Advances in vinification processes, novel compounds and novel reactions | B. Sarmadi, University of Auckland, New Zealand | X. Wang, The University of Adelaide, Australia | S. Dekker, Edmund Mach Foundation, Italy

During the ROUND TABLE #3 – Advances in vinification processes, novel compounds and novel reactions, held at Macrowine 2021 we had three interesting presentations from across the world. 

Bahareh SARMADI – University of Auckland (New Zealand) – Measuring elemental sulfur in grape juice in relation to varietal thiol formation in Sauvignon Blanc wines

Sauvignon Blanc displays a range of styles that can include prominent tropical and passionfruit aromas. Both sensory evaluation and chemical analysis have confirmed the above-average presence of ‘varietal thiols’ in the Sauvignon Blanc wines from Marlborough, New Zealand. The varietal thiols are released from cleavage of non-volatile sulfur-containing precursors or an interaction between a sulfur donor and a C6-compound. Machine-harvesting is the most common harvesting practice used in New Zealand, by which, there is a higher probability to add some leaves to the must. Leaves and grapes can contain elemental sulfur (S⁰), which is commonly sprayed in the fields to protect berries against powdery mildew. S⁰ is known to cause unwanted reductive aromas, including H₂S, in certain wines unless remediation steps are undertaken during winemaking. Also, it was shown that extra S⁰ addition to the crushed grapes could lead to more varietal thiol formation in wines. Despite the clear effects of residual S⁰ present in the must on the final wine quality and aroma, its measurement is not a regular practice undertaken in wineries due to the lack of easy and applicable methods.
An optimized sulfide sensor for S⁰ measurement in grape juice samples and correlation between S⁰ concentration in grape juice and varietal thiols concentration in final wines were studied. A simple apparatus was designed to reduce S⁰ to sulfide. Did the data show very good linearity? Was the reduction process successful?
Find out if the analysis is applicable in a winery setting, to evaluate the potential of grape juices to form varietal thiols and/or reductive compounds in wines.

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Xingchen WANG – The University of Adelaide (Australia) – Alternative fate of varietal thiols in wine: identification, formation, and enantiomeric distribution of novel 1,3-oxathianes

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.
Find out if the knowledge gained helps rationalise the fate of varietal thiols via the production of oxathianes in wine, and if it reveals the stereochemical links between these related compounds. Was a chemical formation pathway to oxathianes verified? Could it be applied to other thiols bearing the 1,3-sulfanylalkanol substitution through the reaction with acetaldehyde?

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Susanne DEKKER – Edmund Mach Foundation (Italy) – Chemical and biochemical formation of polysulfides in synthetic and real wines using UHPLC-HRMS

Sulfur compounds in wine have been studied for several years due to their impact on wine flavour, but the role of polysulfides is a recent topic. Polysulfides in wine are formed when two sulfhydryl groups oxidize, especially in presence of elemental sulfur or metal catalysts from field treatment residues. These compounds are odourless, but can degrade during storage and affect the wine quality. The mechanism of their formation is still largely unknown but different chemical and biochemical pathways have been suggested. Disulfides from cysteine (Cys) and glutathione (GSH) have been revealed in model wines and more recently also higher polymerized forms in real wines. Volatile varietal thiols like 3-mercaptohexanol (3MH) and 4-mercaptopentanone (4MMP) – flavour compounds with tropical or fruity notes – could undergo similar reactions, also with Cys and GSH, subsequently losing their flavour property (fate). Even more concerning is the possible release of H2S from polysulfides during storage, leading to undesired off-flavours. In the present work polysulfides from varietal thiols 4MMP and 3MH were identified for the first time in synthetic and real wines. Additionally, the evolution of glutathionyl and cysteinyl polysulfides was followed during fermentation.
Find out if the UHPLC/HRMS method used in both studies was successfully applied to detect polysulfides in different spiked synthetic and real wines. Were there differences between treatments and matrices? Did they prove the influence of known and unknown compounds playing an important role in polysulfide formation?

Videos of the round table held during Macrowine virtual (June 23-30, 2021)