TL;DR
- Barley contains abundant levels of thiol precursors, but only certain yeast strains can convert precursors into flavor-active thiols.
- Mash hopping is not necessary and can be detrimental; certain hops shouldn’t be used in the mash.
- You can achieve near-perfect control over thiol intensity with just yeast and purified precursor additions.
- There is significant variation in thiol-biotransformation activity across different yeast strains. Berkeley Yeast Tropics™ was developed for highly active and precise biotransformation, yielding strong tropical notes with no off-flavors.
Thiols for Balanced Complexity
Thiols have become a buzzword across the craft-brewing world, and for good reason. Thiols drive an impressive array of tropical-fruit aromas. If you’ve ever tasted a beer bursting with notes of passion fruit, guava, or pink grapefruit, you’ve experienced the magic of thiols.
But is more thiol a universally good thing? Based on hundreds of customer conversations—and many more pints of hazy IPA—we have found that thiol levels are akin to many ingredients in the brewing process: if they are in balance with the recipe as a whole, people will order one pint after another. If they are out of balance, they can be off-putting.
There’s an enormous amount of information on the internet, ranging from accurate to misguided to flat out incorrect. As a result, there is a lot of confusion. We often hear brewers say, “Thiols are polarizing.” The reality is that thiols—at least the target thiols that are tightly associated with tropical fruit—are no more polarizing than a great crop of Citra. The problem is that if you follow much of the advice floating around online, you’ll end up with more off-flavor than actual thiol character.
Don’t worry. In this article, we clear up some of these hazy matters and give you what you need to know to create balanced complexity in tropical-forward IPAs.
What Exactly Are Thiols?
Thiols are a type of sulfur-containing molecule with a sulfhydyl (—SH) bonded to a carbon atom. The most famous ones in beer tend to have high odor activity, meaning that they can be very aromatic at very low concentrations. People often associate thiols with molecules that can be off-putting, such as 3-methyl-2-butene-1-thiol, which smells like skunk and burnt rubber. But thiols such as 3-mercaptohexanol (3MH) can be magical, especially in a well-crafted hazy IPA.
Tropical thiols in beer. 3MH and 3MHA are two of the most important thiols for driving tropical aromas in beer. They are perceived at extremely low concentrations and contribute notes of guava, passion fruit, and grapefruit.
The thiol compounds that are most prized by beer drinkers are 3MH and 3MHA for their distinctive tropical-fruit aromas. These thiol compounds are the primary flavor determinants in a wide variety of fruits, including passion fruit, guava, and grapefruit.
The way thiols present in a finished beer depends on the recipe. In a beer with a sparing hop addition, thiols show up as “tropical” on a blind-tasting panel, tracking most closely with passion fruit, guava, and mango. But what we’ve learned from brewers is that these thiols, in combination with fruit-forward dry-hop additions, can accentuate other fruity flavors too, from orange to pineapple and even stone fruit.
Where Do Thiols Come From?
For decades, it was believed that thiols came exclusively from hops, formed when hop-derived precursors are biotransformed by yeast during fermentation. The reality is that barley is a rich source of thiol precursors, and with the right yeast strain, there’s plenty of precursor to biotransform intense tropical-fruit character. This reality was first hypothesized and documented by Japanese researcher Toru Kishimoto and his team in 2008, but it was not well appreciated at the time, in part because it wasn’t well understood how to biotransform the precursor compounds into their flavor-active form.
In 2020, Berkeley Yeast patented and launched Tropics™, the first commercial strain with high thiol-biotransformation activity. In fact, the biotransformation activity was so high that the precursor levels in barley alone were enough to make an unhopped neutral ale taste more like fruit juice than like beer. This was a pretty groundbreaking discovery because the consensus in the brewing industry at the time was that the flavor-active thiols were coming exclusively from hops. Berkeley’s Tropics gave brewers access to the large pool of thiols in barley that were previously inaccessible.
3MH biosynthesis. Glutathione is conjugated with a product of fatty-acid metabolism to form Glut-3MH, a flavorless thiol precursor. This is a normal process that happens inside almost all plant cells. Glut-3MH is converted to the flavor-active thiol, 3MH, by certain yeast strains during fermentation.
To get a better understanding of where thiols come from, let’s get a little deeper into the biochemistry. Most plants make thiol precursors as part of their natural metabolism. The major thiol precursor–compound type is a “glutathionylated” thiol. Glutathione is a chemical compound that is highly abundant in yeast—in fact, the vast majority of eukaryotic life uses it for cellular detoxification. The glutathionylated precursor of 3MH comes from the natural breakdown of fatty acids in the endosperm and germ of the barley. This cellular process happens in just about all plants at some level, and barley and hops are no exception. So, given the abundance of precursors in the natural world, let’s next talk about the best strategies to get the free thiols into your finished beer.
How to Get Thiols in Beer
Thiol extraction and biotransformation during brewing. Flavorless thiol precursors are extracted from barley during the mash. These precursors are heat stable and survive the boil. Yeast strains such as Tropics, that express highly active carbon–sulfur lyase (CSL) enzymes, convert these precursors into flavor-active thiols during fermentation.
While practically all plants make thiol precursors, barley is a particularly rich source, providing the most potential of the standard brewing ingredients. Accordingly, the most effective strategy for achieving consistent thiol levels is to extract precursors from barley on the hot side, then convert precursors on the cold side through yeast biotransformation. As long as you’re using a neutral base malt (e.g., two-row, pilsner), precursor extraction is consistently high. It’s worth noting that higher-kilned barleys contain fewer precursors, and adjuncts are generally a poor source. Of course, there is some variation among barley lots, and we will continue to share findings as we learn more.
Next, let’s talk about the contribution of yeast because this is where the story gets especially interesting. Traditional and hybrid yeast strains have intrinsically low biotransformation activity and are not effective for converting precursors to free thiols. The reason is that the carbon–sulfur lyase (CSL) enzymes originating from yeast (Irc7 and Str3 are the most well studied) have low activity toward 3MH precursor. Even some strains that are marketed as strong biotransformers most often do not produce sufficient 3MH to make a sensory impact. For thiols to show up in a glass of IPA, concentrations need to exceed 100 nanograms/liter (ng/L), and that doesn’t happen with traditional or hybrid strains. It’s also worth noting that both Irc7 and Str3 play a role in general sulfur metabolism and perform biochemical reactions that may lead to off-flavors. The top yeast experts describe this phenomenon as “a double-edged sword.”
The biochemical difference between traditional yeast and a yeast with strong thiol-biotransformation activity. Bioengineered yeast, expressing a highly active CSL, produces more thiol than traditional and hybrid strains.
So, if traditional yeast doesn’t work, then how do we get more biotransformation? Through years of research, our team at Berkeley Yeast developed a new strain with high and precise biotransformation activity, by reinventing its genetics. To do this, we focused on incorporating a new enzyme into yeast, from a different species of microbe that has very high CSL activity. Then we optimized the enzyme’s structure—through a method called protein engineering—so that it efficiently converts precursors to 3MH, without generating off-target sulfur compounds. Finally, we inserted a gene that codes for this enzyme into the industry’s most widely used hazy IPA strain, London Ale, creating a new strain we call Tropics London.
Validating the Benefit of Precision Engineering
As brewers are all too aware, beer is complex. There are so many chemical compounds in and across beers that affect flavor and quality. Many of these compounds are not well characterized, especially in how they are perceived when mixed together. So, to better characterize the effect of strong thiol biotransformation on beer, we worked with Dr. Tom Shellhammer and the team of experts in his Oregon State University brewing science lab.
The thiol character generated by Tropics is perceived as tropical without off-flavors. Results from a check-all-that-apply (CATA) sensory analysis. The data show the number of panelists that described samples as the given descriptor (N=18), averaged over eight different conditions during a blinded sensory evaluation of beers fermented with standard London Ale yeast and the same parent strain that was developed to express a highly active CSL.
The major finding from our collaboration with Dr. Shellhammer is that in a commercial brewing environment, with an ultra-rigorous sensory science approach, the precursor load from a standard malt bill, when converted by Tropics, drives a strong tropical-fruit flavor profile, with no increase in the perception of off-flavors. For a deeper dive, you can read the full study of how Tropics yeast produces thiols and Tom’s commentary about the implications for industrial brewing. Turns out that—with the benefit of precision engineering—thiols are not a double-edged sword after all.
Thiols and Hops
While biotransformation of precursors from barley is a reliable approach for driving tropical character, hops also contribute to overall thiol levels. Exactly how much the inputs contribute to the final thiol levels depends on the recipe and batch-effects of the materials. To illustrate the typical contributions, let’s consider the theoretical outcome for a standard IPA recipe: if we expect similar extraction efficiency for barley and hot-side hop additions, then we would expect a ~60:40 contribution from barley and hops. This is consistent with experimental data we have previously generated. However, it is important to note that the thiol-precursor levels vary across different batches of both barley and hops. Batch-to-batch variation of inputs can be managed by blending across multiple batches, which is practical at larger breweries. But the variation creates a considerable challenge for delivering a consistent product.
Theoretical maximum thiol potential assuming 100 percent extraction and 100 percent conversion. Most thiol potential comes from extraction of flavorless thiol precursors from barley. Hot-side hop additions may supply additional precursors, typically at lower levels than malt. During fermentation, yeast with high biotransformation activity can convert these precursors into free thiols. Cold-side hopping primarily contributes free thiols and generally adds little to precursor load. Thiol potential denotes the theoretical maximum assuming complete extraction and conversion. Actual concentrations are lower due to limited extraction efficiency, incomplete biotransformation, and losses of free thiols from volatility and chemical reactivity.
For context, let’s look at the absolute levels of free thiol that we expect from a beer fermented with traditional yeast compared with a beer fermented with Tropics. Using traditional yeast and a low dry-hop rate will result in a beer with thiol levels well below 100 ng/L. Using a batch of hops that is rich in free thiols, cold-side additions can lead to appreciable levels, ranging from 0.1 to 0.3 micrograms/liter (μg/L). However, with Tropics, precursors from barley alone are enough to drive the levels of thiol, reaching 2–10 μg/L. This provides a very pronounced juicy character to hazy IPAs that plays well in many recipes. But like any tool, more control is better, as we discuss below.
No Need for Mash Hopping
In the interest of beer quality across the industry as a whole, it’s necessary to clear up some of the confusion around mash hopping in hazy IPAs. Over the past several years, several suppliers recommended and popularized the practice. The reasoning is clear, albeit flawed, that adding hops earlier on the hot side may lead to greater precursor extraction and conversion by peptidases. The reasoning is flawed because we now know that increasing mash-hop additions of high-precursor cultivars can lead to highly detrimental effects to the overall beer quality. By many accounts, adding large hop charges during mashing has wide-ranging negative effects, sometimes resulting in beer that is undrinkable. Many of the varieties recommended for mash hopping carry sensory attributes that don’t play well in a tropical-forward IPA. Hops such as Saaz, Mittelfrüh, and Tettnang can bring spicy, earthy, bell pepper, or vegetal notes that may detract from an otherwise delicious IPA, even at low-use rates. If the goal of mash hopping is to add more precursor, there are better ways to do it.
Get Full Control of the Thiol Dial
Since we launched Tropics™ more than five years ago, the most common questions we’ve received are about control: How do you turn thiols down, so my beers don’t all taste the same? How do you turn thiols up, so they don’t get lost with a 4 lb/barrel dry-hop addition? We have long recommended co-pitching Tropics with other strains as a means of reducing tropical intensity. And we previously developed Tropics Boost™—a super-simple product composed of pure thiol precursor, ethanol, and water—for when you want to increase tropical intensity.
To fully address these questions, here’s the complete answer. We call it the Thiol Dial.
The Thiol Dial. To get full control of thiol levels. all you need is Dry Tropics™, Tropics Boost™, and your house strain.
How exactly should you use the above data for recipe formulation? The simplest approach, and the one we recommend, is starting with a pure pitch of Tropics in an IPA recipe with which you’re already familiar. That will give you a baseline. From there, if you prefer to dial down the intensity, co-pitch at 30 percent (w/w). The thiol character will still have a clear impact on the final beer quality (~0.25x) but will provide a more nuanced juicy quality. If you prefer to dial it up, add Tropics Boost. The more you add, the higher the intensity. Add 50 mL/barrel for a subtle bump (~1.4x), or add 200 mL/barrel to go big. Adding Tropics Boost is especially helpful in aggressively hopped IPAs, where the large hop load can have a muting effect on thiol notes. The Thiol Dial lets you precisely tune flavor profiles across recipes.
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