traditional Japanese cuisine. In addition to rice, koji is also commonly used on barley grains and sweet potato and can be purchased at most Asian grocery stores.

1. 1. Alcoholic beverages: Various koji strains (like white, yellow, and black koji) are used to impart distinct flavors while serving as the fermenting agent in brewed sake, distilled shōchū, and Okinawa-style awamori. It’s also the main ingredient in amazake, a naturally sweet beverage with a low alcohol content that can also be eaten as a porridge-esque dessert.
2. 2. Cooking condiments: Rice vinegar and mirin, a sweet cooking wine, both result from the enzyme activity of koji. Mirin combines koji with short-grain mochigome sweet rice and shōchū, while rice vinegar adds yeast to the converted sugars in koji rice, followed by acetic acid.
3. 3. Miso: Miso paste is the result of combining koji molds with cooked soybeans, salt, and water and allowing it to develop both its signature pasty texture and umami flavor. You can use miso paste as a condiment, a pickling agent, and a base for marinades and dishes like miso soup or nasu dengaku, miso-glazed eggplant.
4. 4. Shio koji: Koji rice is combined with salt and water and left to ferment for a few weeks at room temperature to make koji salt. The result is a crumbly, flavor-packed paste that can be used as a salt substitute in everything from stir-fries to marinades. Learn how to make stir-fry for your next weeknight meal.
5. 5. Shōyu (soy sauce): This Japanese-style soy sauce is made using the same technique as miso paste—applying koji molds to a soybean and salt mash and allowing fermentation to occur—except the mixture is pressed and filtered to create a liquid byproduct.

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