The Secret Life of Yeast

March 1, 2012 By Ken Weaver

To earlier generations of brewers, the microscopic beings responsible for fermentation typically arrived on the scene in the form of a family’s trusty brewing stick, the dank, wooden crevasses pocking the walls of a farmhouse, or straight from thin air, carried along within the currents of cool nightly breezes. While a growing number of traditionally minded brewers in Belgium, the U.S., and further abroad still rely on wild yeasts and bacteria for their spontaneously fermented creations, people today usually encounter yeast as a pack or vial from one of the two main yeast labs, or (just as often) as that murky glop appearing unannounced from a bottle of homebrew.

Granted, it’s hard to be sexy when you’re only a few micrometers across.

But while there have been countless developments improving the science and art of brewing over the past few hundred years—a significantly expanded variety of hops, vast improvements to malt kilning, the invention of numerous technological doohickeys to improve quality control, etc.—the identification, isolation, and selective breeding of yeast has likely been the most formative.

Beer as we know it wouldn’t be possible without yeast. In addition to converting sugars into ethanol and carbon dioxide (providing the booze and the bubbles), different brewing yeast strains can contribute all sorts of different flavor and aroma characteristics to the final product. Hop and malt additions may receive top billing on the average craft beer label, but one sip of a clove-and-banana-laden German-style hefeweizen or a peppery Belgian saison should be sufficient proof of yeast’s ability to carry the team when called upon. Many U.S. brewers do favor relatively neutral yeast strains, but, even without directly contributing significant flavors or aromas, the choice of a yeast strain will still ultimately affect how those other ingredients in the beer are perceived.

The inherent difficulties of managing these colonies of single-celled organisms has influenced the course of brewing history, and today our understanding of these for-hire enzymatic pouches remains a bit patchy. We can’t easily observe them at work. They frequently arrive to us in ways as ill-befitting of their importance as dehydrated packages that look like they might just as easily contain a teabag. Even Germany’s Reinheitsgebot Purity Law initially gave them the shaft (yeast wasn’t scientifically understood for another few hundred years). Yet brewing yeasts still manage to reassert their quiet presence—as well as remind us they’re never entirely under our control.

Over 100 Billion Served

Yeast are miniscule, generally unicellular fungi. They exist to create more yeast, to find sugars and other nutrients to sustain themselves long enough to procreate, and along the way they just so happen to transform mixtures of sugar, hops, and water into something more special than their constituent parts. They usually procreate by a budding process, via which they create genetically identical copies of themselves that grow to the same size as the original within a matter of hours.

They do not, as far as we know, dream of greatness.

Yeast’s role in the fermentation of beer didn’t become well established until the late 1800s, when Louis Pasteur published his Études sur la Bière (Studies on Beer) and Emil Hansen managed to isolate an example of bottom-fermenting yeast at Carlsberg Laboratory in Copenhagen. Up until then, although brewers knew that harvesting the rich foam from a fermenting batch of beer could be used to initiate the next ferment (without the need for divine intervention or breaking out the old brewing sticks), the technological capabilities up until then hadn’t progressed sufficiently far for them to be able to do all that much about it. The international scientific community gradually honed the skills needed to isolate, identify, and transport yeast strains safely, and the organisms responsible for the magical parts of the brewing process were finally becoming understood.

In Amber, Gold & Black, beer historian Martyn Cornell carefully details what happened shortly afterwards. In 1897, the British chemist Horace Brown told a European audience of findings in America that had yielded the ability to package bottled beer that, unlike its bottle-conditioned predecessors, “would remain ‘bright’ in the bottle. It did not take long for the technology to cross the Atlantic.” Breweries were soon marketing these new and improved “sparkling” ales (which were artificially carbonated, instead of naturally carbonated by being bottled with yeast), thereby saving thirsty consumers from yeast’s unseemly appearance. Innovations for removing yeast and improving the stability of the final product would eventually involve all sorts of things, including centrifugal filtration, pasteurization, and fining agents like isinglass, gelatin, and Irish moss.

Yeast was unavailable for comment on these developments.

Making Darwin Proud

It’s one thing to isolate a single yeast cell in a laboratory setting, but it’s another thing entirely to repeatedly use that isolated yeast strain in the brewhouse and maintain a consistent ferment batch to batch. Eventually the resulting beer will start to noticeably change. Perhaps no one appreciates that continuous struggle better than Chris White, President of White Labs Inc. and co-author of the textbook Yeast. White noted, “It’s one of the things that makes what we do important.” White Labs handles 50 to 100 strains each week. Hundreds more are maintained in longer-term storage.

“It was historically difficult to keep yeast from changing,” White added. “If you look at how yeast storage used to be – storing it on plates and slants, so that it’s slowly growing – it will change in months of time. Storing it cryogenically is the best way to have it not mutate, to have it not change. So you have to store them cryogenically, and that’s a great permanent bank, but you have to go back to that bank regularly.”

Pick one: (a) witness evolution, or (b) freeze the little buggers.

Even in controlled environments, setting loose a pure-culture yeast strain (progressively bred from one isolated cell of the desired yeast variety) to ferment a complicated solution in varying conditions brings about an entirely new set of disorderly influences. Not all of the yeast cells will complete the fermentation of a batch in similar health, small probabilities of mutation become a non-trivial concern when yeast pitching rates are measured in the billions, and even the greatest vacuum in the world isn’t going to prevent wild yeast and bacteria from hungrily trolling the perimeters of one’s fermentation vessels. Uninvited participants can start changing the pH and kicking out off-flavors, competing for the same sugars that were intended for that original yeast strain, and soon one’s delicious pale ale recipe is tasting like bad lambic. The road to being able to repeatedly brew with a particular yeast source is, in other words, a mycological minefield.

In the past (and today, though to a considerably less degree), breweries that observed their yeast strains going off the deep end and progressively performing differently during fermentation would typically pop over to a neighboring brewery to replenish their supply – which, again, up until the last hundred years or so was determined by the microorganisms floating in the local air, instead of by what they’d ordered out of a catalog. (This would eventually be known as a “house yeast” or “house character,” as it would be specific to the individual brewery(ies) and often quite distinct.) Most commercial brewers today, whose livelihoods depend on their ability to produce a consistent product, still face a similar decision: control your yeast, or find someone who can.

Today most American craft breweries use a single strain of pure-culture yeast to ferment each individual batch of beer, usually acquired through one of the main yeast suppliers (Wyeast Laboratories, White Labs, The Brewing Science Institute, and a small handful of others). They’ll ferment with it and typically reuse it anywhere from a few times to a few dozen times, depending upon their conditions and brewing schedule, careful to harvest it from each fermenting batch in a way that limits the selective pressure they exert on these ever-evolving colonies. When their yeast population’s fermentation characteristics start to diverge from the norm, and unless they have appropriate microbiological facilities on-site, they’ll simply order more.

One of the more illustrative examples of this tendency to evolve comes from Stan Hieronymus’ Brew Like a Monk. In it he writes, “Left on their own, yeast strains change over time. So, while Wyeast may have kept its 1214 [yeast strain] much the same in the twenty years since it was taken from Chimay, Chimay’s itself likely changed.”

That’s for one strain of yeast. Multiple strains will be slightly more complicated. And when one includes bacteria and wild yeasts in the mix (the latter often mutate more rapidly than cultivated brewing yeasts – see sidebar), it gets even harder to maintain those same original characteristics and proportions between batches.

One glance at the fermentation dynamics of authentic Belgian lambic (Raj B. Apte and others’ work on the subject can be found in Jeff Sparrow’s Wild Brews) provides the general gist of how intricate these relationships can be: cresting waves of ascendance and departure developing over many months, as yeasts and bacteria progress through the smorgasbord of available sugars.

Yeast’s ability to evolve over relatively short periods of time, on a human scale at least, can also prove to be rather beneficial. Over the years, breweries have managed (consciously or less so) to select for the yeasty attributes that work best for beer brewing, including favorable flavor and aroma characteristics, tolerance to alcohol, and the ability to fully complete fermentation cycles (often reabsorbing less-than-delicious byproducts in the process).

Raphael Rodrigues, a beer writer based in Brazil, recently reported an example of how some brewers are taking yeast’s adaptive capabilities one step further. Finding inspiration in a lecture by professor Rogelio Brandão (whose research focuses on fermentation studies), Felipe Viegas and pharmacist José Eduardo Marino decided to do an experiment with a strain of cachaça yeast. (Cachaça, Brazil’s most popular spirit, is made from fermented sugarcane.) After introducing the cachaça strain to a mixed solution of sugarcane and malt sugars, successive generations were fed progressively lower and lower concentrations of the sugarcane parts. Eventually, they were able to selectively evolve a population of yeast that would chow down on malt sugars. The result? A 5.3% wheat beer called Carolweiss, fermented entirely from what was once cachaça yeast.

Apparently, rum yeast is slated to be studied next.

From Thin Air

Though people often reference yeast strains as being “brewer’s yeast” or “bread yeast” or “wine yeast,” the actual distinctions between these categories aren’t always as cut-and-dry as the titles suggest. Many bread yeasts can be used to ferment beer. Belgian strains tend to exhibit closer similarities to wine yeast than conventional brewer’s yeast (both in terms of size and phenolic byproducts). The majority of the types fall into the Saccharomyces cerevisiae species. In the end, the categories generally serve to indicate that a particular strain will get the job done.

Encountering an unfamiliar yeast strain, one feeds it some malt sugars and hopes for the best.

Back in 2010, Dogfish Head’s Sam Calagione and company traveled to Egypt with the hopes of being able to capture a local strain of wild yeast for their recreation of an ancient Egyptian beer. The team set a few dozen petri dishes and swingtop bottles outside (after visiting the Tomb of Ti, famous for its hieroglyphs depicting the related processes of beer and bread making), fitted with local juices and dates and—most importantly—isoamyl acetate. Isoamyl acetate, better known as the banana note produced by German hefeweizen yeasts, also serves to attract the pitter-patter of fruit flies’ yeast-laden feet.

By collecting, isolating, and propagating some of the various wild yeasts that happened into their “yeast traps,” the Dogfish Head team was able to pick out an as-yet-unidentified Saccharomyces strain that could ferment their Egyptian ingredients favorably. In 2011, back home, they collaborated with the University of Delaware to repeat the process at a nearby peach farm, isolating “a wild Delawarean yeast” to ferment their locally sourced beer, D.N.A. (aka “Delaware Native Ale”).

At the 2011 Craft Brewers Conference in San Francisco, Calagione encouraged other brewers to attempt similar projects. “We really see this opportunity to locally source and grow yeast, for each of us wherever we live, as a great frontier that has not been fully explored yet by the craft brewing community.” He later added, “Unlike the wine world, a lot of our terroir is just in the air, not under the soil.” Two questions are quick to arise, though: how hard is it to do this sort of thing, and how likely is it one will end up finding something useful (and tasty)?

Regarding the first question, Chris White replied: “You absolutely can find them pretty easily with simple things: plates, inoculation loops, and patience.” The major limiting factors, he added, are finding strains that have an acceptable tolerance to alcohol and consume a sufficiently wide range of sugars.

Some insight into the second question (for which there’s probably no concise answer) came from Dr. Charles Bamforth, noted beer author and Anheuser-Busch Endowed Professor of Brewing Science at UC Davis. A graduate researcher working with Bamforth, Nick Bokulich, has been studying spontaneously fermented wild ales produced in the U.S. While these beers’ Belgian counterparts have long been revered for their layered complexities (resulting from the hundreds of different wild yeasts and bacteria naturally resident in that region), the beer-friendly microbiological diversity available elsewhere is comparatively unexplored. When asked how the characteristics ended up measuring up between U.S. and Belgian wild ales, Bamforth said, “The successions of the microorganisms involved are very comparable. The similarities are obvious.”

Which bodes well for any brewers wondering where they can get some yeast traps.

The Next Generations

If one could conceivably graph the overall diversity of yeast populations that the average brewer uses as we progress forward through history, it would presumably start taking a nosedive around the end of the 19th century, with perhaps a slight uptick in recent years. The ability to isolate and cryogenically maintain—“domesticate,” in a sense—yeast strains has dramatically slowed their genetic shifts, and this can ultimately be interpreted as a reflection of the commercial realities of our current beer cultures.

Even at our most puristic, we still perceive value in a beer brewed consistently batch after batch.

Speaking about Dogfish Head’s line of Ancient Ales, Calagione said, “one of the liberties that we’ve taken with a lot of these [beers] is we’re going with a single strain of yeast, where, in reality, beers thousands of years ago were almost definitely way more akin to what a lambic would be today.”

There’s been a quiet tradeoff there, and we’re at a point in time where the majority of our commercial beer is brewed with a single strain yielding generally consistent results. In a section of Farmhouse Ales, Yvan De Baets cites a 1920 passage from a beer writer named Marc H. Van Laer, who wrote, “It seems, for example, that if the application of the pure cultures method has improved the average quality of the beer, if it has decreased the chances of infection, it has given us beer with less character than before.”

De Baets goes on to add, with a straightforwardness that’s pretty refreshing, that “the current situation of Belgian beer, some having become caricatures of themselves with their mainstream, standardized flavors, shows us that, unfortunately, Van Laer’s warning was not heeded.” While the historical context of De Baets’ commentary limits it from having quite the same resonance for a predominantly American audience without that same farmhouse brewing culture to fondly look back upon, it still raises an interesting point.

Truckloads of different hops. Pallets of specialty malts and adjuncts. One type of yeast?

White Labs and other brewing laboratories offer a number of blended yeast strains (which are generally produced as pure-culture individual strains that are then blended to the appropriate proportions before shipment). Chris White commented, “It’s a fun way for people to diversify their beers even more.” When asked whether breweries were requesting and using the blended strains more frequently, he answered, “it’s on the rise, for sure.” There have been smaller yeast companies popping up recently, such as East Coast Yeast in New Jersey, which specializes in complex blends with names like “BugFarm” and “Saison Brasserie blend.”

Whether these small pieces accurately forecast any larger movements towards greater experimentation and overall complexity in how American craft brewers obtain and utilize different yeast strains remains to be seen. Then again… craft brewers are usually looking for that next envelope to push, and this one seems to be a relatively uncharted landscape right now. It’s implausible that the ways we use yeast will ever return to their pre-19th century status. But occasionally grappling with yeast’s diversity and wildness seems to be an appropriate way to appreciate what they once were in beer’s history, as well as what they can still be today.