Koji has increasingly caught the interest of western chefs, brewers, and others in the food and beverage world, though it’s not always well understood. “I think in the western world, koji is still in a kind of discovery phase,” says Brandon Doughan, the cofounder and head brewer of the New York sake brewery Brooklyn Kura. “But it’s one of the things that got me into all of this.”
Koji, a mold that thrives in hot, humid environments, was officially recognized as the National Fungi of Japan in 2006, and is used in the production of sake, shochu, miso, and soy sauce. Its role in sake production is unique in the world of brewing, and so vital that the Japanese government’s request to have UNESCO recognize sake as an Intangible Cultural Heritage mentions the mold explicitly in the title.
How exactly does koji—and its various species and strains—influence sake production? Here, sake brewers explain the science behind this unusual ingredient.
Koji’s role in the brewing process
Koji’s job in the brewery is to provide the enzymes that convert the starches of rice into sugars, a process Doughan compares to malting in beer brewing. The sake brewer starts with a powdered form of koji spores, usually referred to as koji starter, tane koji, or seed koji, to differentiate it from the mold itself. Technically, the mold itself is koji, and the koji-covered rice is called kojimai, but most brewers refer to the koji-covered rice simply as koji.
Brewers sprinkle the koji starter onto a portion of the rice to be used for a batch of sake. The inoculated rice is kept bundled overnight in a room favorable to koji growth; around 30 to 32 degrees celsius and 40 percent humidity is typical. Strict hygiene is vital, as many unwanted bacteria also thrive in these conditions.
“The next morning you can see white spots on the rice where the fungus is beginning to grow,” says Doughan. The brewery team spreads out the rice on a large table or into a number of small, shallow trays to disperse the heat and control the temperature. The goal is to get the microscopic filaments of the mold, called hyphae, to penetrate each rice grain evenly and consistently. “We’re trying to make each grain of rice into a little time capsule that will release its enzymes slowly and evenly over the month of brewing, so we get a balance between the activity of the yeast and the introduction of sugars.”
The mold’s job is done once the koji-covered rice is added to the main brewing tank, but the enzymes go on to ferment the starches into sugars at the same time yeast is turning those sugars into alcohol.
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