Koji: The funky mold driving delicious

Have you ever seen or tried the Japanese secret ingredient koji? If you're shaking your head in doubt, you'd be surprised to know you've probably already eaten it, and more than once ...

Think umami-rich soy sauce, mirin or sake. Koji is the magical ingredient that makes them happen.

What is koji?

It's a mico organism, also known as "Aspergillus oryzae", a domesticated mold that's used to initiate the fermentation process that creates those desirable umami flavours.

Koji is a key player in Japanese cuisine, a cornerstone of centuries of cooking, serving as the vital ingredient kicking off the fermentation process in key Japanese ingredients like sake, soy sauce, miso and mirin.

Koji comes in two forms. Confusingly the term koji can refer to both the fungus and the rice or barley inoculated with Aspergillus oryzae fungi.

What does koji taste like?

In its simple form, koji smells sweet and yeasty with hints of chestnut and citrus.

A Japanese obsession 

In Japan, beloved koji is known as the "national fungus" (kokkin), as without it the fundamentals of Japanese cuisine would simply not exist. It even has its own day dedicated to it on October 12, when koji is celebrated during National Fungus Day.

How do you make koji?

Koji is sprinkled onto partially or fully cooked rice and left to ferment over days. As it grows it breaks down the rice transforming the carbohydrates into sugars and producing glutamate, also known as umami.

The koji covered rice can then be mixed it into something else, ie as a starter to make fermented products, like more rice for sake or a seasoning paste known as shio koji.

Follow the easy koji recipe on the Great British Chefs to make your own, or go to town with the team at the Nordic Food Lab.

How can you use koji at home?

At home, koji can be used for a number of simple jobs, where you want to transform flavour or texture. 

Try sprinkling koji into your morning porridge, blending it into smoothies or even baking it into bread.

Koji also works as a tenderiser and odour eliminator for meat and fish. Try these Koji rubbed meats courtesy of Bon Appetit:

How are chefs using Koji?

Western chefs are also now discovering the magical qualities of this ancient microbe.

The team at Silo in London are singing koji's praises and experimenting with the ingredient in their upcycling of dishes.

René Redzepi cites fermentation as the "future of flavour" of which koji is a key driving force. In The Noma Guide to Fermentation, koji is one of the key ingredients, showcased in recipes like roasted koji mole recipe and the sweet Japanese drink, koji amazake, made from rice koji, rice, and water, while in the restaurant he serves a koji taco made from barley.

Chef Garima Arora from Gaa in Bangkok uses sweet white koji from fermented Thai jasmine rice to make bread with a lychee and banana scent, topped with caviar and banana water.