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Tailoring oxygen management strategies to winemaking styles. How much oxygen do we need?

Maurizio Ugliano, Jean-Baptiste Dieval, Stéphane Vidal
Nomacorc Oxygen Management Research Center
Domaine de Donadille, Rhodilan, France
m.ugliano@nomacorc.be

Many key sensory attributes of wines – including color, aroma, and mouthfeel – are affected by the degree of exposure of wine to oxygen. In the modern wine industry, it is largely accepted that inaccurate management of oxygen during winemaking can result in significant loss of quality. On one hand, too much oxygen is associated with the development of unwanted oxidised characters. On the other hand, too little oxygen can be responsible for so called “reductive” faults, characterized by poor expression of pleasant fruity aromas and, in the most obvious cases, aromas of rotten egg, sewage, or struck flint (Ugliano et al 2009). In addition, the complex array of chemical reactions that contribute to softening tannin harshness and stabilizing colour during wine ageing are also closely connected to the oxidative processes that can potentially take place either in the cellar or in the bottle. Although these concepts have been long established in the wine industry, on a practical level it remains difficult to effectively assess the oxygen demand of a wine. In other words, in the vast space defined by too much to too little oxygen, the degree of oxygen exposure that will provide optimal sensory expression of a given wine is still hard to define. At a general level, it is accepted that wines obtained from certain grape varieties are particularly sensitive to oxygen, reflecting the fact that some of the chemical components key to their sensory attributes are strongly modulated by oxygen. Sauvignon blanc is a well documented example of an oxygen sensitive wine. In addition, anecdotal evidence – in some cases supported by scientific literature – suggests that moderate oxygen exposure is crucial to the development of certain key aroma attributes, as in the case of Amarone wines (Fedrizzi et al. 2011). Nevertheless, each type of wine, grape composition, vintage variation, and winemaking practice has an enormous impact on the amount of oxygen that a wine can consume as well as on the sensory consequences of such consumption.

This article discusses some observations emerging from trials carried out as part of the Nomacorc Post-Bottling Chemistry research program, aimed at investigating the key aspects of successful wine oxygen management strategies.

The term “oxygen management” refers to one or multiple operations in which a well controlled amount of oxygen is delivered to the wine in order to achieve, within the expected shelf life of a wine, optimal expression of desirable sensory attributes. In this sense, accidental events leading to unwanted exposure of wine to oxygen are the worst enemy of good oxygen management strategies as they introduce an uncontrolled degree of oxidation, which can compromise the outcomes of further deliberate oxygen management strategies. With this in mind, during the life of a wine it is possible to identify several key stages of oxygen exposures, which in theory should all be the object of careful oxygen management considerations. These are summarized in Figure 1.

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Figura 1: Theoretical representation of some key phases of oxygen expsoure during wine life. OTR= Oxygen transmission rate.

Leaving aside the implications of oxygen exposure of the must, which would require a separate discussion, both the micro-oxygenation (MOX) phase commonly taking place as part of the wine maturation process, and the nano-oxygenation phase occurring during wine permanence in the bottle, are valuable starting points for a discussion on oxygen management strategies. The implications of different regimes of oxygen exposures at these stages, and the interaction of these with other winemaking steps will be discussed here with the aim of providing some clues on how to unravel the concept of wine oxygen demand.

Micro-oxygenation

Micro-oxygenation (MOX) is a form of controlled addition of oxygen to wine, aimed at simulating the slow, gentle oxidation process that takes place in wine barrels. In its most common application, MOX is carried out by bubbling oxygen in the wine at a controlled rate. Wine polyphenolics are greatly affected by MOX, which is the main reason for the frequent use of MOX in wines where a change to the mouthfeel properties is desired. In a recent study carried out within a collaborative research project between Nomacorc and INRA Montpellier, the effects of MOX on the chemical and sensory profiles of Grenache red wines were investigated (Caillé et al. 2010, Wirth et al. 2010). MOX, applied for three weeks at a rate of 5 mg of O2/L/month, resulted in significant differences on colour parameters (Figure 2A) with MOX wines being characterized by higher b* values in CIELab analysis, suggesting more orange tones. Vitisin A, an orange coloured pigment, was found at a higher concentration in MOX wines (Figure 2B), indicating that this compound could be a useful marker of oxidation. Also, MOX wines had a lower content of flavan-3-ol-anthocyanins adducts (Figure 2B). As formation of these compounds should not be affected by oxygen exposure, their lower concentration in MOX wines suggests that the adducts themselves, once formed, could be susceptible to oxidation.

Sensory attributes of the wines were also studied. Results showed that MOX had an influence on amyl, burnt, and red fruit aroma characters. While the intensity of the amyl and burnt attributes was decreased, an increase in red fruit aromas was observed (Figure 3). The increase in red fruit aromas is of great interest, as this attribute is often considered a primary driver of consumer preference. The chemical mechanisms driving the aroma changes associated with MOX remain unclear, although there is some suggestion that MOX can result in a decrease of mercaptans responsible for vegetal and reductive aromas (Nguyen et al. 2010). Conversely, well known drivers of fruity aromas such as esters or 3-mercaptohexanol have been found to not be affected by MOX (Nguyen et al. 2010).

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Figura 2: Effects of MOX (micro-oxygenation) on A) colour properties (CIELab) and B) selected phenolic compounds of Grenache red wines. MOX was applied at a rate of 5 mg O2/L/month during 3 weeks. For a full description of this study, see Ref. 1.

 

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Figura 3: Effects of MOX (micro-oxygenation) on selected sensory attributes of Grenache red wines. MOX was applied at a rate of 5 mg O2/L/month during 3 weeks. All differences were statistically significant at p<0.01. For a full description of this study, see Ref. 1.

Closure OTR

In the food packaging industry, oxygen transfer rate (OTR) is a parameter of primary importance as it defines the oxygen barrier properties of the packaging material. Therefore, OTR is directly linked to the ability of a packaging material to protect from oxidative damage and, ultimately, to product shelf-life. Because for many decades natural cork was the closure of choice for the wine industry, closure OTR was never considered a technologically relevant variable in enology. The permeability of oxygen to cork is in fact intrinsically variable (Faria et al. 2011), which prevents any possibility of a consistent OTR – even among closures from a single batch. While these limitations were largely accepted in the days when natural cork was the only choice, the arrival on the market of alternative closures opened the field for closure OTR to become a topic of significant interest to the wine industry. Besides being free of cork taint, alternative closures such as synthetic stoppers and screw caps can be designed to have specific OTR values, allowing winemakers to address the issue of wine oxygen demand by means of selection of closures with optimal OTR.

The early work carried out at the AWRI showed that OTR can have a dramatic impact on the evolution of wine during bottle storage (Godden et al. 2001). This observation was later confirmed by several closures studies (Skouroumounis et al. 2005, Lopes et al. 2009). All together, these studies supported the view that closure OTR is crucial to wine development and, ultimately, to wine quality. However, when it comes to decision-making, selecting the closure that is able to deliver the most adequate amount of oxygen remains challenging. Wine oxygen demand in itself is difficult to define, partly because oxygen acts at several different levels, and what is a priority in a certain style of wine might not be of primary importance in another style. For example, in Figure 4, the evolution of colour intensity in Grenache wines is shown. Grenache wines are generally characterised by soft palate and intense berry and spicy aromas, which made them commonly used for both monovarietal wines and in blends. However, they often display premature loss of colour during ageing. With higher OTR, colour became more intense while it faded with lower OTR. This difference clearly increased between five and 10 months. It can be concluded that management of oxygen exposure in the bottle, by means of the use of closures with different OTRs, has the potential to affect colour development during bottle ageing. This option could greatly benefit wines with a tendency to lose colour during ageing.

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Figura 4: Effect of closure OTR on the evolution of colour intensity during bottle storage of Grenache red wines. OTR ranged from Very low (equivalent to screw cap with (Sarantin wad) to High (Nomacorc Light). Wines were bottled in 375 mL bottles. For a full description of this study, see Ref. 1.

OTR also has a strong influence on wine aroma development during bottle storage. In the short to medium term, wines under low OTR closures, such as screw caps, generally retain more intense fresh fruity attributes. However, with longer storage periods, a negative “reductive” character appeared which was more intense in screw cap wines (Godden et al. 2001). Among the aroma compounds involving fruity/tropical fruit odours, a key role is played by the volatile sulfur compound 3-mercaptohexanol (3MH). Because 3MH is degraded during wine ageing, there is a considerable interest for practices that allow improving the stability of this compound during bottle storage. In particular, it has been shown that high contents of the natural wine antioxidant glutathione (GSH) can significantly decrease the loss of 3MH during wine storage and, therefore, preserve positive fruity aromas. This aspect was investigated in a recent study carried out within a collaborative research program between Nomacorc and the AWRI (for a comprehensive discussion of the findings of this study, refer to Ugliano et al. 2011). Figure 5 shows the effect of GSH on the concentration of 3MH after six months of bottle storage with an oxygen regime reproducing the permeability of a screw cap with Sarantin wad. Wines that were bottled with higher levels of GSH (20 mg/L) showed losses of 3MH of 19% compared to 55% without GSH, resulting in wines with much higher 3MH after the ageing period. Although GSH cannot be added directly to wine, high YAN values, selection of specific yeast strain, ageing on lees, and careful protection of must and wine against oxidation can all increase wine GSH content at bottling (Dubourdieu and Lavigne, 2004). Additionally, certain yeast nutrient preparations are also enriched with GSH.

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Figura 5: Influence of glutathione (GSH) on the concentration of 3-mercaptohexanol (3MH) following 6 months of bottle storage. Stars indicate % loss compared to bottling. For a full description of this study, see Ref. 11.

On the other hand, excessive levels of the reductive aroma compound H2S, which can accumulate in the bottle during ageing, can result in perceived rotten egg aromas and poor expression of 3MH-related fruity attributes (Lopes et al. 2009). Another sulfur compound, namely methyl mercaptan (MeSH), has also been associated with reductive off-odours in white wines (O’Brien et al. 20029). Figure 6 shows the levels of H2S and MeSH in Sauvignon blanc wines from the AWRI study. Clearly, wines that are bottled with higher GSH content are more prone to accumulate higher levels of reductive aroma compounds in the bottle. In particular, H2S accumulated to a final concentration of 4.5 ug/L, which is much higher than the odour threshold of 1.6 ug/L reported for this compound in white wine. Therefore, although higher GHS at bottling can prevent premature loss of fruity aromas, the risk exists for wines bottled with high levels of GSH to develop reductive aromas that can mask the expression of varietal fruit characters. This might become even more problematic in wines with more neutral aroma profiles than Sauvignon blanc (e.g. Semillon, Pinot grigio, Chardonnay) where varietal fruity aromas are less dominant and reduction can be perceived more prominently. Under these circumstances, selection of an appropriate OTR offers an additional tool to tune wine aroma development in the bottle, with the possibility of achieving a balance of fruity versus reductive aroma compounds tailored to the need of each wine. This is illustrated in Figure 7. Wines with a lower content of GSH developed small amounts of H2S and MeSH, indicating a lower propensity to develop reduction even when sealed with a closure allowing minimal oxygen exposure. At the same time, due to the lower GSH levels, these wines are more exposed to the risk of premature loss of fruity aromas. In this case, a low OTR closure can be chosen in order to compensate for the risk of premature loss of varietal fruity aromas. Conversely, in wines with higher GSH content, while varietal fruity aromas are better preserved, the risk of reductive characters is increased. In such instances, a closure with a relatively higher OTR could be chosen in order to decrease accumulation of reductive aroma compounds as shown in Figure 7.

Figura_1

Figura 6: Influence of glutathione (GSH) on the concentration of H2S and methyl mercaptan (MeSH) following 6 months of bottle storage. Wines at bottling had 0.3 mg/L of H2S and 0.5 mg/L of MeSH. For a full description of this study, see Ref. 11.

 

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Figura 7: Effect of glutathione (GSH) and OTR on the concentration of H2S and methyl-mercaptan (MeSH) in Sauvignon blanc wines following 6 months of bottle storage. For a full description of this study, see Ref. 11.

In conclusion, while we are still learning about the main factors that determine wine reactivity towards oxygen, management of oxygen during wine maturation and bottle ageing has the potential to develop specific compositional profiles that will potentially result in improved sensory quality. Micro-oxygenation (MOX) should be considered to affect mouthfeel properties (e.g. decrease bitterness), but also to improve expression of certain aroma attributes, for example red fruit aromas. While traditionally carried out by means of devices allowing controlled addition of gaseous oxygen, winemakers interested in MOX should also consider alternative approaches such as the use of maturation tanks made of polymeric materials with a specific OTR.

Producers of synthetic and screw cap closures offer different OTR levels, allowing winemakers to select the desired level of oxygen permeability for their wines. While current technology in screw cap manufacturing only allows two OTR levels (namely the ones obtained with either a Saran tin or a Saranex wad), synthetic co-extruded closures (e.g. Nomacorc) offer a range of OTR values to accommodate the needs of different wine styles and market turnaround times. The identification of key markers to assess wine oxygen demand will greatly improve a winemakers’ ability to properly outline oxygen management strategies, but this has not been achieved to date. Nevertheless, our understanding of the influence of certain common winemaking practices on wine oxygen demand makes it possible to indicate some critical situations (e.g. wines bottled with high glutathione content) where selection of closures with the right OTR can contribute to optimal expression of wine aroma and flavour. In addition, winemakers can resort to Nomacorc’s software-based tool to select the most appropriate closure OTR based on wine production protocols.

Bibliografía

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Skouroumounis, G.K.; Kwiatkowski, M.J.; Francis, I.L.; Oakey, H.; Capone, D.L.; Duncan, B.; Sefton, M.A.; Waters, E.J. (2005) The impact of closure type and storage conditions on the composition, colour and flavour properties of a Riesling and a wooded Chardonnay wine during five years’ storage. Aust. J. Grape Wine Res 2005; 11: 369-384.

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Ugliano, M., Kwiatkowski, M., Vidal, S., Capone, D., Siebert, T. Waters, E.J. Evolution of 3-mercaptohexanol, 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. 2011; 59 : 2564–2572.


Wirth, J., Morel-Salmi, C., Souquet, J.M., Dieval, J.B., Aagaard, O., Vidal, S., Fulcrand, H., Cheynier, V. The impact of oxygen exposure before and after bottling on the polyphenolic composition of red wines. Food Chem. 2010; 123: 107-116.


[31.08.12]