Explainer: Koji, Japan’s “National Mould” and the Umami-Maker in Miso

Photo: Courtesy of Pexels / key05

The flossy fungus is foundational in Japanese cuisine, but is becoming increasingly popular in the West. Two fermentation experts break down everything you need to know about this centuries-old ingredient.

What is koji?

“You could say koji mould is Japan’s national mould,” says Yoko Nakazawa, a fermentation expert who teaches traditional Japanese cooking and culture workshops in Central Victoria at her cooking school, Cooking With Koji. “Without it, Japanese food and flavour don’t exist at all.”

Koji is a fermentation culture used in the production of miso, shoyu, rice vinegar, mirin and sake. There are three main varieties: yellow, black and white. Yellow koji, aka aspergillus oryzae, was the most commonly used kind in mainland Japan until the 20th century, when scientists successfully cultivated the hardier, more efficient strains of black koji (aspergillus luchuensis) and white koji (aspergillus kawachii). All three kinds are still used today, but – especially outside Japan – white koji is the kind you’re most likely to encounter.

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Koji is created by inoculating cooked grains (usually rice, but sometimes soy beans and barley) with aspergillus spores and leaving them to cultivate in warm, humid conditions. After a period of time, when the grains are clumped together and covered in a flossy fungus, they’re ready to be mixed with other ingredients in order to kickstart the fermentation process.

Nakazawa says English speakers often use the blanket term “koji” when referring to different stages of production, which isn’t quite accurate. The mould itself is called koji-kin and the inoculated grains are koji. Then there’s shio (salt) koji, a liquid flavour enhancer made by combining koji, water and salt.

The making of koji has been practised in Japan for centuries. And its use in brewing sake and fermenting shochu was granted Unesco Intangible Cultural Heritage status. Today, Australian restaurants, distilleries and even specialty coffee roasters are experimenting with this magical mould.

What does koji taste like?

“[Koji] has a salty, rice-y, umami flavour” says Anthony Schifilliti, head chef at Sydney fine diner Sixpenny. “The smell is hard to describe. It’s intoxicating, but at the same time, it has an unassuming, pleasant smell. The first time I smelled it, it was a very ‘wow’ moment.”

That said, the innoculated grains aren’t meant to be eaten on their own. Koji mould is essentially a bouquet of enzymes, and the real magic happens during the fermentation process when those enzymes break down starches and proteins to produce amino acids, fatty acids and simple sugars. Perhaps the most important of these by-products are glutamates, which give miso and soy sauce their intense umami flavour.

How do you make koji at home?

It’s easy enough to buy a koji starter kit and grow your own at home in a matter of days. But it can take months (and up to a year) for koji to do its thing in a miso or soy sauce. For beginners, Schifilliti says the fastest thing to do (aside from buying the premade bottled stuff) is to make your own shio koji, which takes around 10 to 14 days to ferment “depending on the weather”.

“You stir it daily to promote oxygen and prevent unwanted mould. It turns quite liquid and it’s full of umami. It’s a powerhouse of enzymes so it’s great as a marinade to make the meat tender and juicy. At home, I add it to bolognaise, soups and stews.”

Schifilliti started experimenting with koji during Sydney’s lockdowns. In those days he was inoculating one kilogram of cooked grains at a time, growing the koji in an esky rigged with DIY humidity and temperature controls. Today, he makes it in 20-kilogram batches in Sixpenny’s greenhouse, and keeps it in a storage facility near the restaurant. “I always try to have shio koji on hand so I can make something spur of the moment.”

Where can you try koji if you don’t want to make it?

Traditional Japanese ingredients made with koji can easily be found in most Australian grocers and bottle shops. When it comes to miso, Nakazawa says you won’t find the real McCoy at the supermarket.

“It takes too long to make and because miso is a living thing, it’s very unstable. They either pasteurise it, which kills the koji, or add something to stop the fermentation, otherwise the container would explode.”

Traditionalists like Nakazawa make fresh miso to sell online and at local markets. Up in Sydney, Schifilliti has a side hustle making koji-based condiments under the banner of Cura Seasonings. Over in Victoria’s High Country, distillery Reed & Co in Bright – owned by chefs-turned-distillers Hamish Nugent and Rachel Reed – are making local shochu, yuzushu and other spirits made with koji.

Although Nakazawa follows traditional methods, she appreciates modern takes on the centuries-old ingredient.

“I think it’s interesting when people view things through different eyes,” she says. “We’re in a different location, under different conditions and the ingredients are different from Japan. I like seeing how others use traditional ingredients to try new things.”

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