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The Taming Of The Brew: How Sour Beer Is Driving A Microbial Gold Rush

Whether it's food production, medical microbiology or alcohol-fermentation, one yeast genus holds a near monopoly on research: Saccharomyces.

It's "the most well-studied organism in history," according to Indiana University's Matthew Bochman, a microbiologist specializing in the research of new bacteria and yeast for beer-brewing.

By comparison, we know little about other yeasts. However, an ancient type of beer that continues to grow more popular in America's craft brew scene has sparked a scientific race to unearth and tame new wild yeasts and long-maligned bacteria, which could have wide-ranging implications for other fields.

Sour beer's story starts with Belgium's ancient lambic brewing technique: Wort, the sweet liquid extracted from grain mash, is left in an open vat. Bacteria and yeast from the brew room's wooden ceilings and the Senne Valley air seed the wort before brewers ferment it for up to three years in former wine barrels scrubbed of harsh, bitter tannins. There, slow oxygen diffusion imparts sherry characteristics and softens bitterness. Aromatic compounds leave notes of rose, vanilla and caramelization.

To take some of the guesswork and mysticism out of this process, a few years ago scientists characterized the procession of bacteria and yeast — that is, the microbiome — of these brews. They found that while Sacc. makes most of the alcohol, so-called Brettanomycesyeasts and Lactobacillusbacteria — which normal brewers consider spoilage organisms, but that sour beer circles have nicknamed "Bretts" and "Lactos" — contribute the lactic acid that gives sour beer its flavor.

Colonies of yeast that are capable of making sour beer grow in a Petri dish.
/ Anne A. Madden
Anne A. Madden
Colonies of yeast that are capable of making sour beer grow in a Petri dish.

Nevertheless, lambic brewing is fickle. Brewers hesitate to replace worn panels on the ceiling of the fermentation room for fear of throwing off a batch's flavor. To avoid using potentially taste-altering chemicals, flies attracted to the wort are controlled with an army of spiders.

In America's $55 billion craft brew scene, small brewers can't recreate the exact microbial environment used by Belgian lambic-style brewers, but they do "have the freedom to make a small batch of [sour] beer and see how it's accepted," says Virginia Commonwealth University's Stephen Fong, who's working to identify new potential brewing yeasts.

So, with sour beer becoming a rising star, brewers are partnering with microbiologists like Fong and his wife, Grace, to find (or "bioprospect") new brewing yeasts and bacteria from strange places — or isolate long-maligned stowaways in the microbiome of sour beers and aging barrels.

According to Fong, American brewers typically split sour brewing into two parts: primary and secondary fermentation. During the former, yeast ferments the wort's sugar into alcohol. "For that, we need something that creates ethanol."

There are a lot of variables to consider in finding an ethanol-producing yeast, says Bochman. Potential yeasts must work within a narrow temperature range, and produce ethanol as well as a complex array of flavorful and aromatic compounds, he explains.

"Secondary fermentation is where you produce organic acids and stuff like that, which give it sourness," says Fong. Here, there are way more variables, even with a simple question like: Do you want to sour a beer with bacteria or yeast?

Many sour beers are quite strong for bacteria, says Bochman, "so you need something that will tolerate that much ethanol." In exchange for your troubles, according to Grace Fong, "lactos will do their job really quickly," and some can even brew probiotic beers. But bacteria are nature's favorite infection agent, and your new, resilient bacteria could easily contaminate non-sour beer brewing equipment.

Conversely, souring yeast are more resistant to alcohol and hops, allowing for stronger, hoppier beers. Many also produce a wider array of byproducts, notes Grace Fong, which add to the complexity of the beer. But "A lot of wild yeasts ... don't want to coexist," says Bochman, and of those that do, you need one that's going to be acid-tolerant.

Trial and error abounds. "We've worked with 54 different species from 24 genera," Bochman says, to find five yeasts capable of souring beers. Nevertheless, each new microbe — whether isolated from the microbiome of the Jamestown historical site, or some guy's beard— expands sour beers' flavor palette and allows craft brewers to work with entirely new compounds.

Souring yeast are more resistant to alcohol and hops, allowing for stronger, hoppier beers.
/ Matt Bochman, Indiana University
Matt Bochman, Indiana University
Souring yeast are more resistant to alcohol and hops, allowing for stronger, hoppier beers.

Moreover, sour beer microbiologists may soon consider modifying novel yeast and bacteria, brewing microbes considered wild today, to produce specific flavor compounds and antioxidants. "The technology is already there," says Bochman. "You could conceivably engineer a lactobacillus to be hop tolerant ... [or] species of yeasts to become alcohol tolerant." And for brews in which organisms are filtered out, the final beer wouldn't be a GMO.

In its American reinvention, sour beer has allowed microbiologists an opportunity to find new souring microbes with applications that could extend beyond simply beer. Many of the yeasts and bacteria — and the compounds they produce — being studied for sour brewing have potential applications in manufacturing biodegradable plastics, biofuel, drug delivery systems, antimicrobial substances, breaking down pollutants and antibiotics, and advancing gene and protein expression research.

For now, the largest impact of this sour microbial gold rush is storytelling.

"People are really excited about understanding what hops their beer was made from," says Anne Madden, a bioprospecting microbiologist at North Carolina State University's Robb Dunn lab. "In the very near future, you're going to see consumers asking what yeast their beer was made with." Every brew with a new microbe tells a microbial drama that appeals to aficionados and communicates science.

Bochman, for example, uses sour brewing as a "rubber bullet" to train students who'll transfer their skills to isolating pathogens. "If they drop a sample on the floor, or ruin an experiment, it's not $2,000 down the drain. You're not screwing up some cancer cell line. You just spilled a beer."

Put simply, microbes are the new terroir — one that, thanks to sour beer, tells a more educational story than the same old Saccharomyces tale.

Copyright 2020 NPR. To see more, visit https://www.npr.org.

Ian Graber-Stiehl
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