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The volatile thiol compound 3-sulfanylhexan-1-ol (3SH) is a key impact odorant of white wines such as Sauvignon Blanc. 3SH is produced during fermentation by metabolism of non-volatile precursors such as 3-S-gluthathionylhexanal (glut-3SH-al). The biogenesis of 3SH is not fully understood, and the role of glut-3SH-al in this pathway is yet to be elucidated. The aldehyde functional group of glut-3SH-al is known to make this compound more reactive than other precursors to 3SH, and we are reporting for the first time that glut-3SH-al can exist in both keto and enol forms in aqueous solutions. At wine typical pH (~3.5), glut-3SH-al exists predominantly as the enol form. The dominance of the enol form over the keto form has implications in terms of potential consumption/conversion of glut-3SH-al by previously unidentified pathways. Therefore, this work will aid in the further elucidation of the role of glut-3SH-al towards 3SH formation in wine, with significant implications for the study and analysis of analogous compounds.

Volatile polyfunctional thiol compounds, particularly 3-sulfanylhexan-1-ol (3SH) and 3-sulfanylhexyl acetate (3SHA), are key odorants contributing to the aroma profile of many wine styles, generally imparting tropical grapefruit and passionfruit aromas. 3SH and 3SHA are present in negligible concentrations in the grape berry, juice, and must, suggesting that they are released from non-volatile precursors present in the grape. The exploration of the nature and biogenesis of these precursors to 3SH and 3SHA has proven important for the elucidation of polyfunctional thiol biogenesis during alcoholic fermentation. The development and validation of appropriate analytical techniques for the analysis of 3SH precursors in enological matrices have been extensive, and this review explores the analysis and discovery of these precursor compounds. The development of analytical methods to analyze 3SH precursors, from the selection of the analytical instrument, sample preparation, and methods for standardization, will first be discussed, before highlighting how these techniques have been used in the elucidation of the biogenesis of 3SH and 3SHA in grape wines. Lastly, the future of thiol precursor analysis will be considered, with the development of new methods that greatly reduce the sample preparation time and enable multiple precursors, and the thiols themselves, to be quantitated using a single method.

3S-Gluthathionylhexanal (glut3SHal) is an early precursor to the important wine aroma compound 3-sulfanylhexan-1-ol (3SH), imparting tropical passion fruit aromas, even at trace concentrations. In wine, glut3SHal occurs in equilibrium with its bisulfite adduct (glut3SH–SO3), challenging its quantification. To circumvent the issues encountered when attempting to describe the equilibrium between these compounds, a method for their quantification in wine samples was developed. Separation of glut3SHal and glut3SH–SO3 using solid-phase extraction followed by oxime derivatization and analysis via liquid chromatography–mass spectrometry allowed for measurement of both compounds in wine samples. Analysis of commercial Sauvignon Blanc wines using the developed method confirmed that glut3SH–SO3 is the major species in the wine matrix. The method developed in this work will enable further exploration of the relationship between glut3SHal and glut3SH–SO3 and their contribution to production of 3SH in wines. There is potential to extrapolate this work to explore other aldehyde–sulfonic acid equilibria in foods and beverages.

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