Thiols, Unchained: More Ways to Unlock Tropical Aromas
Here’s how new yeasts are taking advantage of genetic editing and modification to free up thiols—not only in hops, but also in barley malt—for more expressive tropical aromas and flavors.
It hardly seems fair to suggest that Saaz hops have failed to live up to their potential. For centuries they have defined what it means for hops to be Noble, adding floral, spicy, and otherwise fine aromas and flavors to beer. Yet Charles Denby at Berkeley Yeast says he can’t look at recent research without thinking about flavors such as guava and passion fruit that may reside within Saaz, just waiting to be unlocked.
A yeast strain from Berkeley called Tropics may do just that, as might Cosmic Punch from Omega Yeast. The labs didn’t create these strains because the world needs a double dry-hopped Saaz IPA. Instead, they’re driven by the demand for hop-forward beers with tropical character, with the understanding that thiols are a source of unique odor compounds that contribute to that character.
“Thiols are very clearly the shiny new thing with regard to beer flavor,” says Chris Curtin, an assistant professor at Oregon State University. Curtin and Karen Fortmann at White Labs collaborated on a study about the ability of different yeast strains to release polyfunctional thiols, and they presented the results at the 2020 World Brewing Congress.
They aren’t ready to name names—that is, to tell brewers which performed better—but every one of the strains has evolved naturally. Berkeley’s Tropics and Omega’s Cosmic Punch are something different, each a result of genetic modification.
Biotransformation Meets Thiols
The science involved is new. Published in 2011, The Oxford Companion to Beer does not include an entry for “biotransformation.” Brewers understood that yeast played a role in changing hop flavor, but scientists were just beginning to investigate why this occurs.
In 2003, Andrew King and J. Richard Dickinson outlined how yeast transformed monoterpene alcohols, including geraniol and linalool, to create new compounds and flavors described as fruity. Eight years later, summarizing related research, Belgian scientists concluded that most studies were still limited to yeast and monoterpene alcohols, and few investigated the influence of yeast in brewery conditions.
In the years since, brewers have learned that biotransformation can be used to describe multiple ways in which one compound can be converted into another:
Chemical modification of terpenes. This is what King and Dickinson described.
Conversion of organic acids into esters. During hop storage, branch chain acids increase as hops age. During fermentation, yeast may convert these acids, producing fruity ester compounds.
Cleavage of glycosides. Glycosides consist of two parts, one a carbohydrate molecule and the other a non-sugar component called an aglycone. Yeast may cleave glycosides and free this aromatic component.
Release of thiols. It was only in 2003 that Japanese scientist Toru Kishimoto identified thiols as a potential contributor to tropical and other fruity aromas in hops—specifically, three thiols that are present in passion fruit and sauvignon blanc wine: 4-mercapto-4-methylpentant-2-one (4MMP), 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA). They are flavor-active (or “free”) in relatively few hop varieties, such as Citra, Mosaic, Galaxy, and Nelson Sauvin. They are present in far more varieties as bound odorless precursors, and in amounts that dwarf free thiols. However, a β-lyase enzyme can free them and unlock their aromas.
Here’s the catch: Few yeast strains contain enzymes that release a large enough number of thiols to be perceived in beer.
Enzymes, Genes & DNA, Oh My
Last year, the Royal Swedish Academy of Science awarded the Nobel Prize in Chemistry to the scientists who developed CRISPR, which stands for clustered regularly interspaced short palindromic repeats. These “genetic scissors” allow scientists to cut DNA and easily edit genomes.
Omega used the CRISPR technology to create Cosmic Punch, and Berkeley used a conceptually similar method for Tropics. Although the Nobel Prize suggests that many scientists are comfortable with CRISPR—it is different from “GMOs,” which introduce foreign DNA rather than edit an organism’s own genome—it’s not clear how many brewers are ready to use them or how many consumers may choose to drink them.
“We pretty quickly did some dives into what the concerns are—what are the known unknowns,” says Josh Mowry, founder and brewer at Miskatonic Brewing in suburban Chicago. His was one of the first breweries to use Cosmic Punch before the strain had a name. Miskatonic doesn’t hide that Kiwi Coolie is brewed with a genetically modified yeast, and Mowery says that only a couple people have raised doubts about it.
The potential role of genetically modified ingredients, including hops, is worth a story of its own. However, it seems relevant to remember that it wasn’t long ago that brewers who called their products “craft” would have hesitated to use enzymes. Jack McAuliffe took a stand early. When Frank Prial of The New York Times visited New Albion Brewing in 1979, he wrote, “McAuliffe boasts that his beer is a completely natural product. ‘We use malt, hops, water and yeast,’ he said. ‘There are no enzymes.’”
If high-gravity beers didn’t establish that the ethos has changed, brut IPAs certainly did. Lallemand Brewing, known best as a yeast provider, recently introduced ABV Aromazyme, a product composed of β-glucoside enzymes that hydrolyze glycosidic bonds. Researchers at Oregon State University found that enzyme treatment had an impact on hop terpene alcohol release, but it also resulted in higher thiol levels. “We are currently involved in a project about thiol release,” Joan Montasell, technical sales manager at Lallemand, wrote via email. “However, there is not yet any further info [or] data we can provide.”
Genetically modified hops could also be part of the future. John Henning, who has led the USDA public hop-breeding program since 1996, is in the process of putting together a working group to address the question of which genes and markers are linked to various thiols. Several important pathway genes have already been identified, and his group has established their locations in the hop genome.
“Technically, we could modify expression of some of the pathway genes by means of CRISPR technology to produce higher levels of thiols,” he says. “However, one of the burning questions when working with thiols is understanding what levels are appropriate. Some, or many, thiols can be quite offensive if found in high concentration. Think 4MMP and cat urine. So, we would need to figure out what levels are optimum for flavor prior to adjusting expression via CRISPR.
“Perhaps a better way to achieve our goals would be via traditional plant breeding and use of genetic markers to select for offspring containing the favorable sets of genes,” Henning says. “This approach would assure that over-expression of a thiol-gene would not happen.”
Tapping into Potential
Omega Yeast and Berkeley Yeast have taken different approaches in designing strains capable of releasing more thiols, although brewers may care little about the details. Berkeley created the current iteration of Tropics by taking a food-grade carbon-sulfur lyase gene (CSL) from a non-yeast source and inserting it into the genome of a hazy yeast strain. The CSL enzyme breaks the carbon-sulfur bond in thiol precursors.
Tested in a lab, Tropics releases more than 100 times the 3MH found in the parental strain in unhopped beers by freeing the bound 3MH found in barley malt. Hold that thought about barley and be aware that other yeast strains may be better at breaking the carbon-sulfur bond than this particular parent.
Omega focused on a gene, IRC7, that encodes the β-lyase enzyme. “From a regulatory standpoint, the [United States] would not consider [Cosmic Punch] GMO,” says Omega cofounder Lance Shaner. Omega has the details on its website (https://omegayeast.com/news/cosmic-punch-new-thiol-boosting-strain), but the elevator version goes like this: There are many mutations of the IRC7 gene. They took a functioning one from the well-known Chico strain, inserted it into their popular British V strain, and changed the sequence that controls its expression, so that the gene remains turned on, manufacturing more β-lyase enzyme and ultimately releasing thiols.
As a result, Cosmic Punch produces the same flavors as British V, but with 3MH ten times greater than the perceptual threshold. 3MH on its own may be described as smelling of rhubarb, guava, passion fruit, or gooseberry.
Tropics and Cosmic Punch strains are likely going to end up first in beers meant to showcase exotic flavors from hops, but conversations about them should be broader. They are reminders of the role that yeast plays in hop flavor, and they make it clear that barley may be responsible for some flavors associated with hops.
Researchers in France found that some barley varieties contain thiols, particularly 3MH. “You get hop flavors even if you don’t add flavor hops,” says Denby at Berkleley. “I feel like we’ve shown that to brewers. They can use this strain as a tool to get the potential held in both barley and hops.”
Miskatonic’s Kiwi Coolie illustrates that, too. Aided by advice from Omega’s Shaner, Mowry used barley malt from Alberta known to contain thiol precursors. They added 22 pounds of Czech Saaz hops in the mash (for a 17.5-barrel batch) and none in the kettle. The beer was dry hopped with 22 pounds of Rakau after fermentation. Mowry says the most prominent aroma was grapey, like sauvignon blanc, with notes of passion fruit and grapefruit.
“This might be the most interesting thing I’ve been part of,” he says. He’s already thought of at least six more beers he can ferment with the yeast, probably starting with a saison. “I want it all,” he says.
Saaz is one of several varieties that brewers likely don’t realize contain bound thiols. The list is far from complete because there was little reason to measure them.
“It really didn’t matter the [number] of bound thiols there were, until we had strains that would free them,” Shaner says.
Stan Hieronymus