Intro To Thiols: Tropical flavors in beer

Biotransformation has been an exciting discussion in the brewing community over the past decade. Sometimes it feels like a magic black box that makes all these changes and we get a great tasting beer at the end of it! More recently due to researchers, scientists, and curious brewers, we’ve been able to chip away at that black box and shine some light on what’s actually going on during fermentation. The current area of interest is regarding the biotransformation of flavorless, bound thiol precursors to flavor-active, volatile thiols and the production of tropical flavors in beer.

What are thiols?

Thiols are pungent, sulfur-containing compounds that exist in many tropical fruits and are also how we distinguish new school hops or Sauvignon Blanc wine from New Zealand. They don’t smell like rotten eggs or any other smell that comes to mind when you think of sulfur. Instead, they are commonly described to smell and taste like that of grapefruit, passion fruit, and guava. The three compounds (3MH, 4MMP, 3MHA) found in Figure 1 are known to provide these fruit-driven flavors.

The exciting detail about thiols is that they can be perceived at parts per trillion (yes — that’s trillion!). This means that an incredibly small amount available can make a significant impact on overall final beer aroma and flavor. 3MH is the most prominent thiol available in beer, which is why we will focus on this compound specifically.

To get 3MH molecules (and their associated tropical flavors) in beer, brewers have learned that dry hopping using specific hop varieties can amplify these free thiols. Unfortunately, even hop varieties with the most 3MH only contain modest quantities of these desirable thiols in free form, making it challenging to produce beer with strong tropical notes. Fortunately, there exists another way to get 3MH and tropical flavors into beer: Biotransformation of thiol precursors.

Thiol Precursors

Thiols are derived from compounds that are often called thiol precursors. It is now widely accepted that thiol precursors come from hops and barley (they are also commonly found in grape skins and other fruits, but we’ll stick to the more relevant brewing ingredients for now). The most popular 3MH precursors are: Cysteinylated and Glutathionylated thiol precursors, but don’t feel like you need to memorize those — you will most likely see them written as Cys-3MH and Glut-3MH, which feel much easier.

In simple terms, these are basically amino acids (protein building blocks) with thiols bound to them. Now, here is the big kicker . . . these precursors are flavorless and non-aromatic. A bound thiol does not smell like tropical aromas at all. An enzyme is needed to release thiols from the bound, non-aromatic form to a “free,” flavor-active, aromatic form.

Once thiols are in their free form, they smell like delicious tropical fruit . . . obviously depending which specific thiol compound we’re talking about. The biotransformation happening here is incredibly important because malted barley and hops contain way more 3MH precursors than free 3MH. For example, for every microgram (µg) of free 3MH in Cascade hops, there are 35 µg of the Cyst-3MH precursors and 1,574 µg of the Glut-3MH precursors.¹ That’s a lot of aroma potential that is bound up.

Yeast, Enzymes, and Biotransformation

Yeasts produce and release many enzymes during the fermentation process, but the specific enzyme needed to free the bound thiols is a beta lyase (β-lyase). Another name used for this enzyme is carbon-sulfur lyase (C-S lyase), which is just a more descriptive name since that’s what this enzyme does — it breaks the carbon-sulfur bond to release the thiol.

All brewing yeast strains produce this enzyme, but different strains have different enzymatic activities. Some produce more than others, and several yeast producers have tested for this information since it’s been such a hot topic in the industry. However, it has been shown that most existing brewer’s yeast strains tend to be fairly poor thiol biotransformers (compared to certain other yeasts like some wine yeast strains), only converting a tiny fraction of the non-aromatic 3MH precursors into the flavor-active 3MH.²

This is why there have been newly created yeast hybrids and even genetically engineered yeast strains that increase the amount of enzyme produced and/or the activity of the enzyme altogether. Yeast companies will make it apparent to consumers which strains have been developed for these specific purposes.

How to increase free thiols in finished beer

There are roughly three ways that you can increase the free thiols in beer:

1. Use hops with free thiols available
2. Increase the amount of β-lyase enzyme
3. Increase the amount of thiol precursors

However, just note that adding precursors does not automatically lead to more free 3MH and tropical flavor. If you increase the level of precursors, you would also need to increase the amount of β-lyase enzyme to see a significant flavor and aroma difference in your final beer product.

How to increase thiol precursors:

1. Use lighter kilned malts — these are known to have
   more precursors than roasted varieties³
2. Use hop varieties that have higher levels of bound thiols
   (e.g. Cascade, Chinook, Saaz)
3. Add hops to the whirlpool or mash for better extraction
4. Use yeast with a more active β-lyase (see chart below)
5. Use other high thiol precursor products (e.g. Phantasm)

To decrease the degradation of these compounds try to limit the oxygen in packaged beers and limit the potential exposure to metal ions (especially copper).

Biotransformation of thiols is still a relatively new area of research for the brewing industry. It’s exciting to see that many brewers and researchers alike are working hard to shine more light on this black box of fermentation. For now, it is clear that thiols have a major impact on final beer flavor and by learning a little more about thiols and their precursors hopefully you’ll be able to produce beers bursting with flavorful passion fruit and guava notes.

References

¹ Roland, A. et al. (2016). First identification and quantification of glutathionylated and cysteinylated precursors of 3-mercaptohaxan-1-ol and 4-methyl-4-mercaptopentan-2-one in hops (Humulus lupulus). Flavour Fragr. J. doi: 10.1002/ffj.337
² Denby, C. (2020, August). Engineering yeast for more efficient production of hop-flavored beer. Poster presented at World Brewing Congress, Minneapolis, MN.
³ Roland, A. et al (2016, August). First evidence of cysteinylated and glutathionylated precurcurs of 3-mercaptohexan-1-ol in malts. Poster presented at World Brewing Congress, Denver, CO.

Written by Brittney Christianson