In the last seven years or so, koji—like garum—has wormed its way out from the confines of Japanese cuisine, worked its way into the fermentation repertoires of New Nordic kitchens, and become, in fine dining spaces that emphasize scratch cooking, almost commonplace.

Applied to starch mediums—from rice to soybeans to farro, bread, and black-eyed peas—koji transforms these ingredients into shoyus and misos that are worlds apart from those of Japan, preserving, deepening, and encapsulating the terroir of far-flung places.

Over the last year, I made bread miso in Tucson with koji, stuck my head inside a koji-muro outside Richmond, and poked around prep spaces in kitchens across North America that use koji. I spoke to chefs fermenting Arizonan tepary beans and rice grown in Mexico. I encountered shio koji in desserts in Philadelphia, adding brininess to martinis in Mexico City, tenderizing meat in Washington, D.C., and Honolulu, and making leche de tigre milkier—also in Mexico City.

This article has been a long time in the making.

What is Koji Mold?

But first, a primer on koji mold as a blanket term. Koji typically refers to Aspergillus oryzae, which I found to be the most commonly used strain, though there are a few other related fungi. The genus Aspergillus contains only a few strains suitable for culinary use—others can be dangerous to humans. Soybeans, rice, and barley inoculated with Aspergillus oryzae or Aspergillus sojae yield familiar Japanese fermented pantry products and alcohols like amazake and shochu. The process begins with the mold, which starts as a yellow-green dust, being applied to steamed grains and left to ferment for two to three days.

I wanted to see for myself how koji transforms soybeans into soy sauce. For that, I headed to Richmond to visit the San-J plant, where the company produces a variety of tamari and tamari-based sauces.

“Soybeans are soybeans,” shrugged both the manager and the director of plant operations simultaneously. “But koji is the star.” At San-J, koji—not soybeans—is the dominant source of flavor in both soy sauce and tamari. The spores are the only thing San-J imports from Japan to Richmond, where the climate and weather fluctuations resemble those of Japan, making it an ideal location for fermentation.

At the plant, I encountered fermentation tanks three stories tall, each holding 130,000 liters, and silos storing 100 tons of soybeans. I was allowed inside a muro the size of my Philadelphia townhouse, where lumps of soybean meal had spent a day growing fuzzy white koji. The spores had been blasted into the chamber through pipes. A wave of hot, cheesy, sake-scented air hit me in the face, burning my eyes. When I plucked one of the fuzzy soy nuggets from the pile, it tasted like cooked soybeans—funkier and far more umami-rich.

I tasted tamari at various stages: mid-fermentation, post-brine, and just before bottling. A four-month-old batch, still too young to pasteurize, had darkened from muddy beige to pitch black through a process called melanization. I also observed one of the final stages of production, in which the fermented solids are wrapped in massive cloths and pressed under extreme weight.

“Koji has the unique character of containing many enzymes that break down proteins and starches into sugar,” explained Takashi Sato, president of San-J. “This is what gives soy sauce and tamari their flavor. Many chefs also use koji to tenderize meat—it’s a middle ingredient.”

Cross-Cultural Fermentation in Mexico

Restaurants with significant fermentation systems are now growing their own koji from spores and using it to reduce waste in their kitchens—creating garums, shoyus, and misos with a distinct sense of place. In Mexico City, at Balcón del Zócalo, chef Pepe Salinas grows koji on arroz Morelos, a rice native to Mexico. At Quintonil, huitlacoche is blended with koji, then brought to 12% salinity and fermented at 234°F for three weeks to create a huitlacoche garum used to finish a ribeye dish. In these kitchens, Mexico meets Japan in the fermentation tank.

In Tucson, Bread Becomes the Medium

Chef Tyler Fenton at BATA in Tucson also infuses koji into spirits at the bar and “utilizes straight koji to thicken sauces and vinaigrettes. Dehydrated, we’ve used it in crumbles and to make oils. It truly is an incredibly versatile and delicious ingredient.”

But bread might be the star at BATA—or at least it was during my time in the kitchen. “Right now my chef de cuisine, Jackson Dennis, is making a new staple bread for the restaurant that’s entirely leavened with amazake and entirely seasoned with miso. It’s really special and super delicious,” said Fenton.

Last year, I helped Fenton make miso from BATA’s surplus bread using a meat grinder. We blended 450 grams of koji and 450 grams of bread until it had the consistency of cookie dough. “Bread miso has to be 12% salt since it has a higher water content,” Fenton told me. He sped up the fermentation by storing the mixture in the warmest part of the restaurant and walked me through the other staple misos he makes from tepary beans, chickpeas, squash—and “beets, if we get beets that are too woody to use.” I imagine that soon, BATA will be making bread miso from bread made with miso.

Philadelphia’s Honeysuckle and the Fermentation Cycle

The fermentation program at the just-reopened Honeysuckle in Philadelphia is just as complex and cyclical as at BATA, and it’s helmed by their director of fermentation, Jamaar Julal (who also happened to once be a cook at my former restaurant Poi Dog, and whom I’ve interviewed for Fine Dining Lovers when he had first started making garums at Honeysuckle’s previous location).

“We inoculate our grains in house. We had a muro built for us—it’s about three by four feet with eight shelves—and we use Carolina Gold rice and roasted blue barley as the substrates. The Carolina Gold rice takes 48 hours, and the barley takes 36 hours,” said Julal, who also purchases spores specific to the product he wishes to make (like white rice koji for amazake).

“We’re trying to showcase the ingredients of Africa and the Caribbean, and while we don’t grow koji using all those ingredients, we use them as a base for other products,” explained Julal, as he handed me spoons to taste the products he was referring to: black-eyed peaso and burnt yam bread shoyu, made from the last two loaves of bread baked at Honeysuckle’s original location. The shoyu, in particular, illustrates Julal’s aim to use Japanese fermentation processes and connect them to ingredients most central to Honeysuckle—while looking at both in a cyclical way.

The kitchen at Honeysuckle has also taken Julal’s black-eyed peaso and made an intensely flavored, Black approach to miso soup, using potlikker—the leftover liquid from cooking collard greens, which may also contain pork stock, and which you don’t let go to waste.

Shio Koji Goes Mainstream

Home cooks and chefs without fermentation systems in their restaurants can also access the umami depth of koji—most easily by marinating meats in shio koji. At Miro in Honolulu, chef Chris Kajioka marinates sea bass and flank washugyu in shio koji for 12 hours at a time. I tasted the impossibly tender, unctuous, and rich latter.

In Philadelphia, chef Alex Holt veers toward farro koji for both savory and sweet applications, but uses shio koji in most of her sauces at Roxanne.

Shio koji in a martini at Mexico City’s Baldío gives the cocktail an olive brininess without the olive. It also lends a creamy milkiness to the restaurant’s aguachiles. There—and at Oyster Oyster in Washington, D.C.—it’s used as a way to help with waste management within the kitchen.

I spotted a dropper bottle of shio koji at Oyster Oyster when dining at the counter one night and asked chef Rob Rubba how the restaurant uses it. Turns out, it often takes the place of a finishing salt on the line. “We try not to use extra salt. Shio koji goes in our green sauce, composed right now of ramps, asparagus trim, onions, spinach, basil, lemon thyme, and our vegetable stock, which is also made from trims. We get it from Keepwell Vinegar, and we also buy their Carolina Gold rice koji to make miso from our waste products like excess bread, mushrooms, and vegetable trim.”

Also in Washington, D.C., chef Kevin Tien applies shio koji—easily obtainable online through Japanese distributors—to lamb shoulder. “Lamb has a distinct taste, and when we use lamb shoulder, it’s a tougher cut. Shio koji helps to tenderize it.” The ingredient saves him time at the high-volume, 100-seat restaurant. “We make everything in-house and quickly. Koji helps us speed up the process and get those deeper flavors.”

Upon Tien’s advice, I ordered a bottle of shio koji off Amazon for $6. I marinated a whole chicken in shio koji—and nothing else—for two days (mostly because I forgot about it in my fridge for an extra day). I cooked it in my convection oven with a touch of oil for 25 minutes at 325°F.

Roasting the chicken filled my house with the scent of miso biscuits. It was also incredibly tender, as if I had gently poached the chicken—yet it carried a sweet, complex depth that poaching alone could never achieve. I may never marinate a chicken without shio koji again.