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w i n e M A K IN G added with copper at bottling showed much higher H2 S concentrations after six months of bottle storage compared to the same wines that were not treated with copper (the copper addition resulted in an overall increase of copper concentration of only 0.3 mg/L). The mechanisms responsible for this unexpected behavior are not known, but might be linked to the catalytic action of copper on the breakdown of hitherto unidentified H 2 S precursors.8 To further complicate this scenario, a radical suppression of all reductive thiols might have unpredictable and, in some cases, negative consequences to wine aroma development after a few months of bottle storage. Indeed, it is possible (although not conclusively demonstrated) that a "bad" thiol such as H2S might be needed to form potentially "good" thiols such as benzyl mercaptan or furfuryl thiol, meaning that having the bad guys around for a while might eventually have some good outcomes. Controlled oxygen exposure in the bottle has been shown to be one very effective tool to influence different thiols during bottle storage.8,9,10,11 Thiols are very reactive toward the qui- nones formed upon oxidation of certain wine phenolics, resulting in thiol depletion when wine is exposed to oxygen (Figure 3). In the case of "varietal thiols," which are formed during fermentation and not replenished from precursors during bottle storage, this means an actual net loss of the aroma compounds, potentially with consequences for the wine's fruity aroma. However, in wines with a tendency to form reductive thiols due to their intrinsic characteristics (presence of suitable precursors deriving from yeast metabolism), oxygen exposure will act to soften this potentially negative character. Luckily, recent data have shown that H2S seems to be more responsive than 3MH to increased oxygen exposure in the bottle through closures (Figure 4), suggesting that, within certain limits and depending on wine type, oxygen management in the bottle can be used to prevent excessive H 2 S increase with relatively minor loss of 3MH. This can be explained by the fact that H2S has greater reactivity toward quinones than 3MH. Likewise, furfuryl thiol also has higher reactivity than 3MH toward quinone.4 With regard to what we have called smoky/mineral thiols, little is known of the factors influencing compounds such as benzyl mercaptan and furfuryl thiol during bottle ageing and on the complex smoky/empyreumatic characters that seem to be linked to these compounds. In a study involving an independent panel of wine-tasting experts, we recently observed that oxygen permeability of the closure can drastically influence the balance of empyreumatic, reductive and fruity aroma attributes of wine after a period of bottle ageing (Figure 5). In this specific study, the rosé wine showed significantly more intense fruity attributes when exposed to more oxygen, while decreasing oxygen exposure resulted in higher reductive and empyreumatic notes. These observations demonstrate the complex contribution of volatile thiols to wine aroma. Although a larger degree of oxygen exposure in the bottle is expected to decrease the concentration of fruity varietal thiols, overall the wine expresses more intense fruity attributes, as these are not masked by the presence of excessive reductive and empyreumatic thiols. Conversely, the latter become more dom- pr actica l win ery & vin eya r d JANUARY 20 14 7