It’s a rare pleasure to work with an ingredient that’s wholly new to me. I’d read about shio koji, but until I started cooking with it, I didn’t fully appreciate its ability to enhance—and in some cases, even utterly change—the taste of nearly any food.
Shio koji is derived from just three ingredients: water, salt (shio), and cooked rice that’s been inoculated with koji, the ancient, safe-to-eat mold Aspergillus oryzae. “Japan’s national fungus,” as this mold is nicknamed, is also used to make soy sauce, miso, and sake.
After fermenting for a week or so, the mixture takes on a milky, porridge-like appearance and a floral-fruity fragrance, and the shio koji is ready to transform food.
Japanese cooks use shio koji to cure fish, to turn vegetables into pickles overnight, and as an all-purpose seasoning. They also employ it as a marinade that can make any cut of meat taste like the best version of itself.
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More Than a Marinade
When Sonoko Sakai talks about shio koji, her eyes light up. The cook, teacher, and author of Wafu Cooking: Everyday Recipes with Japanese Style (2024) gushed over the ancient ingredient during a video call, describing it as something to “share, cherish, and preserve.”
Although shio koji has been around for millennia, she explained, it’s only come back into vogue in the last 10 to 15 years. “You wonder why it was forgotten,” she said, praising its convenience and functionality.
Nowadays, most Japanese cooks keep a bottle on hand, ready to improve everything from roast chicken to salmon to water kimchi. “I call [shio koji] my secret weapon,” Sakai said, recommending that cooks use it as a replacement for salt. “I put it in everything.”
Take flank steak, an everyday cut that many of us turn to for its relative tenderness and beefy flavor.
But marinate that same steak in shio koji, and its meaty taste will be amplified as it takes on the umami-rich, slightly nutty, and blue-cheese-y flavor of dry-aged beef.
For expert insight on this unique ingredient, I invited Rich Shih, coauthor of Koji Alchemy (2020), the definitive book on the near-magical properties of koji and koji-derived ingredients, to cook with me in the test kitchen. As we stood stoveside marveling at the taste of flank steak that had been soaked in shio koji for a few hours prior to cooking, he grinned between bites, remarking that with shio koji in your arsenal, “you’re no longer limited to premium cuts.”
How Shio Koji Mimics Dry Aging
Dry aging beef for weeks or months allows enzymes naturally present in the meat to break down muscle proteins into smaller molecules, creating remarkable depth of flavor in the process.
Shio koji’s potent concentration of enzymes achieves a similar effect in just hours. The protease enzymes in shio koji act like scissors, snipping at the beef’s long chains of amino acids and liberating thousands of types of new, short chains called peptides, as well as individual amino acids.
Many of the peptides contribute new flavors; for instance, beefy meaty peptide (BMP) is named for its savory qualities.
The individual amino acids also heighten flavor: most notably, glutamic acid, which is associated with umami. Others, such as lysine and tryptophan, take part in Maillard browning reactions that produce rich, roasty colors and flavors when the steak is seared.
Finally, the sodium and chloride ions in shio koji play an important role. Both are quite small and readily migrate into flank steak. The sodium seasons the meat while the chloride prevents the proteins from tightening too much during cooking, which helps the steak hold on to more juices.
Shio Koji–Marinated Flank Steak
Japan’s shio koji makes food taste like the best version of itself. Here’s how to put it to work on flank steak.
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Shio Koji Butter
A dollop of this simple, savory butter perks up everything from steak to vegetables to bread.
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