You May Not Know What Koji Is, but You've Probably Eaten It

This fungus has been used for thousands of years in Japan to make culinary staples like rice, soy sauce, miso and mirin. . Brent Hofacker/Shutterstock

If you’re a fan of Asian cuisine, you've probably eaten koji, though you may not have known it. This little-known fungus is responsible for so much of what makes Asian food so yummy. Check out the ingredient labels on your favorite soy sauce or miso paste, and you'll likely see this tiny, extremely powerful microbe listed.

So what is koji exactly?

Koji is a mold called Aspergillus oryzae. It has been used for thousands of years in Japan to make culinary staples such as mirin and sake. The mold releases enzymes that ferment the food by decomposing its carbohydrates and proteins and breaking them down into sugars and amino acids.

The process is most commonly applied to rice but also can be used on barley, soybeans and other legumes. To make koji rice, the culture is added to the cooked grains. The grains are then placed in wooden trays and left to ferment in a warm, humid environment for up to 50 hours. The result is essentially moldy rice, which sounds gross but tastes heavenly.

Miso is made when koji rice is mixed with cooked soybeans, salt and water. It ferments the soybeans until the mixture is thick and pasty, and it gives it that signature blend of sweet, salty and savory.

Because koji ferments food, it may have health benefits, too: Fermented food can boost your immune system and aid with digestion.

Coming to America

Miso paste
Miso paste is made when koji rice is mixed with cooked soybeans, salt and water. taa22/Shutterstock

Recently, a small number of American chefs have begun experimenting with koji in original and surprising ways, reports Cook's Science. Cortney Burns, co-chef of Bar Tartine in San Francisco, marinates meat and chicken in shio koji, which is a combination of rice koji, salt and water that has fermented for about a week.

Another chef, Jeremy Umansky, who is opening a restaurant in Cleveland, Ohio, this fall, uses koji as a crust on meat, though he has yet to sell it, since his current method of cooking (dehydrating at 80 degrees for 48 hours) doesn’t meet health department standards. Chefs like Umansky and Burns have just begun to scratch the surface of this incredibly versatile product’s uses.

Safety first

Sake, a popular Japanese spirit, is created using a method similar to how beer is brewed. Nishihama/Shutterstock

But even though chefs in America are pushing the limit with koji in their restaurant kitchens, chef Gershon Schwadron, executive chef and owner of a catering company in Boca Raton, Florida, doesn’t recommend trying to culture your own koji at home.

“You can take things that already contain koji, like miso and soy sauce, and play with it in your own kitchen,” he suggests as a safer alternative. “This way, you can get ‘the koji effect’ in your own cooking.” Indeed, koji’s closest relative, aspergillus fumigatis, can be deadly if inhaled to people with compromised immune systems.