Thiols: An Introduction
A brief rundown of the origins, descriptors, and reasons to care about one of beer’s lesser-understood flavor compounds.
Jun 3, 2022
What are thiols?
Volatile thiols are intense aroma compounds that evoke grapefruit, passion fruit, and guava and are found in a variety of tropical fruits, wine grapes, and hops. These thiol compounds exist in two forms — free forms, which are highly aromatic and volatile, and precursor forms (also referred to as bound thiols). The precursor forms, abundant in malt, are non-aromatic and require yeast with β – lyase biotransformation activity to release them.
When looking for thiol precursors in beer ingredients, hop varieties can vary widely — not only in the amount of thiol compounds, but also the percentage that are in the non-volatile precursor form. While Southern Hemisphere hops appear to be the highest in free thiols, there is still much research and discovery yet to be done. Barley is also a great source of thiol precursors. The thiol we’ll be focusing primarily on today, 3‑sulfanyl-1-hexanol (3SH, but also referred to as 3MH) is abundant in barley but does not reach sensory thresholds because it is locked up in the precursor form.
Effectively, these bound precursors are a stockpile of aroma potential with a yeast capable of biotransforming them to the free volatile and aromatic thiol compounds.
Why should I care about thiols?
One of the most interesting and impactful aspects of thiols in beer is their potency. The compound 3SH has a sensory threshold of 60ng/L, or 0.06 parts per billion (ppb), meaning that just a small amount of volatile 3SH in a beer can really pack a punch. When brewers take advantage of yeast capable of freeing thiols through biotransformation, the amount of thiols in the finished beer can increase 200 times — imagine the intensity at that concentration!
Barley is packed with thiol precursor. Discover regional malt terroir by using locally sourced grains, such as this two-row barley from Sugar Creek Malt Co. in Indiana.
Thiols also present an opportunity to embrace the terroir of our ingredients. Barley is grown all over the world, and with each variable change like location, climate, soil nutrient makeup, and more, the available precursors will vary. This approach to terroir will give brewers the chance to discover the potential of local growers and maltsters and learn how those ingredients can directly impact the finished product.
Another way thiols really shine is by adding desirable tropical notes to beer without the need for heavy-handed hopping rates. As contracts for highly prized hop varieties become more and more competitive, brewers can make use of thiols to build complex, fruit-forward flavor profiles. Harness the power of yeast and biotransformation to generate desirable aromatics, rather than exhausting the hop supply, or the planet.
How to identify thiols
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.
Common thiol sensory thresholds
| Polyfunctional Thiol | Sensory | Threshold (ng/L) |
|---|---|---|
| 4MSP (4MMP) | box tree, black currant | 1.5 |
| 3SHA (3MHA) | passion fruit | 4 |
| 3S4MPol | grapefruit, rhubarb | 40 |
| 3SH (3MH) | grapefruit, passion fruit | 60 |
| 3S4MPA | grapefruit, rhubarb | 120 |
One exercise we propose for training yourself on thiols is to assemble a tasting of other things that are rich in thiol-like flavors (and actual thiols). Ideally you’ll be able to get your hands on the actual fruit, but juices and sodas are a fine stand-in. We recently conducted this tasting with guava juice, passion fruit soda, grapefruit juice, and New Zealand sauvignon blanc — often prized for its passion fruit and black currant-like aromas.
A thiol-themed tasting. From left to right: Thiolized® lager, grapefruit juice, guava juice, passion fruit juice, pink guava juice, sauvignon blanc.
Tasting each of these flavors on their own is a great way to familiarize yourself with what to expect from thiols, but in practice it’s unlikely you’ll experience a single thiol at a high enough concentration to be able to isolate it. At the end of our training, we began blending the juices together in one glass. Try different combinations of flavors together, like a bit of guava mixed into a passion fruit soda or a lot of grapefruit juice blended into the guava. Beer with high aromatic thiol content will have other competing flavors, so you could even try blending the juice into a bit of a neutral lager or blonde ale to see how they might meld with the malt and hops used in the brew.
There’s much more to come on thiols — stay tuned.
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