Your Soy Sauce Probably Contains Mold—Here's Why That's a Good Thing

The mold is actually what gives it its delicious, umami flavor.

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Soy sauce is one of the most widely used condiments in the world—chances are you have a bottle in your fridge right now. But whether you're dunking your sushi roll into a bowl or it or adding a splash to a chicken marinade, how much do you really know about this salty dark liquid?

For one, the soy sauce you buy at the supermarket or find on the table at your average sushi joint is likely not the good stuff. That's because the process of making quality soy sauce takes quite a bit of time—a minimum of six months to a full year to ferment properly, says Chef David Santos of Um Segredo Supper Club and Good Stock soup company in New York. "I can't imagine Kikkoman taking the full time it takes to mature soy sauce—that would take up a ton of space," he says. (Kikkoman was not available for comment by press time.)

So why, exactly, does it take so long to make? The process for soy sauce starts off by cooking soybeans in water until they're tender, then draining them and adding to a food processor with 30 to 40 percent flour. "It's [a texture] like cookie dough," Santos says. Here's where things get interesting: The next ingredient—and one that's crucial to the umami flavor development of soy sauce—is mold. It's added to the "cookie dough" and then fermented in a solution of water and sea salt.

The scientific name for the mold is Aspergillus, but it's better known as koji. It's the same bacteria used in saki preparation—and yes, you can actually buy koji on Amazon. Santos, who makes his own soy sauces, gets koji that's been cultured on cooked rice. "It looks like a white mold, almost like the outside of a wheel of Brie cheese," he explains.

Even if you love Brie, you might be a little put off by the fact that a beloved condiment is made with bacteria. But alas, it's not actually as weird as it sounds. "We're surrounded by bacteria that we ingest every day—things like sauerkraut [are made by] bacteria-based fermentation" says Santos. Different bacteria produce different flavors, but kogi is the singular bacteria that is used specifically for soy sauce and sake, he adds. Take bleu cheese as another example: Most of the kinds we know today are inoculated with penicillin mold to give them that umami flavor.

Is Soy Sauce Bad for You?


Recipe pictured above: Stir-Fried Japanese Eggplant with Garlic-Soy Sauce

Koji is crucial to developing the flavor profile of soy sauce, yet during the long fermentation process, things can, of course, go wrong if you're not careful. "Your enemy at this stage is other bacteria," says Santos. Usually, once a "good" bacteria such as koji takes hold, it defends against other bacteria—but there can be stronger bacteria in the air that can attack and overtake the koji, and that's what causes soy sauce to go bad. A sure way to tell is if the mold changes from white to another color, such as green or red, says Santos.The risk involved with home fermenting is one reason health departments don't technically allow chefs to make their own.

Kevin Adey, the Michelin-starred chef-owner of Faro in Brooklyn, does it anyway—but says if he was making soy sauce or kimchi and the New York City Health Department walked in, he'd run it outside the building.

"Although it's the way all soy sauce is made and part of food culture all over the world, there is nothing you can say to the health department to make [growing mold] OK—if they see it being done, they'll lose their mind," says Adey.

soy sauce
Getty / David Samperio Garca / EyeEm

He's spent several years reading up on fermentation from books like Sandor Katz's Wild Fermentation and is by all means an expert in making soy sauce, though he's the first to admit that things can go "very, very bad" if someone doesn't know what they're doing. It took Adey a while to feel comfortable enough with his mold-managing skills to be able to share his soy sauces. "Even as a pro chef, the fear of killing people—mostly myself—was a big deterrent," he jokes.

Provided that soy sauce—even the homemade fermented kind—is done correctly and no "bad" bacteria get into the final product, is there any health risk to consuming it? Probably not. "I haven't heard that the molds and bacteria used to ferment soy sauce cause any health concerns," says Melissa Nieves, LND, RD, MPH, who works with Healthy Meals Supreme. "There are some risks to consuming soy sauce, but the worst are associated with the chemically produced varieties—naturally fermented soy sauce is a better option."

Chemically produced soy sauce, which is made in days (without fermentation) with ingredients like hydrolyzed soy protein and added flavorings, can contain a substance called 3-MCPD, which has been linked to tumors in a rodent study. The FDA sets very low limits on the amount of 3-MCPD allowed in food products, so you shouldn't have to worry.

On the other hand, fermented soy sauce (made with koji) can actually provide health benefits, says Nieves, since fermented foods are considered probiotics—but in moderation. "Since it's very high in sodium, it helps avoid overgrowth of all the molds and bacteria used in the fermentation process. These microorganisms don't really represent any health risks, unless the individual is specifically intolerant to any of them," she adds.

Of course, due to the high sodium content of soy sauce, people with conditions such as high blood pressure, heart disease or renal disease should limit its consumption as much as possible, says Nieves. And if you're gluten intolerant, don't forget that soy sauce contains wheat (here are some soy sauce alternatives for gluten- or soy-free eaters.)

The Bottom Line


Recipe pictured above: Soy-Lime Roasted Tofu

You don't really have anything to fear about consuming small amounts of soy sauce—though if you saw it being made, it might make anyone with a weak stomach a little queasy. "It looks like moldy beans, and it is," says Adey, "and they're going to be delicious."

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