How to Cook with Koji, the Mold Used to Make Soy Sauce, Miso, and More

Koji is arguably the most important ingredient in Japanese cooking. Grains or soybeans that have been inoculated with koji mold are behind some of Japan’s most important fermented foods, including true soy sauce (shoyu), sake, and miso. Koji has also been embraced worldwide by chefs including Noma’s René Redzepi and Larder’s Jeremy Umansky, thanks to it sweet, floral, funky, and intensely savory quality. Here’s what you should know about this Japanese staple and fermented darling of the chef world. 

What is koji?

Koji can refer to grains (such as rice and barley) or soybeans that have been inoculated with Aspergillus oryzae mold. It can also refer to the mold itself, which is called koji-kin in Japan.

While many different yeasts and fungi can facilitate fermentation, they all behave differently and have different purposes. Baker’s yeast, for instance, is a simple single-celled organism that converts sugar into ethanol and carbon dioxide in a bubbly reaction we all know well.

Koji is quite a bit more complex, growing a network of strands or mycelia and creating a number of enzymes in the process, most notably amylase. This converts starches into sugars, giving that sweet taste (and lovely smell) to rice-based sake and soybean-based miso. Other enzymes break proteins down into their amino acids, which is why koji is often used to tenderize meat. 

Japanese home cooks take advantage of a wealth of different dry and liquid koji-based products, which are readily available online or at any good Asian market.

The most basic is koji-inoculated rice, which looks like a big, white block of dried ramen. This can be mixed with rice or grains to make amazake, the sweet, cloudy fermented beverage that is served hot for New Year’s celebrations. Mixed with salt and water, dried koji rice becomes a medium for fermenting quick pickles, or it can be turned into condiments called shoyu koji and shio koji.

What is shoyu koji?

Koji rice can be broken up, mixed with soy sauce, and left to ferment into a condiment called shoyu koji. Making it requires daily attention for a month or so, though jarred versions are available. Shoyu koji is an umami bomb; use it in place of soy sauce or as a marinade for fish, meat, or poultry.

What is shio koji?

As interesting as the dry product is, liquid shio koji is far more versatile. This bottled condiment is completely natural, made with inoculated rice, water, and salt. As a marinade, it tenderizes and helps food brown beautifully. (Pro tip: Rub some on your turkey this Thanksgiving.) As a cooking ingredient, it can to help balance the salt in soups and stews while deepening their flavors.

Add shio koji to dishes as you do salt, a splash at a time, and taste for the balance. As a condiment, it’s a fantastic addition to cooked vegetables and soup noodles. Try tossing a tablespoon into a serving dish of veggies before setting them next out to a single serving of ramen. 

Not all liquid shio koji looks alike and some of it can be quite intense in flavor. Start your journey with the readily available Hanamaruki brand, which is limpid and the color of green tea. Once you’re on the koji train, you’ll be surprised at where it can take you.