Good Beer Hunting

Mother of Invention

An Uncharted Wilderness — Understanding Hop Compound Biotransformation

Chad Yakobson still remembers the moment a beer changed on him. It has happened only once: an unheralded transformation in flavor and aroma so striking, and so unexpected, that it altered the way the Crooked Stave founder and brewmaster thinks about his craft.

The beer in question was an India Barleywine brewed at Odell back in 2009, heavily dry hopped with Amarillo and Simcoe and dosed with Orval’s Brettanomyces strain. When fresh, it tasted classically of orange and tree resin. But within two to three months, it had lost its West Coast hop profile in favor of something stranger and altogether wilder.

“All of a sudden, here was this beer in front of me that was a completely different beer—and it was beautiful,” says Yakobson. “We started looking around, like, ‘Who’s got lemon zest and lavender?’ because that can’t be coming from this beer—it’s too perfect. It was really, really neat.”

Yakobson attributes these radically new flavors and aromas to hop compound biotransformation: a complex chemical process during which yeast cells transform hop components into new, often boldly aromatic compounds. There are numerous forms of biotransformation, and the topic—still argued-about and under-researched, still primarily an uncharted wilderness—is one that is beginning to beguile brewers in search of novelty, or experimentation, or simply a greater understanding of fermentation’s possibilities.

“It was extreme, it was night-and-day, a different aroma and flavor,” Yakobson remembers of that much-transformed beer. “And I found that to be so amazing.”


It’s exhilarating, the idea that hops, already so vivid and potent, may contain additional, hidden aromatics—aromatics that can be unlocked under the right conditions during fermentation. To visualize the process, think of that blue butterfly pea tea that Instagram loves, which, with the addition of lemon juice, turns magenta in the cup. Or compare it to Flavor-Morph Starbursts, which change from cherry to lime in the mouth. Choose your adventure.

These chameleonic possibilities are understandably compelling. They’re also so unexplored—most experiments and studies that focus on biotransformation have taken place within the last decade—that brewers are still in the earliest phases of experimentation. Many have happened upon hop compound biotransformation empirically, accidentally: a hallmark of the recently ascendant New England IPA style is that it is frequently dry hopped during active fermentation, which would create the necessary conditions for yeast and hop compounds to intermingle and produce new flavors and aromas.

“We just kind of stumbled into it accidentally,” says Jean Broillet IV, the founder of Tired Hands. “When we opened, we kind of had to make beer faster than normal. It sounds like a cop-out excuse. We opened with a five-barrel system—at most we could crank out maybe seven barrels per batch—and sometimes we had to dry-hop three to four days into fermentation.”

For Tired Hands, necessity bred the perfect conditions for biotransformation—and the effects were evident. “We noticed early on that the hop profile and aromatic component was way more bombastic,” Broillet says. “It was almost this shortcut we had stumbled across where you get bang for your buck with this rushed product.”

At this point, there have been few attempts to educate consumers on the subject. In 2016, Cloudwater was one of the first breweries to make the topic explicitly public-facing when it released its DIPA versions 4 and 5. The two beers were brewed using the same wort, the same yeast, the same hop bill, the same fermentation time. The only difference was dry hopping: one was dry hopped while fermentation was still ongoing—when active yeast cells would be exposed to hop compounds and potentially able to transform them. The other was dry hopped afterwards.

The two emerged as notably distinct beers. As Zach Fowle wrote at the time in a piece in DRAFT, DIPA v4 was “woody, almost mossy, with notes of overripe orange, mango notes, chopped onions, fresh grass blades, and tangerine pulp,” while DIPA v5 was “lemonade-citrusy and so full of additional orange peel, orange blossom, honey, and lime notes, it should probably be planted in a verdant California orchard.” Cloudwater’s “public experiment” seemed to display compelling evidence about biotransformation’s powerful effects on flavor and aroma. There was palpable excitement about the topic at hand.

Two and a half years later, however, Cloudwater’s investigations seem to have stalled, and owner Paul Jones is markedly ambivalent about the process. “I think [biotransformation] was something we spoke about back then because it seemed we were on the cusp of turning a corner and understanding something—it felt like there was going to be a decent body of evidence emerging. Now things are inconclusive enough that we can't be that clear on what's going on. All we're really aware of, at this stage, is that you change your process and the results can differ.”

It was extreme, it was night-and-day, a different aroma and flavor. And I found that to be so amazing.
— Chad Yakobson, Crooked Stave

Hop compound biotransformation may be a contemporary buzzphrase, despite the clunky way it trips off the tongue, but there are numerous hurdles to understanding it more intimately, from laboratory capabilities and lack of funding to the elusive nature of the compounds themselves. Much supposed evidence of biotransformation, from Cloudwater’s comparative DIPAs to Yakobson’s lavender-scented India Barleywine, has relied on sensory input in lieu of scientific rigor. The vast majority of breweries simply don’t have the capacity to explore the topic in real depth.

“That’s why it seems to have ground to a halt, and we’re basically waiting for researchers and academics to do this work for us, because there are only a few brewery labs in the world that would be capable of running this detailed analysis of transformation of flavor compounds,” Jones explains. “We hear a lot less about it these days than a couple years ago, and I suspect that’s because everyone that’s interested in hop flavor and whether there is biotransformation occurring has realized they can’t actually prove a thing.”


There are many reasons why biotransformation eludes the easy comprehension of seasoned industry professionals. Begin with the name. As Tom Shellhammer, the Nor’Wester Professor of Fermentation Science at Oregon State University, points out, “biotransformation” is a vague, slippery term that can refer to any general alteration of organic compounds—even fermentation itself (the production of ethanol by yeast).

Now, let’s add an additional vector of complexity. Even within the seemingly narrow focus on hop compounds, there are still numerous forms of biotransformation—each of which can have a unique impact on the flavor and aroma profile of a beer. In this context, there are four primary versions of hop compound biotransformation of which to be aware.

1. Esters: Scientifically, esters are a bond between an alcohol and an acid. Sensorially, they can “smell like apples and pears and red berries and cotton candy” and myriad other aromatics. Recent studies have shown that esters can be produced by the interaction between yeast and hop compounds. One example: yeast can transform geraniol—an organic compound that is produced by hops, as well as marijuana and other plants—into geranyl acetate, an ester that smells abundantly, opulently, of roses.

2. Glycosides: Hops also contain glycosides: molecules that are bonds between aromatic compounds and sugars. When bound to sugar, the aromatic compounds are imprisoned, undetectable, waiting release. But hungry yeasts, insatiable in their hunt for food, contain enzymes that can cleave the glycosides in two. As a result, aromatic compounds are made volatile and set free. According to Shellhammer, “there is some thought that a significant portion of the aroma in hops can be these glycosides.”

3. Thiols: Thiols are a class of very potent, sulfur-containing compounds that can exist in free and bound forms. They can be detected by the human nose at a concentration of five parts per trillion. While bound forms behave similarly to glycosides, in this case, their aromatic compounds are linked to amino acids or small peptides instead of sugars. “Thiols are 10,000 times more potent than geraniol—but they’re only there in hops in tiny, tiny amounts,” Shellhammer says. Thiols “have a variety of smells that range from tropical fruit, papaya, and guava, to cat pee and arm[pit] sweat,” and are present in higher concentrations in certain pungent hop strains like Mosaic and Citra.

4. Stripped Compounds: One final form of biotransformation that Shellhammer notes stands in contrast with the rest, as it results in a reduction, rather than an augmentation, of hop aromatics. Both yeast cells and hop compounds are hydrophobic. That is, they may then stick together, in which case the more hydrophobic hop compounds, such as aromatic myrcene, are absorbed into the yeast’s cell surface and “stripped” from the beer. Another type of stripping phenomenon can occur during active fermentation, whereby the carbon dioxide produced by the yeast can potentially reduce the more volatile hop oil aromatics, due to the vigorous scrubbing effect of the CO2 bubbles as they leave the fermentor.


Consider the blooming, fractal complexity of this topic. Consider that each hop has its own makeup of chemical compounds—Simcoe will never behave like Saaz in a biotransformative context. Consider that we have henceforth identified only 450 hop compounds, while up to 1,000 may exist (as noted in a 2017 study written by Daniel Sharp, Yanping Qian, Gina Shellhammer, and Tom Shellhammer). Consider that individual yeast strains, from classic ale yeast to wild Brettanomyces, interact with each discrete compound differently. Consider that we’ve only been aware of this topic—especially newer aspects of it, like hop thiols—for a few short years. Biotransformation is still a shrouded rainforest—scientists are only just beginning to catalogue the trees.

For his part, Shellhammer is intrigued by the possibilities of biotransformation, but is also skeptical of the topic’s overall impact. “I worry sometimes that what happens in the research world—it’s new, right? So there’s a new discovery, and the brewing community views it as the latest answer to everything. And people perseverate or maybe even fetishize [it],” he says. “My gut feeling on this is that [biotransformation] is not a silver bullet or magical discovery that describes all hop flavor in beer.”

My gut feeling on this is that biotransformation is not a silver bullet or magical discovery that describes all hop flavor in beer.
— Tom Shellhammer, Oregon State University

He alludes to a glycoside-focused study that he co-authored in 2017 with Daniel Sharp and Jan Steensels, during which more than 150 yeast strains were tested and ranked according to their biotransformative abilities. They were then tested again alongside raw enzymes. The results were disappointing: even the most active yeast strains showed only a slightly greater ability than the least active ones in transforming hop compounds, and were all markedly less effective than the pure enzymes.

As it is, hops in their natural state are saturated with essential oils. Sticking your head in a bag of fresh pellets will transport you to an olfactory jungle of green. There may be the fantastical lore about hops’ hidden flavors and aromas, but then there’s the unignorable, present-tense reality of their already flamboyantly bold aromatics.

“At the end of our research, we think that...the free hop aroma is probably more like 90% [of the hoppy flavor and aroma in beer], and the glycosides are probably like 10%,” Shellhammer says.

Some are more convinced of biotransformation’s power and influence. Maria Moutsoglou, a fermentation scientist at Sierra Nevada, recently co-authored a study with William Caylor and Andrew Reyes examining the impact of dry hopping at different stages of fermentation, which was presented at the 2018 Brewing Summit in San Diego. The study, which compared beers that were dry hopped in the presence of yeast with beers that were dry hopped after cold crashing found “compelling evidence” of the “flavor impact on beer from biotransformation.”

“According to our results,” she says, “the flavor profiles we obtained in beers dry hopped in the presence of yeast were broadly desirable compared to dry hopping after the tank was chilled and in the absence of yeast. Our panel identified significantly higher citrus, rose/floral, and fruity/tropical flavors in beers with hops added in the presence of yeast. Sensory results were corroborated by the results from our GC–MS volatile profile analysis, where we identified significantly higher relative concentrations of the monoterpene alcohols geraniol, linalool, and nerol, providing citrus and floral flavors.”

Moutsoglou’s study presents credible indications of biotransformation’s influence. But until there’s further corroboratory research, and until there are more breweries or companies willing to front the cash for such research, making concrete declarations about biotransformation’s impact remains exceptionally difficult.


Today’s brewers and drinkers are so riotously entranced with hops that it’s easy to forget how recently they became our darlings. Christina Schönberger, the Head of Education and Innovation at the Barth-Haas Group (the world’s largest supplier of hop products), notes that, until 10 years ago, hops were primarily bred with yield and alpha acid percentages in mind. Today, flavor and aromatics are what dominate the conversation.

And yet, hop breeders still struggle to dial in basic aromatic characteristics in the strains they breed. Selecting for latent hop compounds, at this point, is a virtual impossibility. “Breeding for flavor is extremely challenging because there are no markers…that you can connect phenotypically,” Schönberger says. “There is still very much trial and error in breeding.”

Altering yeast may be a more fruitful way to approach biotransformation. “There’s work to be done on understanding the specific mechanisms behind how yeast perform biotransformation, which can lead to methods of manipulating yeast to perform biotransformation to our specifications,” Moutsoglou says. “It would be naive to discount genetic engineering as a major source for both understanding biotransformation at a high resolution, but also in innovative beer production. Modifying yeast to express enzymes involved in biotransformation using genetic engineering has a great research potential.”

Schönberger agrees. Because many brewing yeasts seem to be limitedly effective at producing new flavor compounds during biotransformation, she says “we probably need to look for known wine yeasts that have this activity, or look for specific enzymes present in those wine yeasts, or talk to enzyme supplies and manufacturers and see if they can produce enzymes that’d do the job.”

Moutsoglou warns that using genetically modified yeast during dry hopping may not appeal to consumers. But for brewers who are hungry to understand the phenomenon, to control it and witness it for themselves, inoculating with modified yeast cells—or even using raw enzymes as a part of fermentation—could be a viable way of fostering discovery.

As for the future, Shellhammer, Schönberger, and Moutsoglou all agree that research into hop compound biotransformation will likely continue apace (provided funding needs are met), and that further revelations will be unearthed in the coming years. “I would think in the next five years we’ll know more,” Shellhammer concludes.

Many brewers impatiently await their findings. “You make an IPA, IPA is all about the hops,” Yakobson says. “If I could make a beer that was all about the biotransformation, that would be the holy grail.” But until more information comes to light, that remains a project for the future. “I hope that this will be a focus of research going forward, because it’s extremely exciting,” he says. “Now I’d like to get the research on it, and see whether it’s myth or fact.”

Others are more content to rely on sensory input, to preserve a sense of the mysterious. "We want to maintain some element of magic in our beer," Broillet says. "It’s not necessarily a positive thing to learn more about what’s happening in terms of the biotransformative elements of hoppy beer."

It’s not necessarily a positive thing to learn more about what’s happening in terms of the biotransformative elements of hoppy beer.
— Jean Broillet IV, Tired Hands

Not that that perspective has prevented Tired Hands from continuing to push exploratory boundaries. Several months ago, the brewery released Sustained Transient Self, an “interconically hopped Double IPA” that was hopped in metered additions exclusively during fermentation. (Broillet likens the results to a creamy gelato.) He’s also excited about a new series of beers the brewery will release later this year, MilkStave, which he describes as “a new mixed-culture riff on our Milkshake IPAs."

“We’ve followed the protocol for a bottle-aged Saison,” he says, but they’ve also used the hallmark ingredients that have characterized the brewery’s dessert-like, ultra-hazy IPAs, including oats, wheat flour, apple purée, and a heavy dose of hops. It will be months before the results of the experiment—and any biotransformative elements therein—are apparent, but Broillet is confident that they’ll be “pretty far out.” They’re also unlikely to be obsessively dissected after the fact.

“For good or for bad, it’s an avenue that no one on our brewing staff has really doubled down on,” he says. “We know [biotransformation] exists, but we just know that’s been sufficient for us so far.”

Words by Claire Bullen
Illustration by Ben Chlapek