Tour a distillery anywhere in the world, and you’ll likely hear about what makes their product different—the shape of the stills, the quality of the grain, the type of barrels used to age the spirit, the climate of the barrelhouse, and more.
Yet for all those variables, yeast are rarely given a place in the spotlight. The tour guide might mention fermentation times or note a unique wooden washback, but the very important little creatures that make alcohol in the first place often remain hidden.
That concealment is partly due to an “ick” factor: Many consumers simply don’t want to be reminded that their favorite spirit begins with a teeming mass of microbes. It can also be difficult to explain to someone outside the industry exactly how different yeast strains influence the final product.
Another reason many producers don’t talk about yeast is that they’re simply using the same kind as everyone else. That is changing, however, as more craft distillers turn to unique yeast strains—and to unique combinations of them—to differentiate their spirits.
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Elevating Yeast’s Role
The impact that yeast have on alcoholic beverages is enormous. More than simple machines that turn sugar into alcohol, yeast are also responsible for many of the esters, congeners, and other volatile compounds that give beer, wine, and spirits their unique and interesting flavors.
Brewers typically know and acknowledge the yeast’s influence—after all, wort produced by the same mash bill but fermented by different yeast strains can turn out wildly different. For many beers, the yeast strain is the delineating characteristic. Hefeweizen, for instance, is defined by an abundance of clove notes, hints of banana, and a cloudy appearance—all products of the specific yeast required for its production. Likewise, saisons and many other Belgian-ale styles depend on diastatic yeasts that produce dry, fruity beers. There are literally thousands of strains that each provide unique flavors and qualities to alcoholic beverages.
So, why don’t distillers take advantage of the myriad yeast strains available? One main reason is that distillers are almost always looking for a factor that brewers are not: full attenuation. Many types of yeast are not fully attenuating—that is, they leave behind some sugar in the wort. That’s usually desirable in the beer world, where residual sugar provides sweetness and a pleasant mouthfeel. However, it’s a major problem in whiskey, where any sugar left behind means alcohol—and, therefore, money—lost. Another reason is that yeast that can fully attenuate but do so slowly need more time in the fermentors—again, money lost.
Scotland’s remote distilleries often cultivated their own yeast on-site, and that likely provided a wide range of flavor profiles. However, by the 1950s, most producers there had switched to sourcing yeast from nearby bakeries and breweries. Yet even that variety shrank as almost all Scottish distilleries started to use the M strain of yeast created by the Distillers Company Limited, the massive corporation that once ruled Scotch whisky. The M strain is fast-fermenting and high-attenuating—two qualities that many distillers prize. That yeast strain has changed slightly since its creation nearly a century ago, and similar M strains have become available, but the basic properties remain the same. Some version of M is used by virtually every Scotch distillery as well as many American single-malt distillers.
When it comes to character and efficiency, however, many distillers have begun to ask, “Why not both?”
It’s not unusual for modern craft brewers to pitch multiple yeast strains. Sometimes this is to combine desired traits—such as pleasant esters, high attenuation, and good clarity—that would be more difficult to achieve with only one strain.
Today’s farmhouse-inspired, mixed-culture fermentations often include diastatic strains that can break down complex sugars to ensure complete fermentations. A recent study by John Nemenyi, published in the Journal of Food Science, concludes that “mixed fermentations using an industrial S. cerevisiae strain and an S. cerevisiae var. diastaticus had characteristics of both strains, including fermentation rate, final attenuation, and volatile compounds.”
Some brewers also combine strains to achieve more complex flavors. The simplest way to do that—because yeast produce most of their flavor compounds in the first few days of fermentation—is to co-pitch those yeasts at the outset. Generally, two ale yeasts with similar pitch rates and fermentation speeds can work in tandem without one dominating the other. Some combination of their traits is virtually guaranteed, although it won’t necessarily be the target profile—further experimentation with different strains, pitch rates, temperatures, and other variables can help the brewer dial that in.
Tinkering with Yeast in the Distillery
For distillers, unfortunately, the story is a bit more complicated. On the surface, it sounds simple enough to pitch two kinds of yeast: an ale strain that provides interesting flavors and a distiller’s strain that ensures all the sugar converts to alcohol. However, yeast have their own priorities.
“The distiller’s yeast will out-compete [the ale yeast], and you won’t get any of that flavor,” says Katherine Thompson-Witrick, a food scientist at the University of Florida. “Or you may get a little bit, but not to the extent you’re wanting.” Distiller’s yeasts such as the M strains are meant to work quickly, and they’ll consume the available sugar before a slower strain has time to provide the desired flavors.
One way to combat this problem is to stagger the times the yeast strains are pitched. For instance, a distiller who wants the banana notes of hefeweizen yeast could pitch that strain first, then come back a few days later to pitch a distiller’s yeast to clean up any remaining sugars.
“If you are looking to add banana during fermentation, start with hefeweizen for one or two days and do it warm—room temperature, and don’t cool it down,” says Andrew MacIntosh, also a food scientist at the University of Florida. “It will produce more of the isoamyl acetate. They’ll be happier, and the more they’re going to produce, the faster they’re going to go. Then, load it up with the distiller’s yeast to get the attenuation that you require.”
How long a distiller should wait to pitch the second yeast isn’t cut and dried. In their book Yeast: The Practical Guide to Beer Fermentation, Chris White and Jamil Zainasheff recommend “the last third of fermentation.” However, each strain is different, and it typically takes some tinkering to get an optimal outcome.
At Holyrood Distillery in Edinburgh, where they ferment with several different brewing yeasts, the norm is to pitch the second yeast one or two days after the first. “For co-pitching, always add the weaker strain first,” says Elliot Rogerson, senior distiller at Holyrood. “Then pitch the distiller’s strain 12 to 24 hours later.”
This method of staggering pitch times comes with potential problems, though. For anyone using a closed fermentor, opening it to add a second pitch creates a slight risk of contamination. Another issue is that not all fermentations progress at the same rate, and that can lead to inconsistencies when staggering pitch times. “You can’t always re-pitch at exactly the same point in the fermentation,” MacIntosh says.
Rather than pitch times, there’s another knob the distiller can turn: adjusting the quantities of each strain while co-pitching yeasts simultaneously.
“I think you’d be much better off getting the right inoculum ratio at the beginning, to give the [ale yeast] a go, let it produce the volatiles, and then be taken over by the other yeast,” MacIntosh says.
Continuing with the example of the hefeweizen strain: If you pitch two yeasts at the same time, MacIntosh says, “you can just up the inoculum of hefeweizen to three parts hefeweizen and one part distiller’s yeast. The distiller’s [yeast] will kick out the hef and take over, but it gives the hefeweizen a bit of a competitive advantage.”
At Holyrood, Rogerson suggests a 60-40 ratio, specialty yeast to distiller’s. That should result in a fermentation that’s fully attenuated but still has the bright banana and clove notes—or whatever traits you want from the ale yeast—that make for a unique and flavorful distillate.
Other Considerations
One potential issue for some distillers is that pitching two strains can make harvesting or reusing yeast difficult. That practice isn’t the norm in distilleries, but it can be a potential cost-saving measure. Harvesting and re-pitching can work well with one strain or sometimes with two similar ale strains, but a powerful and fast-moving distiller’s yeast will quickly become dominant in successive generations.
Even before that happens, it can be difficult to assess the actual distribution of strains after they’re pitched. “The ratio will change at the [tank] bottom,” MacIntosh says, “and there it’d be almost impossible to discern them—which is which, and how much you have.”
Despite those challenges, co-pitching different yeast strains is an often-overlooked way for distillers to develop character in their products while still ensuring they take advantage of all available sugars. There’s still plenty of research to be done to fully understand the interactions of different strains—and the permutations are practically infinite—but craft distillers can be at the forefront of that exploration into microbes, flavor, and their contribution to distinctive spirits.
