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w i n e M A K IN G 18 4 16 14 Increase due to oxygen difference 3 2 1 0 4 1.4 Oxygen entering through the closure in six months (mg/L) 3MH (ppt) 800 Oxygen (mg) H2S (ppb) 5 12 10 8 6 4 2 0 l t ec Se 600 Loss due to oxygen difference 400 200 0 4 Figure 4: Oxygen influence on two wine volatile thiols during bottle aging of Sauvignon-blanc wine (375 mL bottles). Aroma intensity 4 Red fruits** 2 1 0 Empyreumatic* Minéral Se 0 50 t ec l Se 0 30 t ec l Se 0 10 ra xt r tu Na e al N l1 a ur at de ra g st l ra d 2n e ad gr 1 1 r ic M g oa ed at er om gl tu Na + m lo g Ag ed at er Figure 6: Oxygen ingress value of different closures during two years. For each closure type, measures are taken on 10 closures of a single batch, using a Nomasense oxygen analyzer. 1.4 3 l t ec Nomacorc Oxygen entering through the closure in six months (mg/L) Reduced* 0 70 Exotic fruits** Citrus Figure 5: Aroma profiles of a rosé wine after six months of bottle storage with two closures allowing different oxygen ingress. Asterisks denote attributes with significant differences at *95% and **99%. bottled without any sensorially detectable reductive off-odor. However, it has long been known that, during storage in bottle, wine can develop reductive aroma characters again, which, from a sensory point of view, appear to be more complex, with descriptors ranging from struck flint to cabbage to rotten egg. This second stage of formation of reductive aromas is of particular concern for winemakers, as it occurs in the finished product that is delivered to consumers. 6 p r actic al w i ne ry & v i n e yard JANUARY 20 14 A survey from a major international wine competition indicated that reduction is one of the main wine sensory faults observed in commercial wines.3 Origins of volatile thiols and stability during bottle storage Based on Figure 1, volatile thiols can be implicated in a rather broad array of sensory attributes, ranging from the pleasant passion fruit and grapefruit of certain white wines to the complex smoky/ flinty/meaty of bottle-aged wines, but also to the less pleasant reductive odors of MeSH. Other than having rather different aroma properties, wine volatile thiols also differ for their formation pathways and the winemaking steps that are more crucial to their occurrence. Thiols such as 3MH, 3MHA and 4MMP attain maximum concentrations with fermentation, when the yeast can transform the precursors into the volatile compound. Although precursors are still present after fermentation, they are stable at wine pH, so that the concentration of these thiols will not further increase during bottle ageing, and indeed they will decline. 8 Means of converting this residual pool of precursors into odor-active compounds, for example by addition of a suitable enzyme, is currently an active area of investigation. On the other hand, while often not detectable in very young wines, the complex mineral, smoky and roasted coffee aroma characters associated with benzyl mercaptan and furfuryl thiol tend to develop with bottle ageing.7 Likewise, there is a growing consensus that bottle ageing is also critical for MeSH and H2S formation.8,9 The mechanisms be ind this process are h not fully understood so far, but it appears that wine contains one or more precursors that can generate reductive thiols during bottle storage.9,11 Management of volatile thiols From a practical perspective, winemakers are typically interested in minimizing the decline of the fruity varietal thiols and, depending on wine style, promoting formation of the smoky/mineral ones. Conversely, they are mostly concerned with excessive accumulation of potentially negative compounds such as H2S and MeSH. Unfortunately, as the reactivity of thiols in wine is strongly dependent on the presence of their –SH group, the various thiols often display similar behavior during wine post-fermentation and post-bottling, so that it is not always easy to only favor the "good" ones. The addition of copper sulfate to remove reductive off-odors or prevent their accumulation during winemaking is probably one of the most commonly used approaches to the management of reductive thiols in wine. However, as the action of copper on thiols is mostly non-specific, its addition can result in significant losses of 3MH,8 with negative implications for wine aroma. It can also be assumed that other "good" thiols are removed by copper — not only volatile ones, but also non-volatile ones such as glutathione, a powerful naturally occurring antioxidant. Moreover, the effect of copper on H2S is complex, and it was reported that H2S increases more rapidly during bottle storage, when more copper is present. This can be seen in Figure 2, where wines