What are thiols?
In chemical terms, a thiol is the sulfur version of an alcohol. Alcohols have an -OH functional group, whereas thiols have an -SH functional group in their structure. Many thiols have very strong odours which range from tropical fruit, wine and citrus to garlic, sweat and onions.
Because of their impactful aromas they are often used as odorants; for example, natural gas is dosed with ethanethiol so that leaks can be detected easily at low concentrations. Most brewers are already familiar with at least one thiol, 3-methyl-2-butene-1-thiol (3MBT), which is responsible for the ‘skunky’ off-flavour in light-struck beers.
Polyfunctional thiols refer to thiols that also have other functional groups such as alcohol, ketone or acids. Polyfunctional thiols play an important role in the organoleptic properties of wine and beer by imparting tropical fruit aromas. In the beer industry, “polyfunctional thiols” are often referred to simply as “thiols” for simplicity, and we use these terms interchangeably on this website.
Sources of Thiols
Thiols are an important part of the aroma profile of hops. In hops, thiols can be present as a free (aromatic) thiol or as an odorless precursor that is bound to an amino acid such as cysteine, glutathione, or glycine. Thiol precursors are also derived from malt.
The majority of thiol precursors from hops and malt are bound to glutathione (GSH), with a smaller amount bound to cysteine (Cys).
Want to know which hop varieties are high in either free or bound thiols? Check out this resource.
Thiol Biotransformation
Some brewing yeast strains possess the IRC7 gene, which expresses a cysteine desulfhydrase β-lyase enzyme that can release free aromatic thiols from their amino acid bound precursors. Cysteine-bound thiols are imported into the cell where cysteine is enzymatically cleaved. Cysteine is then used for yeast metabolism and the free volatile thiol is secreted from the cell where it contributes to beer aroma.
Additionally, specific thiols can be modified by yeast metabolism to produce different compounds with unique aromas. For example, 3-sulfanylhexanol (3SH) can be esterified by the yeast to form 3-sulfanylhexanyl acetate (3SHA). The sensory threshold of the latter is nearly 10-fold lower than the former, and can only come about through yeast esterification.
Further complicating the matter, in brewing yeast there exists more complete and less complete versions of the IRC7 gene (‘long’ and ‘short’ versions), differing levels of intact promoter regions (which effectively ‘turn on’ the gene), and differing allelic profiles (how the gene is carried on different chromosomes) – all of these things affect the level of activity of this β-lyase enzyme.
Modification
- Yeast imports precusor (3SH)
- Esterase enzyme forms thiol-acetate ester
- Export of thiol-acetate (3SHA)
Release
- Yeast imports Glu-thiol precursor
- γ-Glutamyl transpeptidase enzyme converts Glu-thiol to Cys-thiol
- Yeast imports Cys-thiol precursor
- β-Lyase enzyme cleaves cysteine from Cys-thiol precursor
- Yeast exports free volatile thiol
This enzyme interacts more efficiently with Cysteine-bound thiol precursors, and not at all with Glutathione-bound precursors, unless a pre-requisite step of conversion from Glu- to Cys- via a second enzyme (γ-glutamyl transpeptidase) has taken place. Since Cys-bound precursors are present in smaller amounts than Glu-bound precursors, this means that the majority of thiol precursors are not released by normal yeast metabolism. This presents an opportunity for brewing scientists to explore better ways to exploit the Glu-bound thiol pool.
This β-lyase enzyme is produced intracellularly, so thiol precursors must be actively imported into the cell before being cleaving the amino acid and releasing the free thiol. The amino acid is then available for the yeast to use. The yeast is more interested in the amino acid than the thiol, and current research shows that higher IRC7 gene expression, β-lyase activity, and thiol release are observed in lower nutrient conditions.
The thiol biotransformation potential of a yeast strain is related to the expression level of β-lyase and other enzymes that interact with thiols. We have characterized Lallemand Brewing yeast strains for their thiol biotransformation potential by measurement of free thiols and thiol precursors in an identical wort fermented with different LalBrew Premium strains. A summary of the most up to date strain characterization for biotransformation potential is available here.
Want to know more about the latest in thiol biotransformation research? Check out our R&D resources page.
Thiol flavor impact and sensory threshold
Thiols in beer are usually described as having a tropical fruit flavor, including passionfruit, grapefruit and guava. Thiols have an incredibly low sensory threshold of nanograms per litre. The reason ethanethiol is added to gas is because we can smell it at levels one hundred million times lower than levels of ethanol. Not surprising then, if too many free thiols are created you can get some more undesirable characteristics, such as vegetal, sweaty, rubbery, and over-ripe fruit in your beer.
| Sensory Attributes | Sensory Threshold | Found elsewhere in nature in | ||
| 3SH (3-sulfanylhexanol) |
 |
Grapefruit, passionfruit | 55 ng/L | Grapefruit, guava, passionfruit, white grapes |
| 3SHA (3-sulfanylhexyl acetate) |
 |
Guava, passionfruit | 4 ng/L | Grapefruit, guava, passionfruit, white grapes |
| 4MSP (4-menthyl-4-sulfanylpentan-2-one) |
 |
Blackcurrant | 2 ng/L | Grapefruit, guava, passionfruit, white grapes |
| 3S4MP (3-sulfanyl-4-menthylpentan-1-ol) |
 |
Rhubarb, grapefruit, stonefruit | 60 ng/L |
How to optimize thiol biotransformation
Thiol biotransformation can be optimized through careful consideration of the hop variety, yeast strain, and brewing process.
- Choose hop varieties that are rich in bound thiols for addition in the whirlpool or early in fermentation. This allows for bound precursors to be extracted into the wort or beer and for the yeast to produce beta-lyase enzymes to release free aromatic thiols.
- Hops rich in 3SH can also be added to whirlpool or early fermentation to encourage conversion into 3SHA (yeast esterified 3SH) by the yeast during fermentation.
- Select a yeast strain with good thiol biotransformation potential. Specific yeast strains are known to release higher levels of specific bound thiols, or modify thiol compounds (e.g. by esterification). Download the Lallemand strain table here.
- Use some sugars or adjuncts to decrease wort FAN levels and stimulate IRC7 gene expression and beta-lyase activity. Note: nutrient levels must remain sufficient to support a healthy fermentation.
- Allow a maturation period after fermentation is compete for 3-5 days at >4°C with beer in contact with the yeast. This allows thiols released inside the yeast cells can be secreted out into the beer.
The following chart is a guide to help brewers optimize thiol biotransformation during the brewing process in order to achieve specific hop aroma profile. Hop recommendations are not exhaustive, contact your hop supplier for recommendations for other varieties with similar free/bound thiol profiles. Yeast recommendations may be updated based on current R&D and commercial feedback.
