Unlock the power of thiols

Lallemand walks us through the complex chemistry of biotransformation

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The interaction between yeast and hops during fermentation, better known as hop biotransformation, is a complex process that is currently of great interest to the brewing community. This biochemical process encompasses many types of reactions, such as:

  • Esterification of hop compounds,
  • Hydrolysis of hop glycosides to release monoterpene alcohols,
  • The release of polyfunctional thiols from a non-aromatic precursors (Source: Best Practices).


These reactions are catalyzed by different enzymes produced by the yeast during fermentation.The β-glucosidase enzymes that release aromatic terpenes are well characterized in brewing yeast, and pure forms of this enzyme (i.e. ABV AROMAZYME) are available to boost this activity during fermentation. The β-lyase enzymes responsible for releasing thiols have been less studied but are gaining interest in the brewing community due to their ability to contribute unique, tropical fruity aroma profiles. 

This article introduces the reader to the basics of thiols and how they can be released using LalBrew® Premium dry yeast strains.

WHAT ARE THIOLS? 

Thiols are a family of sulphur-containing aroma compounds naturally found in hops, either as free aroma-active volatiles or as non-aroma-active (i.e. non-volatile) precursors. They represent only 1% of the total hop oil composition and are categorized as the sulphur fraction. Despite their low concentrations, thiols have very low sensory detection thresholds, meaning they are perceived at exceptionally low concentrations.

The main aroma-active thiols are:

• 4-mercapto-4-methyl-pentan-2-one (4MMP or 4S4MP)

  • Described as black currant
  • Threshold: 6ng/L

• 3-mercaptohexanol (3MH or 3SH)

  • Described as exotic, citrus, grapefruit
  • Threshold: 55ng/L

• 3-mercaptohexyl acetate (3MHA or 3SHA)

  • Described as passion fruit
  • Threshold: 5ng/L

Because bound non-volatile thiols exist as an amino acid moiety, the liberation of thiols through β-lyase activity plays an important role during periods of nitrogen scarcity for yeast. Figure 1 demonstrates the primary liberation mechanism of β-lyase, wherein bound, non-volatile thiols are released through nitrogen-mediated enzyme expression of the IRC7 gene. Lower nitrogen concentrations present in grape must promote IRC7 expression, and, therefore, β-lyase activity tends to be high in wine fermentation. The higher nitrogen levels present in brewers’ wort tend to repress IRC7 gene expression and reduce β-lyase enzyme activity. To promote thiol production in beer fermentations, it is important to choose a yeast strain known to have high β-lyase activity.


Figure 1. β-lyase enzyme mechanism and the IRC7 expression. Not all brewing yeast have this ability. In this example, 3MH (an aromatic thiol) is released from a non-aromatic precursor.

WHICH STRAIN PRODUCES THE MOST β-LYASE ENZYME?

Each brewing strain has different enzymatic activities, suggesting that some are more suitable for releasing bound thiol precursors than others. At the Lallemand Brewing R&D Labs, we have characterized our LalBrew® Premium brewing yeast strains for β-lyase activity (Figure 2). By sharing this technical information, we provide brewers with tools to decide which strain would fit best in their recipes to promote thiol production.

FUTURE TRENDS IN THIOLS? 

Thiols are, despite their low concentrations, a fascinating group of aroma compounds that should be further explored in brewing. The wine industry has led the research on thiols and researchers are just beginning to explore β-lyase enzymes in the brewing industry. Recent research suggests thiol precursors are not only found in hops but also in malt, suggesting great potential for brewing yeast to release aroma compounds in beer from non-volatile precursors.


Figure 2. Relative biotransformation activities of β-lyase enzymes in LalBrew® Premium brewing yeast strains. β-lyase activity was measured by growth on selective media containing a specific sulphur-based precursor.  

In addition, there are currently some solutions in the market based on ingredients containing a high concentration of precursors, such as grape skins, and some brewers are reviving older techniques, such as mash hopping and choosing hop varieties containing a high concentration of bound thiols (e.g. Cascade).

Brewers are always pushing boundaries in terms of new recipes and ingredients, looking for better quality beers and tropical-like flavours to attract customers. A greater understanding of hop biotransformation, thiols and β-lyase enzymes will certainly be a major contributor to brewing trends in the years to come.

Q&A WITH JOAN MONTASELL 

As part of this month’s in-depth look at harnessing thiols, we caught up with Lallemand’s Joan Montasell, to learn more about the current state of knowledge around biotransformation.

Q: It felt like everyone was talking about biotransformation a few years back, as the next big area for creating novel flavours and aromas in brewing. Is that excitement still there?

A: It is still there, though I think now people better understand that it’s an emerging field. The thing is that biotransformation is extremely complex. At this point in our knowledge, it's nearly impossible to say ‘okay, if I use this yeast strain with this hop variety, I can achieve this effect’.

Literally everything plays a role here, from fermentation temperature to the selection of yeast strain. The moment in the brewing process where you add the dry hops, at the very start, in the middle or at the very end, has a huge impact. Even annual variations in the hop harvest; Cascade 2021 and Cascade 2020 will have different compositions in terms of the precursors they contain. Then if you use, let's say, fruit in your brew, those will also contain precursors. There are just so many variables, all interacting.

Q: And do you think there'll be a point in the future where we understand all of those variables well enough to harness biotransformation for specific results?

A: At some point, we should be able to achieve that level of knowledge and prediction, yes. For now, we have research articles trying to explain the different pathways and reactions that can happen during fermentation. But it's all just models, just theory for now, and there’s a lot more work to be done in tracing these reactions throughout the process.

For instance, when you release a particular compound – let's say you use a yeast strain, or just a commercially available enzyme that releases a particular aroma – that aroma is not necessarily just staying there. It also undergoes a series of further reactions that are still unknown, and can be transformed into other aromatic compounds. So, will it come out as linalool, geraniol, citronellol? We just can’t predict that yet. 

Q: I know Lallemand has great connections with the brewing industry. Are you working with breweries on cracking this challenge?

A: Yeah, absolutely. We're actually working with both sides; certainly with breweries, to understand the practical side of the whole thing, but also with research institutions, in particular with Oregon State University. In that way, we're trying to connect both worlds, the super scientific, highly skilled world, and then the more practical and hands-on brewing world. After all, this work is only useful if it can help explain what the brewers actually see. And we’re getting there… we’re getting there.

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