The buzziest term in beer today is not one your average drinker knows, or even one that appears on menus or a beer hall’s chalked-up list. Its meaning isn’t explicit to those in-the-know, either, yet mixed fermentation is a process integral to some of the world’s most complex beers, often evoking a carefully honed “house” flavor profile.
“‘Mixed fermentation’ as a buzzword is really more of a past-couple-of-years phenomenon,” says Mike Naclerio, a corporate brand manager for Sheehan Family Companies, a beer distributor whose portfolio includes mixed-fermentation darlings like Minnesota’s Fair State Brewing Cooperative and Maine’s Oxbow Brewing Company. Though the technique has been around for centuries, the term (also called “mixed-culture fermentation” or “mixed cultures” or even “mixed ferm”) and its self-conscious perpetuation are relatively new to the American beer scene.
So—what is it, exactly?
“It’s always kind of vague in a way,” says Naclerio, even though the process is technical enough to necessitate an 8,000-word Wiki page. The TL;DR is this: There are so-called “clean” beers—lagers, IPAs, stouts—that are brewed strictly using a single strain of Saccharomyces, otherwise known as brewer’s yeast. Then there are “wild ales”—spontaneously fermented beers like lambics—created by microbes in the atmosphere like the wild yeast Brettanomyces and lactic acid–spawning bacteria like Lactobacillus and Pediococcus. Combine any or all of these yeasts and bacteria together, and the result is a “mixed” fermentation. Today, the term typically designates a deliberately made, complex fermentation “cocktail,” which results in hard-to-classify beers. Often, they have a noticeable acidity and/or funkiness and, for a lack of a better option, get categorized stylistically as farmhouse or sour ales.
“It has certainly risen to prominence recently, but the term has been around much longer,” says Michael Tonsmeire, who blogs as The Mad Fermentationist and wrote the seminal book American Sour Beers in 2014. He claims that as early as 2012, he sent an email to Chad Yakobson, of Denver’s Crooked Stave Artisan Beer Project, suggesting “mixed fermentation” as his preferred nomenclature for the emerging category of beers that were neither an ale nor a lager, nor purely wild. He believes he likely picked up the term from a then-fellow homebrewer, Nathan Zeender (who ended up at Washington, D.C.’s Right Proper Brewing). Tonsmeire is quick to note, however, that “the actual process certainly goes back hundreds of years, [with] Rodenbach being one of the more famous examples.”
While Rodenbach, the nearly 200-year-old Belgian brewery, seems to have incidentally struck upon the idea of mixing Saccharomyces cerevisiae, Lactobacillus and Brettanomyces together to ferment their archetypal Flanders red ale, today’s breweries are intentionally cultivating and labeling these mixed house cultures. The technique has been especially bolstered by sophisticated fermentation laboratories such as San Diego’s White Labs and Hood River, Oregon’s Wyeast, which specialize in capturing and isolating wild strains that they then sell to breweries.
One of the first modern breweries to adopt mixed fermentation was the Maine farmhouse brewery Oxbow. Co-founder Tim Adams says that they started using the term around 2013 or 2014, but notes that the first commercial beer they ever brewed, Arboreal, an amber-colored sour ale in 2011, was in fact a mixed fermentation. “It was not a very common term back then. It still isn’t, if we are looking at the mainstream,” he says.
Oxbow produces a house ferment by propagating wild cultures from their spontaneous, coolship-fermented beers like Native/Wild, then adding those to a house cocktail of laboratory-purchased saison yeast and Brettanomyces. This creates an array of bacteria and wild strains far more complex than those in a standard wild ale. Oxbow uses the blend in their Albarrique, a fruited farmhouse ale, and Pastoral, a mildly acidic blonde ale.
“The important thing about the term is that it helps to differentiate the confusing realm of ‘sour’ beers,” explains Adams. Most mixed-fermentation beers have a noticeable acidity and complexity compared to the simpler (and more quickly produced) kettle sours, which are usually soured purely via lactic acid bacteria. If kettle sours all taste fairly similar from brewery to brewery, mixed-fermentation beers vary from brewery to brewery, providing a unique flavor profile for each.
The terminology is finally beginning to appear on packaging, providing consumers with a reference point. Recent sightings in the wild include cans of Trve Brewing’s Cursed, which is labeled a “mixed culture sour pale ale,” and Night Shift Brewing’s Rickey Weisse, a “mixed fermentation sour ale aged with raspberries and limes.” Dave Martin, of the acclaimed home brewery Mindful Ales, has become so well-known for making exclusively mixed-fermented beers that he now only employs the term when integrating it into styles you wouldn’t expect, like a mixed-fermentation IPA.
Martin’s mixed culture started as a blend of a saison Saccharomyces strain and a separate spontaneous-ferment, based off of cacti. For four years, he deployed it in spontaneous and open fermentations, adding yeasts off of fruit skins and bottle dregs, until he was content with the result (which is banked and exists as such in perpetuity). Martin thinks his mixed culture presents best in his farmhouse ales Elemental and Dreams (Whatever They May Be).
This process of accretion and honing is one reason mixed-fermentation beers are gaining reverence; once a brewer has hit upon his or her ideal formula, they’ve created a kind of inherently unique terroir.
“Differentiation is becoming a bigger deal now that there are so many breweries,” says Tonsmeire. With about 8,000 beer producers in America alone, he believes mixed fermentation is one of the few ways brewers can truly separate themselves from the pack. Last year, Tonsmeire and a partner opened Sapwood Cellars in Columbia, Maryland, to focus on barrel-aged mixed-fermentation beers. “The nice thing about mixed fermentations is they can be unique … [even] where other breweries may [use] the exact same ingredients.”
Five Mixed-Fermentation Beers to Know
While it can be difficult to know if you’re drinking a mixed-fermentation beer unless it’s explicitly labeled as such, many of today’s top farmhouse and “sour” breweries are producing them. Here are five to seek out.
The Maine brewery’s first-ever mixed-fermentation beer is still a key part of the portfolio, eight years later. An amber sour fermented with Brettanomyces and Lactobacillus and aged in ex-bourbon barrels, it’s funky and vinegary, with heavy notes of sour cherries, balanced by a malty sweetness.
- ABV: 8.5 percent
Russian River Temptation
Perhaps America’s preeminent sour beer producer, the Sonoma outfit was brewing mixed-fermentation beers before they were fashionable, yet doesn’t label them as such. Nevertheless, Temptation, a blonde ale, is fermented with Russian River’s special house yeast before aging in French oak chardonnay barrels. During the barrel-aging processes, Brettanomyces, Lactobacillus and Pediococcus are added to the barrel, resulting in a dry, Champagne-like beer.
- ABV: 7.3 percent
Sapwood Cellars First Blush
This Maryland-born collaboration uses chardonnay barrels from local natural winery Old Westminster. The pale beer is fermented using a combination of Sapwood’s two house mixed cultures, along with Old Westminster’s yeast and chardonnay pomace. Once that’s added to the barrel, it’s topped off with Spanish cedar and cabernet franc grapes, creating a moderately tart and light beer more akin to a rosé.
- ABV: 8.2 percent
Mindful Ales Dreams (Whatever They May Be)
A green-bottle saison brewed with local honey, lavender, lemon peel and black lava sea salt, this garage-brewed offering utilizes Martin’s carefully cultivated house culture, creating a floral flavor profile with good acidity and just a little brininess.
- ABV: 6.5 percent
Kent Falls Alternate World
This Connecticut farmhouse brewery’s wheat-based gose is fermented using a house Brettanomyces strain along with what they call their “souring cultures” like Lactobacillus. A refreshing summer sipper, it’s succulent and a bit fruity, owing to some additional dry-hopping.
- ABV: 4.6 percent