What Exactly Is Mixed-Culture Fermentation?

February 06, 2020

By Diana Hubbell, February 06, 2020

When Vinnie Cilurzo traveled to Belgium for the first time in 1989, he knew he’d found something special. It was years before he would go on to become the co-owner of Russian River Brewing—long before Pliny, either Elder or Younger. Back then, terms like “spontaneously fermented” and “mixed-culture fermentation” had little place in the American brewing lexicon. Cilurzo did not yet understand the mechanics of the complex flavor profiles he encountered, but he was resolved to one day match them. A decade later, he brewed Temptation, a sour blonde ale aged in California chardonnay barrels.

“The idea was to take my favorite components of a Belgian lambic and use them in an American barrel-aged beer,” Cilurzo says. “We were one of the early adopters of mixed-culture fermentation in the modern craft beer era. In some cases, these beers can be highly unpredictable, which is one of the things I like about them.”

Since then, the art of mixed-culture fermentation has gone from the fringes of nerdcore craft beer circles to a trendy calling card for big-name breweries. Thanks to pioneers like Jester King and Oxbow Brewing, tinkering around with a host of wild and cultivated microbes is no longer a novelty. And as the number of breweries in the U.S. soars past 7,000, well-executed mixed-culture fermentation programs have become a way for brewers to stand out from the crowd.

Essentially, what sets mixed-culture beers apart is they start with a diverse ecosystem of microbes, usually including Brettanomyces, the wild yeast that gives lambics their distinctive funk, Pediococcus, and Lactobacillus. Brewers zealously observe and tailor this mixture as wild microbes from wooden barrels and the surrounding atmosphere gradually alter the character of the culture. Over time, the culture evolves, meaning that no two batches of beer it produces are ever exactly the same. In contrast, “clean” beers like IPAs or lagers start with a monoculture of Saccharomyces, or brewers’ yeast. After six generations, brewers toss this yeast and return to an isolate from a lab in order to ensure consistent results batch after batch.

A music professor said our beers reminded him a little bit of John Cage, that we were in this middle ground between the intentional and the avant-garde.”

“One time, there was a music professor who came by Jester King and said our beers reminded him a little bit of John Cage, that we were in this middle ground between the intentional and the avant-garde,” says Jeffrey Stuffings, co-founder of Jester King. "There are factors that are within our control, but never will we have that full mastery over the beer the way you would a lager or a pale ale.”

The brewers at Jester King may have honed their craft carefully, but Stuffings maintains a healthy respect for the fact that they are working with a dynamic, constantly changing set of living microorganisms. While the barley, malt, and hops in your standard pale ale could come from anywhere, the propriety mixed culture at the heart of many of Jester King’s beers is a fundamental expression of the microbiome of the brewery's 165-acre ranch in Austin, Texas.

"What really draws me to it is the ability to create flavors that are totally unexpected, that I know I could not intentionally create," Stuffings says. "I was really moved by the way that you can forge a greater connection to a place through wild fermentation."

After starting with yeast strains from farmhouse breweries in Belgium and France, Stuffings and his colleagues began to adjust it using native Texan flora. They experimented with adding prickly pears, wildflowers, and agarita berries to wort, then seeing what the resulting fermentations produced. At the same time, they gathered wild microbes from the air using a coolship

“Everything we do is sensory-based. If the yeast capture experiment tasted good, then we would add it to the culture,” Stuffings says. “We've been using the same mixed culture since 2013 and over the years it's morphed. Early on, there were notes of curry spices in our mixed fermentations, nowadays we get subtle notes of peach, apricot, and stone fruit aromas.”

That distinctive mixed-culture cocktail is perhaps the purest possible form of terroir in craft beer. It’s a concept that generations of brewers have intuitively understood and harnessed to give their beers a distinct identity.

“We’re interested in placing our beers in a centuries-old lineage of beers. This is how beers were done before modern science was capable of producing an isolate,” Joe says. “When you study sour beer, what you end up doing is looking at all these different historical methods. Everyone’s favorite wild beer is lambic, but there were also all these other different Belgian traditions, as well as different wild traditions in England and Germany.”

Photo courtesy of Tanner ScarrGrimm Artisan Ales has an extensive barrel-aging program

Since brewers have been fermenting grains long before Dutch scientist Antonie van Leeuwenhoek even spotted yeast under a microscope or French biologist Louis Pasteur figured out what it was in 1857, it’s tricky to say exactly how long mixed-culture fermentation has existed in beer. Certainly, spontaneously fermented ales—in which whatever wild microbes happen to be in the neighborhood are allowed to work their magic—have been around for eons. 

At some point though, humans got a little more intentional about combining brewers’ yeast with its wild relatives in the surrounding environment. We do know, for instance, that Brouwerij Rodenbach in West Flanders began spicing its Saccharomyces up with strains of Pediococcus and Lactobacillus in the 19th century. After Eugène Rodenbach, the grandson of the brewery’s founder, traveled to England in 1878 to study porters, he radically revamped the brewery’s program. Today, Brouwerij Rodenbach is still known for mixed-culture beers aged in enormous oak foeders. 

“We started our program with a mix of five different microbes, but over time, the microbes present in the wood itself mean that there’s a natural amount of drift,” says Joe Grimm. “It becomes a matter of paying attention to the direction the brewery itself is taking the culture and adjusting the recipes accordingly to the resident microbiology. You can get it started on a certain foot, but it’s going to take on a life of its own to a certain extent.”

Since 2013, when Joe and his wife Lauren founded Grimm Artisanal Ales as a nomadic brewery, the couple have been on a mission to guide that slowly evolving microbiome. That means meticulously cleaning brewing equipment to avoid unwanted contamination and carefully collecting yeast from each batch of beer during an optimal point in its life cycle. 

“It’s just a different orientation towards the yeast. We’re trying to be good shepherds of it and help it go in the direction it wants to go, instead of going back to the original isolate,” Joe says. “The result is you have an ecosystem of different microbes in the beer that are all producing different flavors and compounds, so you end up with more complexity, because there are synergistic effects between what each microbe is contributing.” 

As many brewers will attest, one of the dangers of this method is that unwanted bacteria can creep in and take the whole culture in the wrong direction. When a yeast culture goes south, the results are often undrinkable. For instance, Tetrahydropyridine, or THP, is a common microbial foe in the natural wine world. Even a minute quantity can produce an off-flavor reminiscent of a rodent’s den.

It’s just a different orientation towards the yeast. We’re trying to be good shepherds of it and help it go in the direction it wants to go.”

“THP appears as an aftertaste about 10 seconds after you take your sip—it’s almost a grainy, Cheerios-y, mousy taste,” Joe says. “I’ve definitely talked to brewers who’ve had their cultures start to produce flavors they don’t like.”

While the methods behind mixed-culture fermentation may be centuries old, there is still often a lack of well-documented information on how to deal with problems like recurring THP. As a result, there’s still an experimental vibe to the practice. Brewers often turn to other members of the community for advice and real-time troubleshooting. 

Whenever their house culture starts to take a turn for the weird, Joe and Lauren return to an earlier generation of it rather than pulling out the mail-order catalog. By now, their house culture is in its 58th generation, which means that it has had time to adapt to its specific surroundings in Brooklyn. A bottle of Gathering Quetsch Plums, a blended, barrel-aged sour, has a flavor profile that no other brewery can replicate. In an industry where the majority of ingredients in beer can come from anywhere, a distinctive mixed-culture cocktail is perhaps the purest possible expression of terroir. It’s a concept that generations of brewers have intuitively understood and harnessed to give their beers a distinct identity.

“Historically, every important brewery has its own specific yeast that has evolved within its own tanks over the generations,” Joe says. “The flavor of iconic breweries like Weinstephaner or Saison DuPont really comes from their own unique house yeast strains.”

Much like sourdough bakers love to tout the age and pedigree of their starters, these historic breweries are justly proud of the cultures they have developed over time. The most famous of these have gained such a reputation that the brewers often sell them.

“We’re playing around with our house culture all the time. Our hope is that after years of doing this, our yeast will be one of those classic yeasts that other breweries will want to steal,” Lauren says. “The mixed culture that we use is constantly on the move and further developing into something that we think is delicious sour beer.”

At the end of the day, “delicious” is still the operative word here. Mixed-culture fermentation is a means of coaxing out additional flavor out of simple ingredients. Whether it’s used in craft beer, natural wine, or even food, it’s calling on a mix of ancient wisdom and modern scientific know-how to create something that tastes like nothing else.

“The flavor of mixed-fermentation beer has a very specific, satisfying tartness and complexity,” Joe says. “All my favorite things to eat, whether it’s soppressata or aged cheeses, these things all have a real richness and complexity that comes from fermentation with a mixed culture. There’s no other way to get that depth of flavor.”

ZX Ventures, a division within AB InBev, is an investor in October
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