The Science of Sour Beer

Beer experts dive into the methods and microbes behind this oft-misunderstood category

Averie Swanson
Averie Swanson. Photo courtesy of Keeping Together.

Sipping a sour beer for the first time is a shock to the senses. The category is instantly recognizable thanks to tart, fruity, funky flavors. Yet the intense acidity associated with sour beers today is only one component of a diverse spectrum of sour beer styles and flavors.

What defines sourness is as much a science as it is subjective taste. As Vincent Cilurzo, the cofounder of Russian River Brewing in Santa Rosa, California, writes in the The Oxford Companion to Beer: “[A]cidity is usually considered a flavor fault in modern beers. When speaking of beer, the word ‘sour’ is usually pejorative.” Yet when beers are intentionally soured, he notes, as in Old-World styles or the New-World replications of those styles, “they can be among the most complex and refreshing of beers.”

Generally speaking, sour beer is defined as that which is high in acidity derived from lactic-acid producing bacteria. Technically speaking, it is beer that has been subjected to bacterial infection. In other words, context is key. 

“When I think of sour beers, I think of any beer that has sourness as part of its primary or secondary flavor components,” says Max Finnance, the senior manager of education and training for Artisanal Brewing Ventures, a North Carolina-based parent company of Northeastern brewers including Southern Tier Brewing, Victory Brewing, and Sixpoint. “There are lots of styles of beer that can have sourness as a part of their overall flavor profile, and other than that, don’t have much in common.” 

Sour beer styles span centuries of Western brewing traditions: Belgium’s rich, full-bodied Flanders brown ales, berry-tart Flanders red ales, and the spontaneously fermented and sharply acidic lambics of Brussels; Germany’s bubbly Berliner Weisse, the “Champagne of the North,” and the salty, softly sour gose; and American and other new-school brewers’ interpretations of these historical styles. In the last decade, Belgian- and German-inspired sour ales have split into endless refractions, bringing on such unexpected trends as fruited sours mimicking smoothies and decadent desserts; and hazy, hoppy sour IPAs. 

Add in the overlapping “wild beer” category, which favors Brettanomyces yeast strains but is not “sour beer” unless acidified, and it’s no wonder retailers and consumers are often left in the dark about what exactly a sour beer will taste like. Where each wild and sour beer style lies on this microorganism-speckled spectrum depends on a variety of brewer decisions and environmental factors.

Kettle Souring

Quick souring, also known as kettle souring, encompasses a vast array of tart, low-alcohol American sour beers. With this method, beer is soured directly in the brew kettle, or stainless steel tank, by adding a lactic-acid producing bacteria to the wort pre-boil. This lactic acid starter gives the bacteria a head start on turning sugar into lactic acid before the brewer pitches yeast. It can also be pitched post-boil for added acidity.

The two main acidifying bacteria used to sour beer are Lactobacillus and Pediococcus. In the case of a kettle sour, Lactobacillus is more favorable, contributing what is considered a “clean” acidity. Pediococcus can offer a more complex acidity over time, but comes with more risk of off-flavors.

Pediococcus might work well in a lambic or gueuze, but in a kettle sour, picking a clean Lactobacillus strain is going to give you a nice, clean, lactic acid sourness. And sometimes that’s all you need to make a really delicious gose or Berliner weisse,” says Finnance.

Although the original Berliner weisse likely got its tartness from a sour mash, yeast cross-contamination during fermentation, or microflora during storage or maturation, brewers typically replicate the style today as a kettle sour using Lactobacillus to acidify the beer during fermentation. This form of quick souring is the least risky because brewers can carefully select their microbes and have more control over the fermentation. It is also the most efficient way to make sour beer; a typical kettle sour can be turned around in two weeks or less, compared to a traditional barrel-aged sour that can take months and even years.

Flavored sours have become popular in recent years, with brewers experimenting with a variety of fruit purees, juices, and yeast strains to amplify citrusy or tropical fruit character derived from fermentation. Stratford, Connecticut’s Two Roads Brewing Co. has an ongoing series of kettle-soured (or more specifically, tanker truck-soured) fruited goses available in a variety of tart-friendly flavors, including Passion Fruit, Pineapple Mango, and Persian Lime. 

Still, others reach for stronger extremes: Victory Brewing’s new Berry Monkey, a fruited sour tripel made with raspberries and Belgian yeast, pushes alcohol to a high-octane 9.5% ABV. 

The Grace of Maybe sour beer. Photo courtesy of Keeping Together.

Souring With Spontaneous Fermentation

A growing number of U.S. craft brewers and blenders are incorporating spontaneous fermentation methods into their sour beer production. To achieve the complexity of a traditional Belgian lambic like Brasserie Cantillon, the single remaining traditional lambic brewery in Brussels proper, the most devoted have even added coolships to their properties. Established regional brands such as Russian River Brewing in Sonoma County, California, Allagash Brewing in Portland, Maine (whom Cantillon has officially collaborated with), and Two Roads’ offshoot Area Two, have produced a dizzying array of both traditional and experimental bottlings.

The Referend Bier Blendery in Kutztown, Pennsylvania, replicates the historical Berliner style in Berliner Messe, a Berliner weisse-style beer that is “spontaneously acidified,” according to the label, and fermented in oak barrels. Each bottling incorporates different barrels and aging periods; the recent Blend No.: 7 was fermented in a mix of neutral French oak barrels and Italian red wine puncheons for between eight and 30 months, or an average age of 18 months at bottling.

Opting for open fermentation, or letting wort cool in a coolship, welcomes the spontaneous inoculation of wild yeast and bacteria in the air. This includes “all sorts of microbes that you wouldn’t normally want in a sour beer,” notes Finnance. Bacteria strains such as Acetobacter, Enterobacter, and Pediococcus will “show up and make what would otherwise be considered some really gnarly off flavors.” 

That’s where Brettanomyces comes in. “[G]iven enough time, the Brettanomyces is going to clean up those off-flavors and turn [them] into some really nice-tasting compounds,” adds Finnance. For example, Butyric acid past its “beer threshold” will give a rotten, cheesy aroma. With Brett by its side, Butyric acid gets broken down into Ethyl butyrate, which “tastes like pineapples.”

Brettanomyces strains work very slowly. That’s one of the reasons that lambic and gueuze take such an extended maturation time to get a product that tastes palatable and tastes delicious and tastes like Belgian lambic or gueuze that we all know and love,” Finnance says.

Spontaneous fermentation is not as simple as “set it and forget it,” however. Averie Swanson, the founder and beer maker of Keeping Together and one of the world’s only Master Cicerones, spent several years honing spontaneous fermentations at Jester King Brewery in Austin, Texas. Now helming her own beer brand, which she is currently transplanting from Chicago to Sante Fe, New Mexico, Swanson has shifted her focus to mixed-culture saisons fermented in stainless steel.

“Spontaneously fermented beer is more challenging than people give it credit for,” says Swanson. “There’s a lot more attention to detail and intention that goes into making those beers, from how you’re building your grain bill, to how you’re going about your mash schedule and the entire hot side process, to where, when, and how you are knocking it out into the coolship and letting it cool overnight.”

Mixed-Culture Fermentation

Sister to spontaneous fermentation is mixed-culture fermentation. “I feel like there was a time in the craft beer scene here in the U.S. where folks were making really aggressively sour beer primarily through mixed-culture fermentation, pitching Lactobacillus, pitching Pediococcus, either post-primary fermentation and into barrel aging, or perhaps just into barrel aging,” says Swanson. “That degree of acidity, though it can be done really well and can make really interesting beers, doesn’t necessarily lend itself to especially balanced or drinkable beers.”

While many mixed-culture fermentations can reach too-tart territory, Swanson prefers “to create acidity in a beer that acts as a balanced flavor element as opposed to the primary flavor element.” She does this by manipulating variables such as IBUs and temperature throughout the brewing and fermentation processes. 

“[Y]ou can control for when acidity will develop,” she says. “I want to see Saccharomyces in my culture dominate for the first part of fermentation, but I want that bacteria to be present in the background so that when the Saccharomyces begins to slow down a little bit, or once that primary fermentation is coming to an end, then that lactic-acid producing bacteria can begin developing a little bit of acidity.”

For even greater acid development, Swanson says, “I would put it into an oak barrel and allow that beer to sit in there for a period of time—four to six to eight months, if not longer—and allow the concentration of lactic acid or other types of acid to increase through that bacterial activity over time.”

Brewers can also opt for lab cultures, selecting specific Brettanomyces strains to achieve their desired flavor profile. “We have so many different types of Saccharomyces yeast that can make all sorts of different flavor profiles and all sorts of different beers—whole categories of Belgian yeast and English [yeast]—and it’s almost true for Brett as well,” says Richard Preiss, the cofounder of Escarpment Laboratories, which provides yeast cultures to Canadian craft brewers from its lab in Guelph, Canada. Preiss describes a “whole rainbow” of Brettanomyces strains among Escarpment’s offerings, each selected for flavors ranging from “horse-blanket funk” to pineapple, plum, pear, cherry, citrus, and white wine.

Escarpment Labs
Photo courtesy of Escarpment Labs.

New Frontiers

In addition to a broad yeast strain collection, Escarpment Labs has been developing and launching new products that target more niche needs, including “Lactic Magic,” an acid-producing yeast strain developed with sour beer brewers in mind. “We started seeing this interest pop up with brewers [for] yeast that can be used to sour the beer, because normally they have to use bacteria,” says Preiss.

Lactic Magic is developed from a species of yeast called Lachancea thermotolerans. “We were able to dig into our strain collection and screen a few different strains of that species. We found one that we really liked that was doing a good job of producing acid and that we felt had a really unique flavor profile,” says Preiss. “We’re excited about it because it’s just different from a Lacto sour. You can do different things, like make a sour IPA.”

It has potential for barrel-aged sours, too, Preiss says, adding that this could be one of the strain’s best applications. 

“There’s no right or wrong way to make a sour beer, as long as you end up with a product that tastes good and isn’t showing off too much of a specific off-flavor—or any, ideally,” says Finnance. Though sour beers can be crowd-pleasers, there is much to be learned—and tasted—in the realm of these tart, tangy fermentations.


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Cat Wolinski is a writer and editor specializing in the beer and beverage industries. Her work has appeared in publications including SevenFifty Daily, Wine Enthusiast, Craft Beer & Brewing, and VinePair, where she was formerly managing editor. She lives across the street from a brewery in Brooklyn, New York and is WSET Level 2 certified.

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