The Future of Yeast
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For hundreds of years, brewers have slowly guided yeast strains to do their bidding, developing some that flourish in temperatures above 80 degrees, and others that prefer the chill of a winter cellar. These changes happen incrementally, over human generations. In Northern California, a trio of postdoctoral researchers at UC Berkeley founded a company engineering next-gen yeast that behaves in astonishing new ways—and takes months rather than decades to evolve.
Further north, on the eastern outskirts of Portland, another trio founded a company taking the opposite approach, nurturing their flock of fungi entirely organically. The team at Imperial Yeast are using cutting-edge technology as well, but only to preserve newly-discovered strains and breed new strains in a time-honored manner, not to manipulate their genomes.
The contrasting approaches, the traditional and the technological, represent the ancient push and pull that has buffeted brewing for thousands of years. The tension is evident in malt and hops as well. In small-batch malthouses today, maltsters reproduce 19th century methods of Czech and British predecessors while state-of-the-art facilities churn out freight trucks of the latest barley varieties, bred for diastatic power and starch content. Hop growers are happy to sell brewers whole cones of hundred-year-old varieties, or advanced “hop products” from strains they’ve just bred. Nowhere, however, are the paths more divergent than in the way yeast is prepared, nor is any ingredient as capable of radically changing the landscape as the new strains, natural and genetically modified.
Millions of Years in the Making
Yeast is everywhere. It rides currents of air and coats myriad surfaces. It lives in the stomachs of bees and on our skin. Members of this fungus family are legion—well over a thousand—and have names like Malassezia, Pseudozyma, and Wickerhamomyces. And of course Saccharomyces. That tiny creature has arguably done more to shape human history than any other save humans themselves. The agent that activates both bread and beer, it has been domesticated, if not fully understood, for thousands of years.
The Sightglass series started with How a Hop Earns its Name--the fascinating story of hop breeding. See also The Keys to Kveik, which summarizes the complexity of that farmhouse family.
The old axiom goes that brewers make wort, but yeast makes beer. It hints both at yeast’s central place in the process, but also its mysterious, almost magical activity. The word “yeast” comes from the verb in Old English gist, “to boil.” Anyone peering into a roiling fermenter, heaving and bubbling like a witch’s cauldron, will immediately see why. The yeasts, which explode in population as they gobble up the wort’s sugars, raise the temperature during fermentation while sending clouds of carbon dioxide upward, and molecules of ethanol back into solution. It looks magical, and the way it purifies, self-carbonates, and transforms sugar water into a mood-altering drug suggest the qualities of alchemy.
More importantly, yeast transforms the sugary, breakfasty tea that comes from steeping grain into all the things we recognize in our cans: an alcoholic, fizzy, crisp beverage. Beer, in short. Imperial Yeast’s co-founder Owen Lingley offers this succinct overview.
“What does yeast do? Yeast takes sugar and in the process of fermentation, it releases acid, carbon dioxide, alcohol, and flavor compounds. The acid is important because it is going to acidify the liquid to a stage where things like botulism or other things can’t grow in it.”
While the alcohol and carbonation are the obvious yeast products, flavor compounds are equally important in creating a quality of beeriness. At the beginning of fermentation, yeasts gobble up all the simple sugars they can find. “They’re going to take in oxygen first to build up their cell walls so they can control what’s coming in and what’s going out,” Owen said, describing the process. “Then they’re going to create some precursors to flavor compounds. When the yeast start getting to end of fermentation, they’ll consume those precursors and transform them into esters or phenols.” In Belgian beers and Bavarian weizens, these compounds are the most striking, but they exist in all beers, even lagers, filling out the flavor profile with little daubs of fruit and spice.
Fermentation involves a dance of myriad biochemical reactions, one sensitive to conditions and processes. Writers have devoted whole books to describing them. Scientists have catalogued hundreds of potential flavor and aroma compounds yeast may produce. Brewers may not be able to ferment the beer, but they can guide which flavors the yeast will express. Strain selection is the most important factor. Some are more neutral (the “Chico” strain is the most important example), while others, particularly in Belgium and the UK, are more expressive. Some, like the yeasts that make Bavarian weizens, have a gene that creates a spicy, clove-like flavors. Yeasts in this category are known as POF+ strains. Lager strains are more subtle but contribute an unmistakable quality.
Process is a big part of the equation. Temperature and fermenter geometry will repress or encourage yeast to kick off flavor compounds. Sometimes they do it because they’re happy, sometimes because they’re stressed. How many yeast cells a brewer pitches is another big factor. “In general, the ester and phenol production is a stress reaction,” Owen told me. “So if you underpitch, you get more of them. If you over-pitch, most of the strains will start to get fairly neutral, where the fermentation finishes prior to the expression of a lot of flavor compounds.”
Yeast adapts to its environment over time. Most of the commercially available yeast strains came from specific breweries where they developed distinctive characteristics over years or decades. Until the 20th century, breweries managed their yeasts by repitching them—thousands of times in the case of older breweries—and the older ones developed unique flavors and behaviors. Modern labs have banked these strains, and they compose the “biodiversity” brewers enjoy. If you pick up a pitch of Imperial’s Juice (or 1318 from Wyeast or White Labs WLP066), you’re brewing with descendants of yeast from Boddington’s Brewery, as one example. (Actually, there’s a big debate about whether Boddington’s was actually the source—but some English brewery was.)
As yeast labs continue to grow and evolve to meet the needs of brewers, one way they expand their portfolio is acquiring new varieties. That Boddington strain is a great case in point. It’s been around forever, but has gained incredible traction in recent years because of how it behaves. A soft fluffy yeast, it leaves a haze behind in the beer. More importantly, it seems especially good at “biotransforming” hop terpenes—transforming them from one flavor compound into another, and goosing the perception of fruitiness. Thanks to the trends in juicy and hazy IPAs, it became the go-to strain. But finding overlooked, existing strains is not the only approach.
The yeast strain at Samuel Smith’s has evolved with the old slate squares in which the beer is fermented. It has become so good at falling out of suspension that the brewery pumps it back over the wort from those fan-like appendages so it will keep fermenting.
Engineering Flavor
Ales limned with acid represent one of the biggest growth categories in beer, whether we’re talking about lemonade-like summer sours or fruit-saturated smoothie ales. Beginning in the middle-teens, breweries started making these by kettle-souring their wort. The method offers breweries more control over the process of lactic fermentation, producing cleaner beers that create less problems in the brewery than older methods. The Canadian yeast company Lallemand wondered if there was an even faster, easier way to sour a beer.
In 2019 they released the answer to this question, Sourvisiae, a regular Saccharomyces strain that had been genetically modified to produce lactic acid. Conventional yeast already produces acid, as Owen explained above, so getting it to produce more required only a small genetic tweak. Recent advances in gene “scissors” like CRISPR made this a snap. According to Great Notion, on Portland’s leading edge in making these kinds of beers, it’s a great solution. I spoke to co-founder James Dugan, who heads up the original, smaller brewpub facility.
“You pitch it just like you would an ale yeast. But it’s kind of weird, because the next day there’s really no activity, you don’t see active fermentation happening. And then you take a reading and you don’t see a gravity drop, either. What it’s doing is creating the acid first and then the Saccharomyces kicks in and does your fermentation.”
Although breweries haven’t been quick to talk about it, Sour-V (as Dugan affectionately calls it) is in wide use. “Clearly, this made them a lot of money. Without a doubt it really changed the brewing industry. Everyone uses it now.” Because of the high acid content, it can’t really be repitched, and it’s expensive. Yet it still saves Great Notion money because they gain a day not having to kettle-sour. It’s also reliable. “The acidity is super clean. The lactic acid is very bright and clean and doesn’t have any THP [off-flavors]” James reported.
Lallemand may have gotten the first-mover advantage on acid-producing yeast, but they’re not the only ones making them, nor is this the only kind of bio-engineered yeast out there. In the Bay Area, Berkeley Yeast was founded by a group of academics already working in the field of bioengineered yeasts, and that’s all they do. The strains they produce offer breweries a buffet of amazing, next-gen fermenters. In addition to the acid-producers (they have two), they offer yeasts that create specific terpene profiles to taste like Cascade hops or melons—no dry-hopping needed—a strain that turns up the thiols for super-charged biotransformation, and probably the most revolutionary, strains that don’t produce diacetyl.
I spoke to one of the co-founders, Nick Harris, about his process. It’s a little mind-bending to think about a yeast strain that can make a beer taste like hops, and I wondered how the process works. Nick is one of those rare scientists who can explain technical processes clearly for the layperson. Here’s how he described their work.
“The majority of molecules in the natural world are produced by enzymes. Enzymes are just proteins that catalyze reactions and make reactions happen. Every enzyme is encoded by one gene. A gene is just a sequence of nucleotides, a piece of DNA, that encodes a string of amino acids. When those amino acids are together, they make up an enzyme. Every single process in your body, for example, is mediated by enzymes. And those enzymes are turned on whenever those genes are turned on.”
“The way it works is DNA gets turned into RNA and RNA turns into protein, and proteins can make small molecules. That’s the central dogma of genetics: DNA makes RNA makes protein. Genes turn on based on environmental conditions, and so what dictates whether a gene is on or not is another piece of DNA immediately before the gene called a promoter. It promotes gene expression. When you talk about turning a gene on what we mean is ‘are you making RNA’? In the most basic sense you can think of promoters as dimmer switches. As genetic engineers, we’re able to swtich out promoters from any gene—and these genes can come from any edible food source—so you can control the extent to which a gene is turned on and therefore how much of whatever compound it’s going to make.”
To go back to acid-producing yeasts, the genetic engineers can control how much acid they produce by adjusting the promoter. In Sourvisiae, Lallemand is using a strong promoter that creates beer with a pH of 3.3 or lower. Berkeley’s strains are calibrated to produce less acid. In the case of the terpene-producing strains, the scientists are taking genes from plants that produce those terpenes and inserting them in the yeast, and then dialing in the right amount with the “dimmer switch.” It requires a lot of trial and error, but they eventually get there.
“It started off as a project at UC Berkeley. Charles and Rachel, two of my other co-founders, started the project in Jay Keasling’s lab, which is pretty well known in the field of synthetic biology or metabolic engineering. They worked on a project to have yeast produce the primary flavor determinants of Cascade hops—linalool, geraniol, and citronellol.”
In the case of their diacetyl-free strain, they’ve altered the way the yeast behaves. Instead of expelling the precursor compounds that cause diacetyl, they stay inside the yeast cell. “So whenever the yeast is consuming sugars, like this refermentation process you get with dry-hopping, Nick said, “the yeast, as soon as they start to produce the ALDC (acetolactate decarboxylase) enzyme, they will instantly convert it into acetoin. So it bypasses that whole diacetyl step.”
This could be a game-changer. Everything about the yeast remains the same—it just doesn’t produce diacetyl. So whether it’s a lager, a clean strain like Chico, or a hazy strain, diacetyl-free versions of which Berkeley offers, the only thing that changes is the time it takes to make a beer, and the knowledge that diacetyl won’t appear post-packaging.
“Diacetyl just sucks,” James at Great Notion said. “You waste a lot of time waiting for it to clear up.” Though they’re sticking with Imperial’s Juice rather than Berkeley’s equivalent diacetyl-free strain, Great Notion was impressed with Nick’s yeast. “His yeast was pretty awesome.”
Strains that produce those amazing juicy esters now used in American IPAs developed their character in open, flat square fermenters like this one.
Nature’s Toolkit
Nature produces its own buffet, however. One of the newest dishes on offer comes from Norway in the form of a family of yeasts called kveik that produce both a dazzling range of flavors—or none at all. Back at all-organic Imperial, Owen Lingley and crew have been fascinated with these strains’ potential. On the one hand, the strains offer a kaleidoscope of flavors. Before releasing Loki, the kveik in question, Imperial fermented with the yeast in a variety of conditions. “We ran fermentation tests at 60-64 degrees [16-18C], 64-68 [18-20C], 68-70 [20-21C], and all the way up to 90 [32C],” he said. “We tasted all of them and they’re very different beers.”
Existing yeast strains may not be able to produce the precise aromas and flavors of a beer dry-hopped with Cascades, but they possess incredible elasticity—especially the kveik strains. “I call Loki Princess Peach because there’s a two-degree temperature window from 68 to 70 where you get a peach ester that I’d never gotten in any other beer,” Owen said. For a company producing conventional yeasts, there are still opportunities for unusual expression strain to strain. “We’re excited about educating people on how to get the flavor characteristics with a combination of pitch rate and temperature. What I’m looking for is different flavor compounds that haven’t been found in beer yet—that’s what I’m excited about.”
Kveik has such potential because it goes the other way, too. A number of breweries are experimenting with kveik “lagers.” Fermenting at the cool end for an ale yeast, some strains of kveik produce fewer esters and all of the characteristic crispness one expects from a lager. Parker Rush of the Narrows in Tacoma, WA recently made a batch he called Pseudolager. Uncertain how it might taste, he hopped it with fruity Ella, which concealed some of the clean lines. Nevertheless, it was surprisingly crisp and lagerlike. He pitched at 70F and let it rise a couple degrees a day. He waited to transfer until day ten and then let it “lager” for a week. “I was shocked at how bright it got in such a short time frame,” he reported. He used Omega’s Lutra yeast, isolated from the Hornindal kveik strain. Omega is another lab working with both bioengineered and conventional yeasts.
A 17-day lager would be as much of a game-changer as a diacetyl-free yeast strain. A typical lager will take around 42 days, or more than twice as long. Breweries could move twice as much quick lagers (kveik-lagers?) through the brewery, saving a lot of money on steel and square feet.
Beyond kveik, much potential lies in lesser-known strains and the classic old method of breeding. At Imperial, they use a technique called “sporulation” to hussle matters along. “If you take yeast and put it in a nutrient-deficient media, it will sporulate instead of divide.” Owen said. “Normally their reproduction is division. But if you stress them out enough, some strains will create spores, and then those spores can interact with another strain that isn’t sensitive to the same conditions. And then if you use a super-fancy microscope, and move the individual cells around so the spores touch, and then you put them in a dark room for a little bit, you can see what comes out.”
Genetic engineers have a tool to more precisely change yeast, but all labs are looking for a way to make a more interesting strain.
Trade-offs Either Way
Whether a lab genetically modifies a yeast or breeds it, the choice comes with pros and cons. The downsides for organic methods are fairly straightforward. They depend on chance and mycologists have limited control in what emerges from their breeding or searches. Yet hidden in these downsides are certain advantages. Two of the most sought-after yeasts right now, the Boddington descendant and kveik, are both natural strains. Sometimes what can be found is every bit as interesting than what can be engineered.
The downsides for engineered yeast are less obvious, but one emerged in my discussions with brewers. While brewers can still manipulate a strain by using different techniques and temperatures, a strain that tastes like Cascade hops is meant to taste like Cascade hops. One of the challenges with Sourvisiae is its aggressive acid level, which is a result of the way it was designed. At Great Notion, they love the yeast, but have had to jury-rig a process to inhibit the massive pH drop. “Now what we’ve been doing is back-to-back brew days,” James said of Great Notion’s work-around. “On day one, I’ll just pitch the dry Cal ale and on day two I’ll pitch the Sour-V. Giving the Cal ale that day, the pH finishes in a really cool spot, 3.5, 3.6.”
Of course the biggest disadvantage is these yeasts’ very nature: they’re genetically modified. Consumers have been generally skeptical about GMO foods, and the EU basically forbids them. This would seem to be a barrier in the beer industry—except that so far it hasn’t been. Great Notion is fine with it, and Nick Harris at Berkeley was perfectly happy to talk about it. Moreover, his customers don’t seem to care, either. “We’ve found that pretty much every brewer is down to use our yeast,” he said. This may be partly a function of the fact that yeast is more an agent than an ingredient, and partly because GMO foods have been around almost 30 years, anyway. And it’s clearly true that customers aren’t motivated by organic beer the way food-buyers are. Still, breweries don’t seem to be advertising these yeasts, either.
The Future is Flavor
Writing in the first edition of the Beer Bible in 2013 about American brewing, I made this observation: “In the United States, breweries typically use neutral ale yeasts.” At that point at least half the breweries in the country were using the neutral “Chico” yeast (Sierra Nevada’s) as their core ale strain. The thinking then was: get out of hops’ way. Breweries used neutral grains as well, with probably a dollop of caramel malt to sweeten things up, but little else. The juice revolution in hopping changed that calculation. Breweries pursued anything that increased the sense of fruitiness, and yeast became a key player.
In those eight years, new hop varieties have poured out of breeding houses as breweries seek ever more unusual, distinctive flavors. A similar pattern is unfolding in yeast. Initially, breweries were focused on a few strains to increase the juiciness of their IPAs. Yet as the years roll along, they have begun to look for variety, much as they had in hops. The fruity ester profile of Juice is fantastic, but what if you want, say, a strawberry or peach note?
Like hop breeders and hop varieties a decade ago, yeast labs and yeast varieties are now mushrooming, each lab offering new, interesting strains. Some have been discovered, some bred, and some engineered. And as in the hop world, there will be winners and losers. At the moment, it’s difficult to say where we’ll fall on this question of organic versus genetically-modified. But one thing seems certain—the quest for flavor is just beginning.