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w i n e M A K IN G MANY FACES OF VOLATILE THIOLS IN WINE SH Passion fruit with a hint of mineral, some smokiness but kind of stinky Nomacorc France, Domaine de Donadille, Rodilhan, France Author email: m.ugliano@nomacorc.be U ntil the early 1990s, volatile sulfur compounds in wine were mainly seen as agents of aroma defects, the only exception being dimethyl sulfide (DMS), which was known to contribute positively to wine aroma when present in low concentrations. As we approached the turn of the last century, however, things changed quite drastically In the late 1990s researchers at the University of Bordeaux highlighted for the first time the primary role of various grape-derived volatile thiols in the typical aroma of Sauvignon Blanc wines, and this work completely changed our understanding of key aroma compounds in wine. Today, it appears that nearly every piece of scientific work investigating wine aroma addresses volatile sulfur compounds, which chemists typically refer to as thiols. At the same time, terms such as "flinty," "mineral," "smoky" and "passion fruit" are becoming a common part of today's wine-tasting jargon. These terms are related to volatile thiols, but certain volatile thiols are also responsible for "reduction," another term long used to designate wines exhibiting unpleasant rotten egg odors. This article discusses the contribution of volatile thiols to the presence of these aromas in wine, with some thoughts a b o u t h o w winemakers can modulate thiol-related characters by means of controlled oxygen exposure. 4 p ractic al w i ne ry & v i ne yard JANUARY 20 14 e.g. 3-Mercaptohexanol Roasted, Smoky e.g. Furfuryl thiol Flinty, "Mineral" SH SH e.g. Benzyl mercaptan SH Cabbage REDUCTIVE e.g. Methyl mercaptan Rotten REDUCTIVE Reductive characters associated with stinky thiols that develop in the wine post-bottling are considered detrimental to perceived wine quality. But preventing their occurrence in a correct way is not straightforward, as we risk to lose more than what we gain. The right dose of oxygen can be the key to fine-tune this complex balance. BY Maurizio Ugliano, Jean-Baptiste Diéval, Stéphanie Bégrand, Stéphane Vidal, OH Fruity e.g. H2S + Methyl mercaptan H2S + SH Figure 1: Main volatile thiols in wine and potential sensory contribution from pleasant (green) to unpleasant (red). Figure 1 shows the different characteristics of the main volatile thiols found in wine. From a chemical perspective, these compounds are all characterized by the presence of the–SH group typical of thiol compounds, which is key to their reactivity in wine, as will be discussed below. Other volatile sulfur compounds can be implicated in wine aroma, for example dimethyl sulfide (DMS). However, DMS is not a thiol and therefore its reactivity in the wine environment follows different mechanisms than thiols. Fruity varietal thiols The compounds 3MH, 3MHA and 4MMP are often referred to as "varietal thiols." This is probably linked to the fact that these thiols were initially identified as key contributors to the aroma of certain grape varieties, in particular Sauvignon Blanc. It was then shown that they contribute to the aroma characteristics of many white and rosé wines, and probably play an important role in some red wines too. Their presence in wine has been associated with pleasant tropical fruit/passion fruit aromas that seem to be highly sought by today's consumers. Precursors of these extremely potent odorants are in the grapes and can be further formed during must preparation. Yeast activity will then transform these precursors into fragrant compounds. Therefore, precursor content in grapes and must, yeast strain selection and fer- mentation management are crucial to the optimization of varietal thiol content in wine. However, as we will see in more detail below, thiols are, in general, relatively unstable in wine, and therefore post-fermentation and post-bottling factors are equally crucial to wine content of varietal thiols. Volatile thiols and wine aroma complexity Today, the definition of specific regional or terroir characters seems to be rather topical in the wine community. For example, searching Google for "wine minerality" reveals that seven of the first 10 links speak about minerality in relation to specific wine regions such as Burgundy and the Loire Valley in France, Mosel in Germany or Priorat in Spain, suggesting that minerality might be seen as characteristic of certain terroirs. An exact definition of minerality does not exist yet, and the debate is still open as to whether this is an odor or a mouthfeel attribute. Nevertheless, a recent survey carried out in Burgundy with 2,000 participants made up of wine professionals and wine enthusiasts indicated that "mineral" wines are mainly characterized by struck flint and chalk odors, implying that minerality is essentially an odor attribute.6 Benzyl mercaptan, a powerful thiol imparting struck flint aromas (odor threshold 0.3 ng/L), is suspected to play a primary role in wine minerality. 6