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Passion fruit with a hint of mineral, some smokiness but kind of stinky: 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.

Until 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 about how winemakers can modulate thiol-related characters by means of controlled oxygen exposure.

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 rose 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 fermentation 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 mouth-feel 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)

"Empyreumatic" is another term sometimes used to describe complex wine attributes as smoky, roasted meat or toasty bread crust, which can be present in barrel-fermented wines, but also, for wines that have not been in contact with oak, after a certain period of bottle ageing. Such empyreumatic aromas have been associated with volatile thiols such as 2-methyl-3-furanthiol and furfuryl thio1,7 the latter a key compound in coffee aroma.

Volatile thiols and reduction

In spite of the potentially favorable outcomes of having certain volatile thiols in wine, there is another side of volatile thiols that is not associated with development of pleasant or complex characters.

"Reduction" is a term often used in wine tasting to designate wines having unpleasant odors of cabbage and rotten egg, which have been associated with the presence of certain thiols such [H.sub.2]S and methyl mercaptan (MeSH). The origin of these thiols in wine is complex, and their occurrence and concentration depend on multiple factors.

Yeast fermentation is frequently associated with the occurrence of a rotten egg off-odor due to production of [H.sub.2]S by the yeast. While this reduction can be quite intense, most wines at the end of fermentation exhibit low levels of perceived reduction, and typically low H2S levels.

Anecdotal evidence indicates that, during further processing and storage of wines in the cellar--for example, during tank or barrel maturation with or without lees--reductive characters might reoccur and require specific intervention to be eliminated. Generally speaking, under the condition commonly adopted in the modern wine industry, most wines are 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.

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 [H.sub.2]S formation. (8), (9)

The mechanisms behind this process are 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, winema kers 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 [H.sub.2]S 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 [H.sub.2]S is complex, and it was reported that [H.sub.2]S increases more rapidly during bottle storage, when more copper is present. This can be seen in Figure 2, where wines added with copper at bottling showed much higher [H.sub.2]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.sub.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 [H.sub.2]S 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 quinones 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 [H.sub.2]S 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.sub.2]S increase with relatively minor loss of 3MH. This can be explained by the fact that [H.sub.2]S 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 rose 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 dominant when oxygen exposure is lower. Overall, these observations seem to confirm that reductive and empyreumatic thiols are more responsive to oxygen exposure than fruity thiols, consistent with Figure 3.

From- a practical perspective, it is important to emphasize that the amount of oxygen needed to shift a wine aroma profile from fruity to more reductive or empyreumatic characters is rather small. This can be seen in Figure 5, where approximately 1 mg of oxygen was already sufficient to obtain these two distinct profiles after only six months in the bottle. In order to modulate the different thiol-related characters according to wine style, it is therefore necessary to precisely control oxygen ingress within a rather narrow range.

Figure 6 shows data relative to oxygen ingress of different types of closures, expressed as the sum of oxygen released from the inner cavities of the closure after bottling compression (a process often referred to as outgassing), plus the oxygen entering through the closure due to its intrinsic permeability (commonly referred to as oxygen transmission rate or OTR).

While OTR is commonly taken as the reference parameter for assessing the potential influence of a closure on a wine's post-bottling development, out-gassing should not be neglected, as it can account for a significant portion of the oxygen entering the bottle in the first year (up to 50% or even higher depending on closure type), therefore influencing wine thiol composition even in the short-term.

For example, we have observed that the amount of oxygen derived from desorption alone can account for a 50% loss of 3MH in six months of bottle storage (in 375 mL bottles).

Consistent with other studies, the data in Figure 6 confirms that oxygen ingress is strongly dependent on closure type. However, for certain types of closures it appears not possible to define a specific oxygen ingress value, due to the large variability observed across the single batches analyzed. This is the case for natural cork and 1+1 closures, in agreement with the findings of other studies. (2), (5)

Other types of closures are generally more consistent, although variations in the range of 2-3 mg of oxygen over two years were still observed in the case of agglomerated or micro-agglomerated closures.

Conversely, co-extruded synthetic closures (Nomacorc Select series) displayed the most consistent oxygen ingress values, therefore offering improved performances in the management of volatile thiol evolution during wine bottle storage.

Take-Home messages

* Volatile thiols contribute to both pleas-ant/complex and negative/reductive aroma attributes.

* Removal of reductive thiols by copper is likely to negatively impact fruity thiols. Moreover, copper addition at bottling can enhance [H.sub.2]S formation during bottle ageing.

* Selection of closure with optimal oxygen ingress can allow removal of excess reduction.

* A co-extruded synthetic closure allows precise and consistent oxygen ingress post-bottling, reducing the risk of bottle-to-bottle variation in wine thiol profile.

Conclusions

Volatile thiols are powerful aroma compounds with a primary role in wine aroma composition. Their sensory contribution is rather diverse, ranging from pleasant passion fruit/exotic fruit (3MH and 3MHA) to complex smoky/mineral (furfuryl thiol and benzyl mercaptan), to unpleasant reductive ([H.sub.2]S and MeSH) notes.

Bottle ageing is crucial to formation and degradation of different volatile thiols, but achievement of an optimal balance remains challenging. Although often adopted to remove unwanted reductive aromas, copper addition also removes pleasant aromatic thiols and can have unpredictable effects during bottle ageing, including an increase in H2S accumulation.

Careful selection of closure oxygen ingress can allow winemakers to manage the different aroma nuances associated with varietal thiols, representing a valuable solution for management of wine volatile thiols.

While preservation of volatile thiols generally requires closures with relatively low oxygen ingress, unpleasant reductive thiols seem to be the most responsive to oxygen, increasing the risk of reductive notes with closures allowing too little oxygen.

In addition, a non-negligible variability of oxygen ingress values exists for certain types of closures even within the same production lot. This could lead to significant bottle-to-bottle variation and should therefore be carefully considered for wine styles that are closely dependent on volatile thiols.

Caption: Figure 1: Main volatile thiols in wine and potential sensory contribution from pleasant (green) to unpleasant (red).

Caption: Figure 2: Distribution of [H.sub.2]S concentration after six months of bottle storage for Sauvignon Blanc wines added or not with 'copper before bottling. The two groups were significantly different at 95%.

Caption: Figure 3: Oxygen influence on wine volatile thiols and consequent sensory implications.

Caption: Figure 4: Oxygen influence on two wine volatile thiols during bottle aging of Sauvignon-blanc wine (375 mL bottles).

Caption: Figure 5: Aroma profiles of a rose wine after six months of bottle storage with two closures allowing different oxygen ingress. Asterisks denote attributes with significant differences at *95% and '99%.

Caption: 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.

Bibliography

(1.) Dieval, J-B., S. Vidal, 0. Aagaard. 2011 "Measurement of the oxygen transmission rate of co-extruded wine bottle closures using a luminescence-based technique." Packag. Technol. 24, 375-38.

(2.) Faria, D., A. Fonseca, H. Pereira, 0.M.N.D. Teodoro. 2011 "Permeability of cork to gases." J. Agric. Food Chem. 59, 3590-3597.

(3.) Goode, J., S. Harrop. 2008 "Wine faults and their prevalence: data from the world's largest blind tasting." Proceedings of LesXXes Entretiens Scientifiques Lallemand.

(4.) Nikolantonaki, M., I. Chichuc, P-L Teissedre, P. Darriet. 2010 "Reactivity of volatile thiols with polyphenols in a wine-model medium: Impact of oxygen, iron, and sulfur dioxide." Anal. Chim. Acta 660, 102-109.

(5.) Oliveira, V., P. Lopes, M. Cabral, H. Pereira. 2013 "Kinetics of oxygen ingress into wine bottles closed with natural cork stoppers of different quality." Am. J. Enol. Vitic. doi: 10.5344/ ajev.2013.13009.

(6.) Pasquier, G. 2013 "Mineralite. Avant de la comprendre il faut la definir." Rev. Oenol. 254, 48-49.

(7.) Tominaga, T., G. Guimbertau, D. Dubourdieu. 2003 "Role of certain volatile thiols in the bouquet of aged Champagne wines." J. Agric. Food Chem. 51, 1016-1020.

(8.) Ugliano, M., M. Kwiatkowski, S. Vidal, D. Capone, T. Siebert, J.B. Dieval, 0. Aagaard, E.J. Waters. 2011 "Evolution of 3-mercaptohexa-nol, hydrogen sulfide, and methyl mercaptan during bottle storage of Sauvignon blanc wines. Effect of glutathione, copper, oxygen exposure, and closure-derived oxygen." J. Agric. Food Chem. 59, 2564-2572.

(9.) Ugliano, M., J.D. Dieval, T.E. Siebert, M. Kwiatkowski, 0. Aagaard, S. Vidal, E.J. Waters. 2012 "Oxygen consumption and development of volatile sulfur compounds during bottle aging of two Shiraz wines. Influence of pre- and post-bottling controlled oxygen exposure." J. Agric. Food Chem. 60, 8561-8570.

(10.) Ugliano, M., J.B. Dieval, S. Begrand, S. Vidal. 2013a "Volatile sulfur compounds and 'reduction' odour attributes in wine. An update on why some wines 'stink,' some others have 'complex mineral aromas,' and what winemakers could do about it." Wine Vitic. J. 34-38.

(11.) Ugliano, M. "Oxygen contribution to wine aroma evolution during bottle aging." 2013b J. Agric. Food Chem. 61, 6125-6136.

BY Maurizio Ugliano, Jean-Baptiste Dieval, Stephanie Begrand, Stephane Vidal, Nomacorc France, Domaine de Donadille, Rodilhan, France Author email: m.ugliano@nomacorc.be
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Title Annotation:WINEMAKING
Comment:Passion fruit with a hint of mineral, some smokiness but kind of stinky: reductive characters associated with stinky thiols that develop in the wine post-bottling are considered detrimental to perceived wine quality.
Author:Ugliano, Maurizio; Dieval, Jean-Baptiste; Begrand, Stephanie; Vidal, Stephane
Publication:Wines & Vines
Date:Jan 1, 2014
Words:3212
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