Chefs generally want to keep mold out of their kitchens, but they make an exception for koji. For around a decade, the fungus has been a secret weapon for trailblazing chefs like Rene Redzepi at Noma in Copenhagen, used to ferment grains, cure proteins and impart umami to dishes both sweet and savory.

Now, the culinary world’s most popular mold is poised to become the biggest trend in the specialty coffee world. Enterprising producers believe the multipurpose ingredient can improve on mediocre coffee beans, and produce a better-tasting caffeinated cup.

It’s an opportune time to maximize coffee bean quality. The price of Arabica beans, the most popular in the world, has more than doubled in the past year and a half.

“The biggest thing that makes koji great is the potential to grow sweetness in coffees that are lacking, or to enhance a coffee to a higher grade,” says Mason Salisbury, co-founder of Nevada-based Luminous Coffee, one of the country’s first koji coffee adopters. Salisbury started selling his fermented beans this spring; a 200-gram bag goes for $30.

A handful of specialty coffee shops around the world have begun releasing bags of koji coffee, including Ohio’s Phoenix Coffee, The Netherlands’ Manhattan Coffee Roasters and Hatch, in Ontario, Canada. Manhattan Coffee Roasters sold out of stock quickly, moving 100 kilos (220 lb) of koji coffee in 72 hours.

Whether brewed via the filter method or pour-over, part of the appeal of the koji process is that, when done right, it doesn’t add a flavor of its own. For industry professionals, the coffee is revolutionary for its ability to boost the quality of a basic bean and turn it into a better version of itself. For the consumer, koji means a rounder, silkier, sweeter brew.

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The process garnered serious attention after Finnish barista Kaapo Paavolainen of Helsinki’s One Day Coffee brewed the unconventional koji beans in public for the first time at the World Barista Championship in Milan in October.

“The championships … are so big that it can define entire crops of coffee for years to come,” Paavolainen says.

El Vergel Estates, a boutique coffee farm in Colombia that’s known for high-end, exotic cultivars, is credited with producing the first successful batch of koji-treated beans. The farm collaborated with a team of koji enthusiasts to use the mold — a strain of the aspergillus oryzae fungus, best known for its ability to turn rice into sake and soy beans into miso paste — for coffee. They included Salisbury; Christopher Feran, director of Phoenix Coffee; chef Jeremy Umansky (his Cleveland, Ohio deli Larder, sells products like koji-cured pastrami); and Koichi Higuchi, a seventh-generation koji spore producer based in Osaka, Japan.

The process is deceptively simple. It starts by sprinkling freshly picked coffee cherries with yellow-white koji powder — a substance that looks like flour — then gently mixing with a paddle. The berries then sit for two days, producing a fluffy white fur, before being sun-dried for two to three weeks. They’re milled to remove the outer husk, and shipped to open-minded roasters.

The process, which Feran dubbed the Koji Supernatural Protocol, led to the world’s first koji-processed coffee beans in 2020. Paavolainen brewed the second harvest on stage during last year’s World Barista Championship.

Following the demo, “there was massive amounts of interest for the processing amongst my colleagues,” says Paavolainen. Many were incredulous that a coffee bean’s quality could be improved with mold.

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Koji coffee’s accentuated sweetness is created when fermentation produces enzymes that break down proteins in the coffee cherry into amino acids, and the starches into fermentable sugars, Feran says. These help “increase a coffee’s perception of sweetness, fruitiness and complexity,” he says.

Koji has “the power to transform even the most mundane of ingredients,” Umansky says. The process allows coffee to “become more brilliant than the original bean,” says Rabi Aouam, founder of Spain’s Kima Coffee, another early adopter.

Fans eager to try the brew can expect a slightly pricier cup of joe. In general, koji coffee costs around twice as much per pound compared with regular coffee, which amounts to $5 to $7 per cup in most shops. The coffee is now creeping onto menus at places like Luminous in Brooklyn; in Bristol, UK, Sweven occasionally stocks the beans. And as koji-processing has expanded to farms in Colombia, Guatemala, Costa Rica, Thailand, China, Vietnam, Brazil and Indonesia, the fermented coffee is set to become more widely available worldwide this fall.

For Paavolainen, the transformative potential of koji coffee is easy to encapsulate: “Be all that you can be,” he says.