Adventures with polyfunctional thiols: biogenesis, reactivity, and chirality

Abstract

Polyfunctional thiols are a class of potent odourants that contribute distinctive aromas to various wines.¹ Aside from “varietal thiols”, typically associated with tropical and citrus characters, other polyfunctional thiols can impart roasted coffee, roasted nut, or struck flint characters as a result of different grape varieties and winemaking practices. Given their aroma impact, including in other foods and beverages,² knowledge of the formation and fate of polyfunctional thiols has received significant attention over the past two decades.³ They are reactive compounds present at ultratrace levels and require specialised methods of analysis.⁴ As an additional complexity, certain thiols exist as enantiomers that differ in odour quality and olfactory detection threshold. Stereochemistry is also an important feature of the odourless precursors found in grapes, which are transformed during fermentation under the control of various enzymes.^5,6 The liberated thiols are nucleophilic and subjected to various nonenzymatic transformations in an acidic and redox-active wine matrix, which could further modulate the sensory properties of wine.

Dedicated to understanding wine chemistry, our work has spanned from precursor identification and grape processing effects to fermentation studies, compound synthesis, and analytical method development. Investigations have focused on Sauvignon Blanc clone and region, the extent of precursor conversion, and stereochemical relationships between thiol precursors and volatile products. This includes exploring the reaction of polyfunctional thiols containing a 3-sulfanyl-1-alkanol arrangement of functional groups. Cyclisation mediated by acetaldehyde leads to oxathianes as a new class of chiral volatile sulfur compounds in wine, predominantly found in the cis-configuration. The work extends knowledge of the potential fate of polyfunctional thiols, which could have practical consequences for expression of wine aroma. Aspects of these “adventures” with polyfunctional thiols and precursors over almost 20 years will be presented. This will highlight the progression of a body of work dedicated to understanding the formation and modulation of these chemically interesting and sensorially important compounds in wine.

References

1. Roland, A., Schneider, R., Razungles, A. & Cavelier, F. Varietal thiols in wine: Discovery, analysis and applications. Chem. Rev. 111, 7355-7376 (2011).

2. Waterhouse, A. L., Sacks, G. L. & Jeffery, D. W. Thiols and related sulfur compounds. In Understanding Wine Chemistry (eds A. L. Waterhouse, G. L. Sacks, & D.W. Jeffery), 99-111 (John Wiley and Sons Ltd, Chichester, UK, 2024).

3. Waterhouse, A. L., Sacks, G. L. & Jeffery, D. W. S-Conjugates. In Understanding Wine Chemistry (eds A. L. Waterhouse, G. L. Sacks, & D.W. Jeffery), 293-303 (John Wiley and Sons Ltd, Chichester, UK, 2024).

4. Chen, L., Capone, D. L. & Jeffery, D. W. Analysis of potent odour-active volatile thiols in foods and beverages with a focus on wine. Molecules 24, 2472 (2019).

5. Jeffery, D. W. Spotlight on varietal thiols and precursors in grapes and wines. Aust. J. Chem. 69, 1323-1330 (2016).

6. Bonnaffoux, H., Roland, A., Schneider, R. & Cavelier, F. Spotlight on release mechanisms of volatile thiols in beverages. Food Chem. 339, 127628 (2021).

Acknowledgements

Co-authors and collaborators involved with this work over many years are gratefully acknowledged.