Alternative fate of varietal thiols in wine: identification, formation, and enantiomeric distribution of novel 1,3-oxathianes
AIM: This study aimed to explore an alternative fate of varietal thiols by identifying and characterising cis-2-methyl-4-propyl-1,3-oxathiane (cis-2-MPO) and cis-2,4,4,6-tetramethyl-1,3-oxathiane (cis-TTMO) in wine. Elucidating these new pathways could aid in explaining the loss of varietal thiols and would further our understanding of the stereochemical relationships between oxathianes and varietal thiols.
METHODS: GC-MS was used to identify cis-2-MPO,1 and a stable isotope dilution assay (SIDA) was developed to quantify its enantiomers after separation with a chiral β-cyclodextrin GC column.2 Varietal thiols and their enantiomers were analysed by SIDA with HPLC-MS/MS to determine their relationship with cis-2-MPO. Production of cis-2-MPO and its correlation with 3-SH, 3-SHA, and acetaldehyde was studied by profiling the evolution of these volatiles during alcoholic fermentation (AF) of Sauvignon blanc (SB) juice fermented with J7, VIN13, and their co-inoculum.3
RESULTS: cis-2-MPO, derived from 3-SH and acetaldehyde, was identified and then measured at up to 460 ng/L (equivalent to 385 ng/L of 3-SH) in a set of wines. Analysis of (2R,4S)-2-MPO and (2S,4R)-2-MPO, arising from thiol enantiomers (3S)-3-SH and (3R)-3-SH, showed respective concentrations of up to 250 and 303 ng/L. The enantiomeric ratio of (2R,4S)-/(2S,4R)-2-MPO was 43:57 whereas that of (3S)-/(3R)-3-SH in the same wines was 51:49.2 Strong correlations were revealed for both 3-SH and cis-2-MPO and their related enantiomeric pairs.The AF study showed cis-2-MPO was produced from an early stage of AF and reached a peak of 847 ng/L (VIN13 ferment) before gradually declining to 50-65 ng/L. Its evolution profile was identical to that of acetaldehyde and 3-SHA, with moderate to strong correlations found for the analytes.Additionally, cis-TTMO, derived from 4-MSPOH and acetaldehyde, was identified in wine as a single enantiomer at concentrations of up to 28 ng/L (equivalent to 23 ng/L of 4-MSPOH). An aroma detection threshold of 14.9 µg/L was determined for cis-TTMO, and this new volatile was described as ‘citrus’, ‘green’, ‘sweet/caramel’, and ‘mango’, shifting toward ‘onion/sweaty’ and ‘sulfurous’ at higher concentrations.2
The knowledge gained helps rationalise the fate of varietal thiols via the production of oxathianes in wine, and reveals the stereochemical links between these related compounds. A chemical formation pathway to oxathianes was verified and may also apply to other thiols bearing the 1,3-sulfanylalkanol substitution through the reaction with acetaldehyde.
Issue: Macrowine 2021
Department of Wine Science and Waite Research Institute, The University of Adelaide (UA), PMB 1, Glen Osmond, SA 5064, Australia,Liang, CHEN, Université de Bordeaux, Unité de Recherche Œnologie, EA 4577, USC 1366 INRAE, Institut des Sciences de la Vigne et du Vin, 33882, Villenave d’Ornon cedex, France Dimitra L., CAPONE, Department of Wine Science and Waite Research Institute, Australian Research Council Training Centre for Innovative Wine Production, UA, PMB 1, Glen Osmond, SA 5064, Australia Aurélie, ROLAND, SPO, Univ Montpellier, INRAE, Institut Agro, Montpellier, France David W., JEFFERY, Department of Wine Science and Waite Research Institute, Australian Research Council Training Centre for Innovative Wine Production, UA, PMB 1, Glen Osmond, SA 5064, Australia
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3-sulfanylhexan-1-ol, 4-methyl-4-sulfanylpentan-2-ol, acetaldehyde, chiral stationary phase, odour detection threshold, sauvignon blanc, stable isotope dilution assay, gas chromatography–mass spectrometry