As any decently informed drinker knows, beer gets its flavor and aroma from all kinds of different elements during the brewing process. Malted barley may contribute chocolate notes to stouts, while hops might bring tropical fruit to an IPA. But humble yeast also generates hundreds of compounds that provide flavor and aroma notes, like banana from isoamyl acetate (think German hefeweizens) or clove notes from 4VG.
Different yeast strains—or the same strain under different brewing conditions—can produce quite different compounds, in theory allowing brewers to use yeasts as a key to unlock a vast array of flavors and aromas. Yet for centuries, most brewers haven't really gone rummaging in their yeasty toolbag in a focused, scientific way. But now bioscientists are creating 21st-century yeasts that promise novel aromas, hikes in specific flavors—and even finally making flavorsome lagers. Yes, really.
Speaking of lager, Denmark’s Carlsberg brewery established one of the earliest yeast biology labs in 1875, isolating the first pure culture of a brewing yeast in 1883. The lab had another burst of yeast creativity in the 1930s and 40s, breeding a few new strains with useful practical brewing traits. But most brewers use just a few familiar yeast strains, like cold-tolerant Saccharomyces pastorianus for lager, Brettanomyces for lambic sour beers, and Saccharomyces cerevisiae for pretty much everything else.
This lack of adventure frustrated bioscientist Kevin Verstrepen when he moved to the Belgian beer capital of Leuven to create a beer-focused lab at VIB (Vlaams Instituut voor Biotechnologie/Flemish Institute for Biotechnology) in 2009, after academic spells in the US at MIT and Harvard. Yeast research at the time seemed geared toward technical troubleshooting for brewers rather than opening new vistas of flavor and aroma. He summed up the situation in a 2018 New Scientist article as “nobody doing any biology, really.”
So Verstrepen turned his Leuven beer lab into a state-of-the-art yeast arena in search of tailor-made flavors and aromas, drawing on freezers housing around 30,000 yeast strains. Around 1,000 of these are already used by brewers, bakers, and others worldwide, while another 1,000 are myriad wild isolates.
Verstrepen's team probe this yeasty bonanza for genes known to influence taste and aroma, then mate different strains to produce new yeasts that hopefully achieve a desired specific aim—a novel aroma maybe, or a particular boosted flavor. So far, the VIB team have created more than 150 new brewing yeasts. “We are crossing beer yeasts with wine and Champagne yeasts, as well as yeasts used for sake, Chinese liquor, and wild yeasts isolated from the craziest places—old basements, jungles, and flowers!” Verstrepen says.
The true test, of course, is making beers from any new yeast showing promise, which is where the VIB team gets a boozy work bonus in twice-weekly blind tastings. “We have installed a special pilot brewery engineered to be as flexible as possible in making different beer styles,” explains Verstrepen. Black tulip-shaped glasses means tasters can't be swayed by seeing what they're sipping. As with many experiments, though, high hopes don't always translate into great results. (Like one beer they made that tasted of onions.)
But the VIB team's efforts to transform the flavor and aroma range of contemporary beers has been applauded by many brewers. Among them is Peter Bouckaert, formerly at Belgian's Rodenbach and then Colorado's New Belgium Brewery, before starting Purpose Brewing and Cellars in the same state in 2017. “Kevin’s research is a little bit out there,” he told Nature in 2016. “But that doesn’t mean it will not translate to something that could be huge in the future.”
In 2014, one brewery asked VIB for a yeast to boost the banana- and pear-like esters prized in its Belgian-style beers. The Flemish team did some research and found that existing yeast strains varied a massive 34-fold in how much of these esters they made. But by cross-breeding three of the top producers, they came up with a new strain producing 50-percent more of the desired compound, which added the desired heft to the resulting beer.
VIB engages with all sizes of producer, too. “We are working with more than 30 small and large breweries, including AB-InBev, Orval and several Trappist breweries,” says Verstrepen, who reveals that at least ten of VIB's new yeasts are now used commercially.
But it isn't easy coming up with yeasts for every desired flavor or aroma. A request for a yeast to boost chocolate aromas is still stumping the VIB team, for example. “It would require hundreds of compounds to accurately mimic these aromas,” explains Verstrepen.
Some US craft brewers are very open to GM yeasts. And if a sufficient number start using the yeasts, they might slowly become more mainstream, allowing the larger breweries to also start.”
One way to bypass these hurdles may exist, however. “To get chocolate flavors, we would likely have to insert several genes and pathways from the cocoa plant,” says Verstrepen. “That would be doable—but it would be GM….”
Genetically modified yeasts are an issue for some brewers, however, particularly multinationals—though more due to public wariness than safety issues. “While we are 100-percent sure these yeasts are safe, we have so far not given out any GM yeasts for commercial production, because we want to make sure the brewery would follow all the appropriate guidelines and legislation,” explains Verstrepen.
Things may change, though. In the US, Brazil, and some other countries, gene-edited organisms that do not contain any foreign DNA sequences are exempt from GM legislation. “Some US craft brewers are very open to GM yeasts,” says Verstrepen. “And if a sufficient number start using the yeasts, they might slowly become more mainstream, allowing the larger breweries to also start.”
The VIB team may also transform the beer landscape with yeasts to improve flavors and aromas in low-alcohol brews, which are currently made in one of two ways. One just stops fermentation early, yielding less alcohol—but also fewer of the pleasing yeast-derived compounds. With the other method, you brew normal beer and then remove the alcohol—and many of those same flavors and aromas, too. But new VIB yeasts are being trialled that allow a beer to be brewed normally while ending up with low alcohol but retaining desirable flavor and aroma esters.
Complementing this is the effort to finally give the world flavorsome lager—the beer that accounts for 90 percent of global consumption. Lager is fermented at low temperatures, where the usual brewer's yeast S. cerevisiae does poorly, so brewers have historically crossed it with cold-tolerant wild yeasts to create lager-making hybrids. But because lager makers still rely on just two of these old yeasts—Saaz and Frohberg—there is a lack of genetic yeast diversity that explains why commercial lagers taste so similar.
So VIB's Stijn Mertens created 31 new cold-tolerant yeast hybrids in 2015. And while some didn't score well for flavor and aroma, one in particular—catchily-named H29—combined fast fermentation and high attenuation with complex, pleasing fruity notes in the resulting lager.
Though the work at VIB combines the potential of yeasts with scientific rigour, the complexities of how individual new yeasts work during brewing means there are always unexpected results—which means you can't target specific flavors or aromas with absolute certainty. But Verstrepen is fine with this. Why? “It stays a bit of an art.”