It’s impossible to overstate the role of yeast in wine, beer, and spirits production. Without yeast, there would simply be no alcohol. During the fermentation process, the single-cell organism feeds on sugar provided by grape juice or mash, a mixture of grains and water, converting it to carbon dioxide and all-important alcohol.

It’s not just alcohol that yeast brings to the table during fermentation; it’s flavor, too. This is true for all alcoholic beverages, but it’s a topic that’s seldom explored when discussing distilled spirits. While wine lovers debate the merits of indigenous yeast, and beer drinkers enjoy specific yeast-related styles, spirits enthusiasts instead focus on alcoholic proof, the types of still used in production, and aging practices.

The role of yeast is not lost on distillers, of course. But the exact manner in which the microorganism influences their products remains a topic of debate. Nowhere is this more true than the bourbon industry.

The Role of Yeast in Spirits Production

“I think yeast may be the single most important thing in [spirits production],” says Ian Glomski, founder of Virginia distillery Vitae Spirits. Glomski doesn’t produce bourbon at his distillery, but as a former professor of microbiology at the University of Virginia School of Medicine who trained at the Pasteur Institute in Paris, he’s well versed on yeast and fermentation science.

Fermentation provides the essential “building blocks” for all distilled spirits, Glomski explains. While subsequent processes like distillation and barrel aging also influence the final profile, many of the flavors present in a spirit can only be created during fermentation. “If yeast doesn’t make it, you can’t pull it out of the hat,” he says.

The fermentation process itself is just as nuanced and complex as the alcoholic beverages it yields. Multiple factors influence the concentration and complexity of flavor compounds produced, including the source of sugar, temperature the reaction takes place at, and duration of fermentation. Then there’s the strain of yeast selected for the process. Individual strains perform in different ways depending on fermentation conditions, which ultimately impacts the “building blocks” produced by the strain.

Distillers call upon two sources of yeast for distillation. Some use their own “proprietary” strains, live-culture yeasts that have been continually produced at their facilities for generations. Others purchase from dedicated commercial producers, who offer both dried and live strains.

One such company is White Labs, which has facilities in San Diego, Calif., Asheville, N.C., and Copenhagen, Denmark. Founded by Chris White in 1995, White Labs services the brewing, winemaking, and distilling industries. Some distilleries buy from the company’s huge catalog of yeast strains while others contract with White Labs to produce their own proprietary strains for them. White Labs also “banks” proprietary strains for distilleries that continue to produce in-house, ensuring they have a secure backup off-site in case of any major incidents at their facilities.

Proprietary Yeast Strains

If consumers are aware that yeast has an impact on their favorite bourbons, then it’s almost certainly because they’ve read of a distillery talking about its proprietary strains — often in almost mythical terms.

Among the major bourbon brands, Four Roses leads the yeast discussion. The Kentucky-based distillery has a library of over 300 strains, but uses just five of these, along with two mash bills, to create the 10 base recipes for its bourbons.

The mash bills and yeast strains used at other distilleries are often shrouded in mystery. Four Roses, by contrast, is completely transparent about its process. The distillery showcases the recipes for each of its bottlings on its website, using five-letter codes to indicate the yeast strains and mash bills used. Listed alongside the letters for its yeast are short descriptors of the character each imparts. The “K” strain, for example, adds a subtle spice, while the “V” strain adds delicate fruit.

Historically, all distilleries in Kentucky and Tennessee produced their own proprietary strains, but that’s now becoming much less common, says Chris Morris, master distiller at Woodford Reserve, which is owned by Brown-Forman. While many major distilleries have switched to using commercially produced dry yeast, Brown-Forman isn’t one of them. “Our brands have their own strains of yeast that are sacrosanct,” Morris says.

Morris, who began working at Brown-Forman in 1976, says he was brought up in the company with the understanding that the first flavor sources — water, grains, fermentation (i.e., yeast), and distillation — contribute 50 percent of whiskey’s overall profile. If you don’t get it right with those four sources, you can’t make it better in the barrel, he says.

While he acknowledges that many other factors in fermentation influence the final product, Morris says “yeast dominates the flavor formation.” He adds, “In Woodford Reserve, specifically, it builds a tremendous array of fruit and spice character that makes our whiskey what it is.”

When he started at Brown-Forman, Morris recalls visitors entering the distillery’s yeast production facility would have to keep their hands by their sides where the staff could see them. “We were worried about someone stealing our yeast strain,” he says. “We didn’t want anybody wiping the walls with a handkerchief and putting it in their pocket.”

Not all distilleries are as protective over their yeast strains. Pat Heist wears multiple hats within the distilled spirits industry. He co-owns and operates Wilderness Trail Distillery and Ferm Solutions, a company that supplies yeast strains (among other microbiological services) to over 600 breweries and distilleries.

Far from protecting the yeast strains used at Wilderness Trail, the distillery markets them on their bottles. “You can go on our website and buy the yeast strains we use,” Heist says. “We’re about as transparent as you can possibly get.”

Flavor Versus Efficiency

Of the nine strains Ferm Solutions markets to distilleries, the company doesn’t differentiate by stating things like “strain X will provide pronounced toffee character” or “strain Y will lead you to baked apples.” Those types of nuances, Heist says, are defined more by the fermentation conditions and subsequent aging than specific yeast strains. It would take a highly trained palate to detect the difference of impact between one strain and another, he says. And ultimately, every batch of distillation yields a slightly different product, regardless of whether all the processes were carried out exactly the same.

So why offer nine yeast strains and not just one?

Each of Ferm Solutions’ strains has been developed with specific conditions in mind. Some strains have higher temperature tolerances, while others are capable of completing fermentation at breakneck speed. Some are particularly efficient at maximizing alcohol production. All of these factors have significant financial implications.

“It takes a lot of energy to keep large fermenters cool, so it makes a big difference in our energy bill if we’re fermenting at 80 degrees versus letting them go to 94,” Heist explains. “If I only have to occupy a fermentation vessel for two days versus a whole week, I get three-and-a-half uses out of that tank in a week instead of one.”

“We want good flavor, but we also want good production,” Heist says.

Further complicating the yeast debate is the fact it isn’t the only ingredient present in fermentation that influences the process. Many older distilleries use wooden fermenters; the walls of these vessels are “imbibed and impregnated” with different bacteria and yeast, Heist says. These “contaminants” affect the flavor profile and impact yields because they also consume sugar.

With bourbon demand soaring in recent years, some producers may be tempted to upgrade to more efficient stainless-steel equipment, which can be easier cleaned of contaminants. But then they would also risk losing some of the flavor compounds that define their products. For the same reason, many distilleries persist with their proprietary yeast even though there might be more efficient strains available.

Do Consumers Care?

Ultimately, the topic of yeast selection is just as much a philosophical debate as it is scientific. It stands to reason that a heritage distillery would wish to preserve a treasured “house style” that consumers know and love. It’s also completely understandable that a smaller distillery, which may have a lower profile, would instead seek to maximize efficiency.

So far, this is a conversation that’s largely taking place within an industry bubble. Besides Four Roses, no other major distilleries devote much effort to educating consumers on their proprietary strains or the impact of yeast on flavor profiles. Perhaps they’re missing a trick here.

If a proprietary strain can indeed negatively impact yields, why not profit from it in different ways? If a company owns a unique product, surely that could be marketed as a unique selling point. But whether or not consumers are ready to respond to this is a different question.

Blake Riber, author of Bourbonr blog and owner of online craft spirits retailer Seelbach’s, says even devoted bourbon enthusiasts didn’t care about yeast strains until Four Rose started speaking about them. Nowadays, however, it’s common to find bourbon hunters scouring secondary markets in search of Four Roses bottles made with specific strains, such as V or K.

While none of the profits of these illegal sales end up in the pockets of the distillery, Riber says there’s still value in this for the brand. “It drives the interest back to their products and makes them easier to sell,” he says. Nevertheless, he concedes this market is still “a small percentage of a small percentage” of bourbon drinkers.

Perhaps consumers will never care about yeast as much as distillers do. After all, its effect on the final product is so much less tangible than elements like mash bills and alcohol proof, and processes such as aging and cask finishing. For distillers, too, it’s easier to market these distinctions, than going to great lengths to educate drinkers on fermentation science — especially when such a small percentage will be receptive to it. At the end of the day, no one can contest the fact that, without yeast, none of this would all be possible.