The Case for Thiols:
Yeast strains genetically engineered to release thiols are the most exciting area of research in brewing today. A tiny amount of thiols can elevate a beer with vibrant passion fruit and grapefruit notes that enhances traditional hop aromatics.
The Case Against Thiols:
Yeast strains genetically engineered to release thiols are the most over-hyped area of research in brewing today. A tiny amount of thiols can overwhelm a beer with a one-note artificial fruitiness that obscures traditional hop aromatics.
We started pitching thiol-expressing yeast at Sapwood Cellars (Columbia, Maryland) in 2020. Since then we’ve used a dozen different strains across 50 recipes. We’ve brewed collabs at breweries smaller than us and ones 100 times our size: 3S4MP (Omega Yeast Cosmic Punch IPA with Phantasm powder and Nelson SauvinTM hops at Fidens), Cone Concentrate (Cosmic Punch DIPA with Simcoe® at Other Half DC), Celestial Paradox (Berkeley Yeast London Tropics IPA with Strata®/GalaxyTM/Citra®/Simcoe® at Toppling Goliath), Yasokeee (Omega Yeast Helio Gazer DIPA with Peacherine®, HydraTM, and Phantasm at Cushwa), Heisenberg (Helio Gazer DIPA with GalaxyTM and Nelson SauvinTM at Commonwealth), and most recently Wedding SZN (Jasper Yeast Particle Wave Rice Ale with Phantasm at Olde Mother). Clearly some of the best breweries in the country are intrigued by the potential for thiols!
Initially there was a push to maximize thiols with techniques such as mash hopping, Phantasm (thiol-rich powderized New Zealand Sauvignon Blanc grape skins), and cold fermentation. Then yeast labs developed new strains with more potent genes that could make 200 times the flavor threshold of these tropical aromatics solely from compounds in the malt itself. After that initial excitement, today we only pitch thiol-expressing yeast strains sparingly, on specifically targeted recipes where the thiols reinforce flavors already in the beer or add complexity.
Through this article we will dig into what thiols are, how to control their production, and what beers they suit best!
What Are Thiols?
Thiols are some of the most potent aromatic compounds found in nature. This can be a positive thing (3-Mercaptohexan-1-ol, or simply 3MH, brings passion fruit and citrus notes from hops or yeast) or negative (skunky smelling mercaptan in light-struck beer is also a thiol). The positive role of thiols has come into focus since the release of genetically modified yeast strains from Omega Yeast and Berkeley Yeast (and now others like Escarpment Labs and Jasper Yeast) capable of freeing them from malt, hops, fruits, adjuncts, and other ingredients brewers may use that contain precursors.
Thiols are sulfur-containing molecules that are often potent aromatics. The ones brewers are excited about are tropical, wine-like, and citrusy, while other thiols are intensely unpleasant with aromas of garlic or rotten eggs . . . which is why mercaptan (methanethiol) is added to natural gas to alert people to leaks.
Unlike many other beer aromatics that require concentrations measured in parts-per-million or parts-per-billion, most thiols have aroma thresholds in the range of 4–70 parts-per-trillion (ppt). This means that they don’t require much to be apparent, but also that “high” concentrations quickly become overwhelming.
In terms of positive beer and wine aromatics, the thiols that get the most attention are 3MH, 4MMP (4-methyl-4-mercaptopentan-2-one), 3MHA (3-mercaptohexylacetate), and 3S4MP (3-sulfanyl-4-methylpentan-1-ol). These have perceptions that range from passion fruit, to grapefruit, to rhubarb. These are the intense aromatics that give New Zealand Sauvignon Blanc wines their distinct aromas (at 3MH levels as high as 19,000 ppt1) and are found free at low levels in many New Zealand hops (as well as many other varieties from around the world).
Most of the thiols found in hops, malt, and other botanicals are bound to amino acids and thus are not active aromatically. Enzymes are required to free them. There are wine strains naturally capable of this, but getting those genes into brewer’s yeast requires more work.
You’ll see the same thiols abbreviated with either an “M” for “mercapto” or “S” for “sulfanyl,” both are acceptable. Here are some of the most common (with aroma thresholds in parts-per-trillion in parentheses*).
4MSP/4MMP – Boxtree, Black Currant (4.2 ppt)
3MH/3SH – Passion fruit, Guava (60 ppt)
3MHA/3SHA – Passion fruit, Grapefruit (4 ppt)
3M4MP/3S4MP – Passion fruit, Tropical (70 ppt)
To put those numbers in context we had three beers fermented with one of the “high” thiol expressing strains measured for thiols. Quantum Juicinator measured 4,490 ppt of 3MH (standard IPA), White Dragon was 7,140 ppt 3MH (DIPA with Phantasm in the whirlpool), and our Passion Fruit Rings was 16,330 ppt 3MH and the only one with 3MHA present at 150 ppt thanks to 1 lb./gallon (120 g/L) of passion fruit puree.
Where Do Thiols Come From?
Bound thiols are found in both malt and hops, but levels vary widely. The bond in need of breaking comes in two “flavors” — cysteinylated (Cys-) and glutathionylated (Glu-) named for the amino acids cysteine and glutamine respectively. The vast majority (90%+) of bound thiols in malt and hops is Glu. The IRC7 gene in certain wine strains and Omega’s Cosmic Punch can only work on the less common Cys-type. As a result, mash hops are only potentially beneficial for Cosmic Punch, as the enzymes in the mash (especially in the acid-protein rest temperature range) can convert some Glu to Cys. Luckily some less expensive hop varieties have the highest levels of bound thiols. We’ve used Saaz, Cascade, and CalypsoTM with good results.
Omega’s Cosmic Punch, a cisgenic yeast, was developed by inserting IRC7 genes from their West Coast I yeast strain and a highly active promoter sourced from the Saccharomyces cerevisiae reference genome into their British V strain using CRISPR/Cas9 gene modification.
The more intense strains like Berkeley Yeast Tropics line and Omega’s Helio Gazer, Star Party, and Lunar Crush all include bacterial genes. Rather than having more copies of the IRC7 gene, they are given wholly different genes that can free Glu-thiols directly. The Omega strains have been transgenically modified by inserting patB genes from bacteria into brewing yeast using CRISPR/Cas9, and the Berkeley strains have been transgenically modified by inserting bacterial tnaA genes that code for carbon-sulfur-lyase (CSL) into yeast using CRISPR/Cas9.
Because the enzymes produced by these transgenic strains hydrolyze Glu-type and can simply be added to a standard recipe with or without mash hops thiols, they have a more pronounced effect on beer aroma than Cosmic Punch. At Sapwood Cellars, which I co-founded with Scott Janish in 2018, we fermented a kettle sour that only had a small dose of hexalone (isomerized hop extract for head retention) with London Tropics. The result was intensely passion fruity, so much so that it could almost pass as a fruit beer (which came entirely from malt-derived thiols as the hop extract has no thiols).
Genetically modified (GM) yeast strains aren’t allowed in many countries (e.g., Europe and New Zealand). Canada does allow cisgenic strains like Cosmic Punch, but not transgenic strains like Berkeley’s Yeast Tropics line or Omega’s Helio Gazer, Star Party, and Lunar Crush. As a result there are labs working with wild isolates capable of freeing thiols for co-fermentations (e.g., CHR Hansen SmartBev NEER, which is primarily marketed for non-alcoholic beers) and breeding strains with heightened thiol freeing capabilities (e.g., Escarpment). Omega initially worked on yeast breeding between an English ale strain and a wine strain capable of freeing thiols . . . our trials with it (batches including Designer Baby and Cheaper Hops) were interesting, but had too much of the wine strain’s idiosyncrasies present (poor flocculation especially).
The Case Against Thiols
We’ve reached the part of this article where I’m going to editorialize and share opinions based on the dozens of batches I’ve brewed using thiol-expressing yeasts. Take it for what it’s worth.
My biggest issue with thiols is that they can come across as one-note in a delicate beer. Without something to compete against, I find Berkeley Tropics and Omega Helio Gazer can be too dominating. You get all the aroma of passion fruit, but without the acidity or sweetness. This tends to completely obscure subtle malt or hop character, as well as other aromatics from the fermentation. The result is most similar to adding “natural” passion fruit flavors to a bland base beer.
Even in a heavily-hopped IPA or DIPA where the thiol flavor doesn’t dominate, I find that these expressive strains tend to muddle the hops. I love “varietal” hop aroma, e.g., a Simcoe® IPA that has big mango-popsicle notes along with resin. High thiol levels tend to make all hazy IPAs taste generically “tropical.” This is great if you just want a bright, fruity beer, but it will make all your beers taste more similar.
The other potential issue with these strains is harvesting and repitching if you are someone who likes to use generations of a strain across numerous beer styles. The strains work fine on subsequent generations, but unlike other strains you may use for hazy IPAs such as London Ale III or the Conan strain, you may not want to harvest them for use in a stout, English bitter, brown ale, etc. Granted, this may be more of an issue for commercial brewers who rely on repitching for the significant cost savings. You might be able to get away with using a less expressive strain like Cosmic Punch as long as you skip the mash hopping, but that isn’t something I’ve experimented with. I’d love to see a packaged thiol-expressing enzyme developed that you could add to a specific batch, and then repitch the yeast without the enzyme. In some cases you’d be just as well off adding extracts to taste rather than the lack of control a fermentation provides.
It’s not universal, but we often find beers brewed with thiol-expressing strains have higher sulfur aromatics (rotten egg, burnt match, etc.). This has been especially true with lager strains, which tend to retain more sulfur. We were forced to bubble carbon dioxide through our one large-scale batch with thiolized lager yeast to scrub the sulfur. The addition of copper some breweries use to reduce sulfur would also work to catalyze the oxidation of the positive thiols.
The Case For Thiols
If you are brewing on a budget, thiol-expressing yeast strains give you more aroma and character without significant added cost. They can save you money by reducing the amount of fruit or hops required to brew a similarly characterful beer. Thiols aren’t just a single flavor compound that you are “dosing” into the beer, they are aromatics present in the wort from the malt and hops that can give a variety of aromatics, some of which still haven’t been identified!
Some of our best results have come when we’ve leaned into the aromatics. For example Tropical Pop, a kettle-soured beer with frozen mango and passion fruit. The base beer without the fruit likely could have passed as a fruit beer, but with a “restrained” dose of real fruit it was terrific. Similarly, using a thiol-expressing strain with an IPA hopped with varieties that already lean toward passion fruit like GalaxyTM or HydraTM can make the beer taste more “varietal.”
In practice we also like Cosmic Punch because it isn’t so expressive that it dominates. Scott Janish (my brewing partner and author of The New IPA) thinks it just makes a “brighter” IPA without some of the doughy notes or vanilla richness we get from our usual house English strain. It makes a fun base for mixed-fermentation barrel-aged beers as well. My favorite was Field Learning, a collab we brewed with Bissel Brothers, which included Hallertau Blanc and Phantasm aged in fresh Sauvignon Blanc barrels and finished with local white wine grapes (see the clone recipe below).
Without a thiol-expressing yeast you are throwing money away on precursor products like Phantasm, which we’ve found to have very little character on their own.
Thiols & Oxidation
Thiols can be highly susceptible to oxidation, making brewing one of these beers a good excuse to tighten-down your post-fermentation dry hopping, transferring, and packaging processes. Again though in practice, even our barrel-aged “thiolized” beers retain a sensory impression of tropical fruit after more than a year of micro-oxygenation.
There is debate over how much “protective” effect thiols have on your beer. Given their ultra-low concentrations in the 10–1,000 parts-per-trillion they are likely not enough to reduce oxidation, which is often measured in the 50–100 parts-per-billion (50,000–100,000 ppt).2
Thiol-expressing yeast can be a valuable arrow in your brewing quiver. Employed in the right recipe they create “wow” flavors and get the most out of your ingredients. However, they don’t enhance every juicy IPA or fruited sour. These strains are just the most visible piece of the genetically modified trend in brewing yeast. Already on the market there are strains modified to create specific esters, lactic acid, haze, or incapable of making off-flavors like diacetyl and phenols. Don’t feel you have to brew with them, but just as technology has always pushed music and art forward, so too brewing. Whether it is new hop varieties created by diligent hop breeders, black “patent” malt created by a new drum roaster. . .
or yeast created in a lab rather than from a chance mutation!
1 Piano, F. “Studies on the occurrence of thiol related aromas in wine.” Graduate School in Molecular Sciences and Plant, Food and Environmental Biotechnology.
2 Giles, D. et al. “The aerobic and peroxide-induced coupling of aqueous thiols—I. Kinetic results and engineering significance.” Chemical Engineering Science, Volume 41, Issue 12, 1986.
Thiolized Clone Recipes
Sapwood Cellars & Bissel Brothers Brewing Co.’s Field Learning clone
(5 gallons/19 L, all-grain)
OG = 1.052* FG = 1.008
IBU = 0 SRM = 4 ABV = 6.3%
* Gravity prior to the addition of wine grapes, ABV is after the addition of wine grapes
This beer uses fresh grapes at the end of the process to layer the flavors onto the Phantasm powder used at the start of fermentation.
6 lbs. (2.7 kg) Weyermann Pilsner malt
1.6 lbs. (0.73 kg) Best Chit malt
1.6 lbs. (0.73 kg) Crisp Naked malted oats
1.6 lbs. (0.73 kg) Rahr white wheat malt
2.5 oz. (71 g) Hallertau Blanc hops (hopstand)
4 oz. (113 g) Phantasm powder (hopstand)
15 lbs. (6.8 kg) Chardonnay grapes (destemmed and crushed) or a bottle of white wine of your choice
1 oz. (28 g) medium-toast oak cubes
Omega Yeast OYL-402 (Cosmic Punch)
Bissell Brothers bottle dregs (or a commercial blend such as Yeast Bay Mélange, Bootleg Biology Sour Solera Blend, Wyeast Lambic etc.)
5 g Lalvin 71B wine yeast
1–1 1⁄4 cups corn sugar (if priming)
Step by step
Mash at 157 °F (69 °C), adding enough calcium chloride to achieve 150 ppm chloride. Add lactic or phosphoric acid if needed to achieve a mash pH of 5.2. Collect wort and boil for 60 minutes. Cool wort to 160 °F (71 °C) and add hops and Phantasm. Allow to steep 20 minutes.
Chill to 68 °F (20 °C), aerate, and pitch Cosmic Punch and desired souring culture. Allow to ferment in primary for two weeks at 70 °F (21 °C). Transfer to secondary with 1 oz. (28 g) of medium-toast oak cubes or into a white wine barrel. Once the gravity stabilizes (6–12 months), transfer onto grapes in a purged bucket or carboy. Pitch fresh rehydrated 71B yeast. Allow to referment for one month. Alternatively, you could blend in a white wine of your choice to taste (a New Zealand Sauvignon Blanc would be a fun thiol-rich option). Transfer to a purged keg or bottle prime in Champagne-style bottles for as much CO2 as you are comfortable with (3–4 volumes).
Partial mash option: Substitute the Pilsner malt with 3.6 lbs. (1.6 kg) Pilsen dried malt extract. Mash the crushed malts in a large grain bag in 2.4 gallons (9 L) water at 157 °F (69 °C) for 60 minutes. Remove the grains and place in a colander. Rinse grains with 1 gallon (3.8 L) of hot water. Top kettle up to 6 gallons (23 L) and add malt extract off heat. Stir until fully dissolved, then bring up to a boil. Follow all-grain instructions, being sure to top fermenter up to 5.25 gallons (20 L) before starting fermentation.
Sapwood Cellars’ Cosmic Rings clone
(5 gallons/19 L, all-grain)
OG = 1.068 FG = 1.022
IBU = 5 SRM = 5 ABV = 6%
We love GalaxyTM and Citra® together, but when we can’t source good GalaxyTM, using a thiol-producing yeast with Citra® can bring a similar balance of tropical, citrus, and
8 lbs. (3.6 kg) Briess 2-row Brewers malt
2.5 lbs. (1.1 kg) Briess pale ale malt
1.5 lbs. (0.68 kg) Great Western white wheat malt
1.5 lbs. (0.68 kg) flaked oats
0.75 lb. (0.34 kg) Best Chit malt
0.5 oz. (14 g) Taiheke® hops (New Zealand Cascade) (mash hop)
2 oz. (57 g) Taiheke® hops (hopstand)
1.25 oz. (35 g) Idaho 7® hops (hopstand)
1.25 oz. (35 g) Columbus hops (hopstand)
10 oz. (283 g) Citra® hops (dry hop)
Omega Yeast OYL-402 (Cosmic Punch)
3⁄4 cup corn sugar (if priming)
Step by step
Mash with the hops at 156 °F (69 °C), adding enough calcium chloride to achieve 150 ppm chloride and enough gypsum to achieve 75 ppm sulfate. Add lactic or phosphoric acid if needed to achieve a mash pH of 5.2. Collect wort and boil for 60 minutes. Cool wort to 180 °F (82 °C) and add hopstand hops. Allow to steep 20 minutes.
Chill to 68 °F (20 °C), aerate, and pitch Cosmic Punch. Allow to ferment in primary for two weeks at 72 °F (21 °C). Once gravity stabilizes, cool to 34 °F (1 °C). Hold for two days, then transfer into a purged vessel with the dry hops at 34 °F (1 °C) for three days, agitating once each day to resuspend the hops. If you are unable to do this, add the hops to primary fermenter as the fermentation slows to reduce oxidation. Transfer to a purged keg or bottle condition in bottles aiming for 2.4–2.5 volumes of CO2.
Partial mash option: Substitute the Brewers and pale ale malts with 6 lbs. (3.3 kg) extra light dried malt extract. Mash the flaked grains and crushed malts in a large grain bag in 2 gallons (8 L) water with the mash hops at 156 °F (69 °C) for 60 minutes. Remove the grains and hops and place in a colander. Rinse grains with 1 gallon (3.8 L) of hot water. Top kettle up to 6 gallons (23 L) and add malt extract off heat. Stir until fully dissolved, then bring to a boil. Follow all-grain instructions, being sure to top fermenter up to 5.25 gallons (20 L) before starting fermentation.