What Is Koji, and How Can It Impact Gut Health? | Well+Good

Unwrapping a package of koji for the first time can easily make you feel like a professional chef that’s about to slay the competition on a heated cooking show (or a kid on Christmas morning). After all, you’re handling an ingredient exploding with umami goodness that will take your culinary skills from zero to 100.

What is koji, you ask? Well, it’s a safe-to-eat-fungus, also known as Aspergillus oryzae, that’s traditionally used in Japanese cuisine to make everything from sake to miso to soy sauce. It’s also been linked to improving digestion, skin, and gut health. To learn more about the benefits of consuming koji, we spoke with several experts, including a Japanese food historian, a registered dietitian, and a culinary chef, to get the full rundown on this fermented food. Plus, a recap of how consuming koji for a week impacted my digestion and sleep—FYI, it was a 10 out of 10 in my book.

What is koji, according to a Japanese food historian

To learn more about the cultural significance of cooking with koji, we spoke with Eric Rath, PhD, a professor of premodern Japanese history and dietary cultures at the University of Kansas. “Koji refers to a variety of molds used in fermentation…[it] contains some 50 enzymes, and these break down the starch in grains like rice into sugars to make them available for yeasts to consume in fermentation,” Dr. Rath says.

According to the professor, the fermented rice-derived product dates back centuries in Japan’s history—as far as the eighth century—and is used to make many of the Japanese ingredients we know, love, and consume daily today. “In Japan, koji is essential for making alcoholic beverages such as sake and awamori, flavorings such as miso and soy sauce, and fermented foods such as traditional forms of sushi and pickles,” says Dr. Rath.

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In recent years, chefs and food enthusiasts worldwide have started to experiment more and more with koji, as it’s an easy and delicious way to impart tons of umami-rich flavor to proteins and vegetables, and can also be used to cure meats. “One modern use of koji is as ‘salt koji’—known as shio koji in Japanese—which is a mixture of salt, koji-infused rice, and water,” Dr. Rath says. “Many chefs and consumers are surprised at the many ways that koji can impart both sweetness and umami to foods.”

A registered dietitian weighs in on the health benefits of consuming koji

Aside from its rich history and deliciously sweet-savory flavor, know that koji is really good for you. “Like other fermented foods, koji is beneficial due to the probiotics it contains, which are friendly gut bacteria that can improve digestion and overall health, says Laura Iu, RD, CDN, CNSC, a registered dietitian nutritionist. “It’s also rich in essential B vitamins, which are vital in the regeneration of skin cells, energy, and brain function.”

Iu explains that koji can be found in many different food products and is a gut-healthy way to add umami flavor to just about any type of food. “For example, common uses for shio koji is to marinade and tenderize proteins, while shoyu koji is a soy sauce that can be delicious in salad dressings. As a super versatile ingredient, koji adds rich flavor to plant-based dishes and enhances meat-based ones,” she says.

As far as who can (and should) eat this flavor-boosting food, Iu says that been around for centuries and is a staple in Japanese cuisine that’s considered safe for most people when consumed in moderation. Studies have also indicated that this ingredient contains a compound called glycosylceramide that functions as a prebiotic that might be the connection between Japanese cuisine, intestinal microbial flora, and longevity. Although, as always, she recommends consulting a nutritionist to ensure it’s well-suited for your personal needs.

How a professional chef recommends cooking with it at home

Christopher Arturo, chef-instructor of Culinary Arts at the Institute of Culinary Education, has a love for koji so deep that he started cultivating it on his own in the fermentation lab at the school. With a balance between ingredients, science, and lots of patience, Arturo discovered the best ways to incorporate koji into his everyday cooking. He explains that moisture and temperature are two key elements he has to control when making koji from scratch.

In the lab, Arturo grows koji in a temperature and moisture-controlled cedar chamber set at about 85°F and 70 percent humidity, which allows the mold spores to propagate on the cooked rice. Though it can take several weeks to cultivate, the chef enjoys making it from scratch, as it gives him the liberty to control the flavors more precisely and gives him access to the freshly-made ingredient at all times. Of course, he notes that if you’re pressed for time or don’t have the fancy equipment to make it yourself, he says you can find plenty of excellent packaged koji products at Japanese grocery stores, specialty markets, or online. To scope out a high-quality product, Arturo says that a good batch of koji smells like a newly-opened bottle of sake and has a “sweet funk.”

One of Arturo’s favorite ways to use this ingredient is “flash dry-aging” steak, where he uses shio koji as a marinade that transforms the protein into a super tender and umami-rich dish. “Koji is a super thirsty mold, and through the wonderful science of osmosis, it’s going to keep pulling out moisture from the protein in order to drink,” he says. As this occurs, Arturo explains that the mold undergoes an enzymatic reaction that breaks down the exterior of the protein to form glutamate, which is one of the building blocks of umami, yielding a sweeter, richer, and even more flavorful food.

Although making a koji-marinated New York strip steak is Arturo’s way to go, he says you shouldn’t stop there. He’s also experimented with pickling everything from peppers to cucumbers to plums using koji’s transformative powers.

I tried eating koji every day for a week—here’s what happened

Growing up in a Japanese-American household, eating koji-derived foods like soy sauce and miso was the norm. However, with limited access to imported Japanese foods in my hometown, koji on its own wasn’t something I ever used. Now, living in California, where there are specialty markets with much more accessibility to international ingredients—and thanks to next-day delivery on Amazon—getting my hands on a package of shio koji was beyond easy. To really understand the benefits of this fermented food, I decided to take the advice of our helpful koji experts to try a variety of koji-infused dishes for a week—here’s what I discovered.

First, my taste buds were leaping with joy upon taste-testing the shio koji made with just four ingredients (rice, sugar, salt, and koji). I quickly realized what the hype was all about. The paste tasted nutty, savory, and a little funky, much like creamy salted butter mixed with a hint of miso-y tangy. I used the product to marinate a New York strip steak, pickle thinly-sliced carrots, and roast sweet potatoes. Every one of these dishes was absolutely spot on. The shio koji enchanted each recipe with its enzymatic powers to create super tender steak, tangy yet sweet carrots, and decadent sweet potatoes. My favorite part? The absolute versatility of this ingredient. No matter the protein or produce, the koji imparted so much umami that I was on cloud nine all week long.

Aside from my taste buds feeling great, my gut also benefited from this fermented food. Shio koji, which already has salt in it, meant that I reduced how much sodium I used while cooking, as the product is beaming with flavor on its own. My gut felt well-balanced, and digestion was highly regular—but perhaps that was from being so relaxed due to all of the good food I was enjoying.

Want more koji-infused fermented goodness? Check this out: