Sour in an Hour?

Brewers are experimenting with white koji to speed up the time to acidify their kettle sours.

John Holl Jun 22, 2019 - 3 min read

Sour in an Hour? Primary Image

White Koji

Craft brewing is all about innovation. Whether it’s adding new flavors to a recipe or finding new ways to brew, there’s no shortage of paths brewers are willing to explore. That’s how many found themselves last month in Denver at the Craft Brewers Conference standing at the White Labs booth talking with Todd Bellomy about a microorganism that will speed up the process for kettle-soured beers.

Bellomy, a long-time brewing-industry professional turned sake entrepreneur, was touting the benefits of white koji, which is technically a fungus and is used to make a wide variety of fermented foods.

Most of us know about koji’s role in making miso, soy sauce, and, of course, sake. Koji breaks down substances such as rice or soybeans to fermentable sugars. Yellow koji is most commonly used in sake. White koji has been used to make shōchū, a cousin to sake.
However, when you add koji into the mash of, say, a Berliner weisse, it will dramatically cut the time needed to properly kettle sour a beer.

“You talk to so many brewers who pitch lactobacillus (sometimes yogurt) on Friday, let it grow all weekend, producing lactic acid, and then come back on Monday to boil the wort,” says Bellomy. “With test brews that we’ve done with white koji, if you make it 20 to 25 percent of your grain bill, you could make a sour beer in a standard 1-hour mash time.”


It’s like the Fotomats of old: Sour in an Hour.

“White koji already contains the citric, lactic, and succinic acids, which impart a beautifully clean acidity to the beer,” Bellomy says.

To demonstrate he gives the example of a Berliner made mostly with Pilsner malt, a tiny bit of wheat, and 25 percent white koji and later fermented with neutral German ale yeast. It comes out at 3.2 pH after just an hour mash.

“You get this clean acidic profile that you can control,” he says.

As more brewers experiment with the ingredient, there’s the possibility for new avenues to open up and more white koji will make its way to U.S. shores from Japan.

“I think for small to mid-size brewers, this will help them turn around kettle sours faster and even control the process a little better,” Bellomy says. “It is more expensive per pound than malt, but the cost savings of freeing up 3 days in the brewhouse makes sense.”

John Holl is the author of Drink Beer, Think Beer: Getting to the Bottom of Every Pint, and has worked for both Craft Beer & Brewing Magazine® and All About Beer Magazine.