Thiols: Choose Your Own Adventure

Thiols are intense aroma compounds that evoke grapefruit, passion fruit, and guava and are found in hops and a variety of tropical fruits. Thiols are also responsible for the roasty aromas of coffee, the skunky aroma of cannabis, and that classic je ne sais quoi punchiness that makes the Marlborough region of New Zealand famous for their sauvignon blanc. 

Thiols, like esters and phenols, are powerful aromatics that can contribute depth, flavor, and complexity to your beers. One such compound, 3SH, has a sensory threshold of 60ng/L, or 60 parts per trillion (ppt) — it is so potent that if it were just one drop in an Olympic-sized swimming pool, you would be able to perceive it.

Thiols exist in two forms — free forms, which are highly aromatic and volatile, and precursor forms (also referred to as bound thiols).

Free thiols can be found in some varieties of hops, like Nelson Sauvin, Citra, Mosaic, and Simcoe, to name a few. This means that when you use hops like these, it’s highly likely that the finished beer will have perceptible tropical fruit-like aromas.

The precursor forms are non-aromatic and require yeast with carbon-sulfur lyase biotransformation activity to release them. Some hops have thiol precursors, and barley is highly abundant in precursors. Thiolized yeast acts as the key to unlocking these precursors from their bound form and releasing them into beer as aromatic thiols. 

Malt terroir and thiol potential

While the research is still in progress, we have seen positive results when brewing with grain bills that primarily employ malted barley. Through our experimentation, we have found that barley malts almost certainly contain high amounts of thiol precursor, where malted wheat and oats have substantially less. 

The actual amount of thiol precursor varies among different barley varieties as well as the potential for malting practices to impact thiol precursors. Anecdotally, lesser-kilned malts have provided more thiol output. This isn’t a strict observation as our research has found that some lightly kilned Munich malts can still provide the same precursor amounts as a pilsner malt. 

Run sensory tests and take note of thiol intensities when testing out new malt suppliers or working with local maltsters to cultivate your own sense of thiol precursors. We’re hopeful that brewers and maltsters (and yeast labs, of course!) will continue to experiment with different regional grains and discover the full picture. 

Mash hopping

While it may seem counterintuitive, adding hops to the mash is a great way to increase the level of thiols in the finished beer when that’s the flavor profile you are aiming for. We ran trials using Cascade hops in the mash and found a 20% increase in 3SH levels when fermenting with Cosmic Punch, the Thiolized version of British V that uses the IRC7 gene.* 

While some hops are more rich in precursors than others, mash hopping is a reliable way to form a larger pool of thiol precursors. We recommend lower alpha acid hops to avoid over-bittering (assume 30% of the IBU levels that you would get with a beginning of boil addition) and to avoid expensive aroma hops because much of the other volatile hop aroma compounds will be lost in the boil and beginning of fermentation.

*Note: patB strains (Star Party, Helio Gazer, Lunar Crush) generate intense thiols on their own from barley malt — we haven’t found it necessary to mash hop when using these strains.

Precursor-rich products

Apart from malt and hops, there are other products available to brewers that are replete with thiol precursors. Many thiol aromas remind us of tropical fruit because those same compounds are found in the fruit themselves, so try blending in mango, passion fruit, pineapple, guava, etc. to see how those flavors meld with your recipe when using Thiolized yeast. (See ​“Thiol Sensory”)

The beer world’s relationship with thiols is just beginning to truly flourish, but the wine industry has a long and storied history of studying thiols. Decades of wine research have shown that the types and amounts of thiol compounds vary by grape variety, growing region, and cultivation practices, and we know from recent research in beer that this plays out similarly with hops and barley. We have experimented with different varieties of grape juices and grape skin-derived products (like Phantasm®) when brewing beer and found thiol compounds to reach a stronger aroma intensity when used in conjunction with Thiolized yeast strains.

The best experiments change one variable at a time. For starters, we recommend starting simple and layering thiol flavors into your recipe. Build from what you know: if you have a tried and true recipe and you’ve fully dialed in every element, use that as your control sample. Try swapping out the yeast strain for Thiolized yeast and see how that compares. This will give you a baseline and help you think about other ways to modify your recipe. Use the tips and guidelines below to enhance or integrate thiols into your beer.

These three experiments are designed to help you understand a few key concepts regarding thiols:

1. What are thiols? Compare two beers made with practically identical recipes but fermented with different strains — 100% base malt, minimal/whirlpool-only hopping, and fermented separately with the parent and Thiolized strain. This experiment will help you understand the types of flavors you can expect when using Thiolized yeast.
   
2. How does dry hopping affect thiol intensity? Compare two beers made with the same grain bill and fermented with the same Thiolized yeast strain, but with only one of them dry hopped (we recommend starting with 2 lb/bbl). This experiment will demonstrate the way that dry hopping affects the intensity of thiols in the finished beer.
   
3. How do thiols affect the perception of the dry hop profile? Compare two dry-hopped beers fermented with the parent and Thiolized strain (for best results use the same strains you used in experiment #1). This will help you understand how thiols affect the perception of the hop profile when expressed in dry-hopped beer.

Make sure to compare Thiolized yeast with the corresponding parent strain: if you want to test out Cosmic Punch or Helio Gazer, run trials side by side with British V; compare Star Party with Chico, Lunar Crush with Mexican Lager.

When using Thiolized yeast strains in a recipe, remember that thiols are the most potent component. Consider how these intense thiol flavors will work together with the other parts of your recipe.

Of the four strains, Cosmic Punch has the least intense thiol output while still generating thiol levels approximately ten times above sensory threshold. The other three strains (Helio Gazer, Star Party, and Lunar Crush) generate even higher thiol levels roughly 250 – 300 times above sensory threshold because they use a different thiol-freeing enzyme — more on that in the next section. 

Additionally, blending Thiolized strains with their non-Thiolized counterparts will result in lower thiol output proportional to the blend (e.g., 50% Thiolized yeast in the blend will result in about half the thiol output). Starting with blending is a great way to fine-tune your beer to achieve your ideal level of thiols.

The four Thiolized strains leverage enzymes to make thiol aromas pop. There are two genes at work here: IRC7 and patB.

While many beer enthusiasts can easily identify ester- and phenol-driven flavors, thiols haven’t historically commanded quite as much attention as other fermentation flavors. 

The best way to learn flavors is to experience them firsthand — if you’ve never tasted a banana, you’d have no reference point when tasting classic hefeweizens! The same goes for thiols. We can talk at length about typical thiol aromas — guava, grapefruit, passion fruit — but without ever having experienced these flavors on their own and in combination, it can be difficult to pinpoint them in beer. We’ve set up a tasting activity that you can run through on your own or with your brewery team or homebrew club to get a better grasp on thiol flavors.

Materials:

• New Zealand sauvignon blanc wine
• As many of the following as you can find: 
• guava juice, passion fruit juice, grapefruit juice 
• (substitute flavored sodas if juice is not available)
• Pale lager
• Dry-hopped IPA

Tasting procedure:

1. First, familiarize yourself with each of the fruit juices. These fruit flavors are typical of what you should expect when tasting thiol-forward beers.
2. Taste the New Zealand sauvignon blanc. These wines are the prototype for the flavor profile many brewers are aiming for.
3. Now taste through the beers on their own: first the pale lager, then the dry-hopped IPA. Take notes on the flavor profile, focusing on the ester and hop profiles (or lack thereof).
4. Finally, the fun part: begin blending! Mix a bit of the different fruit juices together to see how those flavors meld and interact when combined.
5. Blend a bit of fruit juice in with the beers. Take note of how the fruit flavors interact with the lager, and how the hop profile is enhanced or diminished as these blends come together.
6. Repeat with various combinations to see how these flavor intensities change.