For more than 10 years, many scientific studies have shown that closure oxygen ingress can modulate the aromatic evolution of wines during their preservation (Laurie et al., 2012 ; Wirth et al., 2012 ; Ugliano, 2013 ; Prieto et al., 2014 ; Ugliano et al., 2014 ; Ugliano et al., 2015). Better known as ‘closure permeability’, the overall contribution of a closure to oxygen ingress is the sum of two components: the desorption, which is the oxygen from the closure that is released after bottling, and the OTR (Oxygen Transmission Rate), which is the amount of oxygen that permeates through the closure over time. Closure oxygen ingress has been modelled (Diéval et al., 2011). The perfect definition and homogeneity of oxygen ingress through the closure is mandatory to properly manage wine evolution and choose a closure to obtain an organoleptic profile after a certain shelf-life. This article focuses on results presented in 2014 at the American Chemical Society (Ugliano et al., 2015) showing the aromatic impact of low O2 intakes through closures and proposing hypotheses to explain them. Concrete examples of wine developments under different oxygen ingress will be taken for illustration.
Low differences of oxygen exposure levels modulate the fruity character of wines
35 unwooded wines (10 whites, 12 rosés and 13 reds) were bottled under controlled conditions with Select Green 100, 300 and 500 (Nomacorc closures) allowing during bottle storage three distinct and perfectly defined oxygen ingress levels. These wines were tasted by the same expert and regularly trained sensory panelists, at different stages of bottle ageing (9 to 19 months for whites, 5 to 11 months for rosés, and 12 to 48 months for reds). For the same given wine, the differences in oxygen exposure levels during their preservation created by the different closures were small, in the range of 0.7 to 1.6 mg/L for white wines, 0.5 to 1.6 mg/L for rosé wines, and 0. 8 to 1.9 mg/L for red wines. A total of 126 wines were subjected to sensory analysis and aroma analyses were also carried out.
Figure 1 shows for each aromatic descriptor the frequency to which it contributed significantly to differentiate different levels of O2 intakes. It highlights the aromatic descriptors that are the most impacted on these wines (dark bars) by different closure oxygen ingress. In the case of white wines, aroma intensity, fruity attributes (white fruits and citrus fruits) and reduction were the most cited aromatic descriptors to describe the differences generated by the level of oxygen exposure on the same wine. In the case of rosé wines, oxygen mainly influenced the aromatic intensity and notes of red and exotic fruits. In this study, rosé wines from Provence were mainly used, which explains the high incidence of the cited attributes, which are typical of the sensory profile of these wines. In the case of red wines, the aromatic intensity, red fruits, cooked fruits, spices, and reductive notes were mostly impacted by the different oxygen ingress through the closure. From a global point of view, these results confirm that the fruity attributes and those related to the reductive character of the wines are highly sensitive to oxygen.
Different sensory evolutions depending on the type of aromas
However, all descriptors do not always evolve the same way with the degree of oxygen exposure. For example, fruity attributes may decrease or increase with O2.
Figure 2 illustrates the complexity of the relationship between oxygen exposure and the fruity attributes of wines. Indeed, while in wine 1, a higher degree of oxygen exposure promotes the expression of exotic fruits, in wine 2, the same degree of exposure to oxygen leads to a loss of exotic character. These differences in the response of wines for the same degree of oxygen exposure are common and show that the evolution of aromas is wine dependent. Wine 1 tends to reduce (RedOx 1 on the modality with the lowest closure oxygen ingress): the contribution of a larger amount of O2 decreases the reduction notes, which generally tend to mask the fruit. On the contrary, wine 2 does not show reduction notes, including under the lowest closure oxygen ingress, while the higher oxygen exposure tends to decrease fruit descriptors.
The impact of increased oxygen exposures on a wine will depend on:
- aromatic molecules present in wine,
- reactivity of these molecules with O2 or with molecules derived from oxidation mechanisms,
- masks or synergies effects between molecules.
Scientific literature provides hypothesis on the potential origin of some of these developments. The fruity aromas of varietal thiols like 3SH (grapefruit) decrease when exposure to oxygen increases, due to their reactivity with polyphenol oxidation products (Ugliano et al., 2015). Their decrease may therefore explain the decline in fruity attributes when observed. Similarly, to varietal thiols, concentration of H2S (rotten egg) and MeSH (cabbage), the main markers of reduction-off flavors in wines, decrease when exposure to oxygen increases. These molecules are known to mask the fruity attributes. A higher level of oxygen exposure decreases their concentration and allows a higher expression of the fruit, as in the example of wine 1 (Figure 2). An interesting point to note is that oxygen exposure more easily impacts reduction compounds such as H2S and MeSH than varietal thiols at the degrees allowed by the closures (Ugliano et al., 2011). Managing the reduction of a wine through the closure oxygen ingress is therefore an effective way to make this reduction disappear with minor impact on the desired positive character of varietal thiols.
Other aromatic compounds, such as β-damascenone (apple compote), increased with higher exposure to oxygen (Ugliano et al., 2015). This compound is known as a fruit enhancer (Pineau et al., 2007), especially for red fruits. This synergistic effect could explain the increase in fruity characters with higher oxygen exposure. In addition, “cooked” notes could also enhance impression of ripe fruits, cooked fruit/jam in red wines (Ugliano et al., 2015).
Aroma analyses carried out in this work (Ugliano et al., 2015) have shown that fermenting esters, largely associated with fruity aromatic notes in wines, are not affected by oxygen in the exposure levels studied. Therefore, they cannot explain the differences observed between closure oxygen ingress levels on the same wine. On the other hand, other studies (Pouzalgues et al., 2013) have shown synergies between esters and varietal thiols. For example, when the thiol/ester ratio is favorable to thiols, the aromatic note perceived on the tasting recalls grapefruit but approaches exotic fruits when this ratio is more balanced. A decrease in thiols during bottle ageing while esters are not impacted could therefore also contribute to the evolution of the aromatic profile of the wines according to the closure permeability.
The evolution of the aromas in the bottle recalls the evolution of the aromas in the grape during maturity
This work provides an explanatory point for practical observations. In a first dimension, the variation in the concentrations of aromatic compounds according to the levels of O2 brought through the closure (decrease in the concentration of thiols for example) and their synergies (olfactory interactions thiol/ester or enhancement effect of the β-damascenone) change the nose of wines towards aromatic universes that can be described as “riper”. To describe the differences between wines, it is convenient to place them on an aromatic maturity scale, metaphor for the aromatic sequence of grape ripening (Deloire 2013, Šuklje et al., 2017). It allows the taster to describe an overall impression resulting from the presence of different aromatic notes. This maturity scale begins for whites on vegetal profiles, then goes to grapefruit/citrus sides, turns into white fruits notes, exotic fruits notes and then dried fruits. In practice, a white wine with a thiol profile, exposed to increasing doses of oxygen, sees its aromatic profile where the citrus notes dominate evolve towards white and yellow fruits (linked to the decrease of thiols).
A second dimension for the description of the evolution of bottled wines corresponds to the effects of aromatic masks linked to the reduction, or the oxidation, even if no oxidation was observed in the work described above. Also called the “redox level” in traditional tasting, this scale ranges from rotten egg/cabbage notes to honey/nut notes, going through empyreumatic notes.
The mapping presented in Figure 3 allows us to highlight the evolution of wines during their storage in the bottle with different closures by dissociating these 2 dimensions. On the first example, a rosé wine from Bandol (Provence, France), increasing amounts of oxygen cause a change in the fruit maturity level, without the appearance of a reductive mask.
On the other hand, in the second example (Figure 4), a Chardonnay from Hungary, reduction aromas appear at some point of bottle ageing and then their masking effect gradually disappears. It is then a ripening of the fruit that is observed. To be precise, it is not the amount of oxygen that contributes to the formation of these reduction notes but the oxygen transfer rate into the bottle that does not seem sufficient to prevent their formation. Indeed, the reductive mask is not observed on the wine after 6 months of storage in the bottle, having received 0.93 mg/L of O2 through the closure. But it appears a few months later, after the wine has received 1.6 mg/L, which corresponds to an additional 6 months of storage with the same closure.
This work confirms the value of using the closure, especially its oxygen ingress properties, as an effective means to modulate the sensory evolution of the wine in the bottle. The intake of small amounts of O2 allows to modify the concentration of various aromatic molecules that are present in wines. This of course leads to a modification of said molecules’ direct contribution but also of the sensory universe of the wine via mask or synergy effects.
Thanks to the coextrusion technology, Vinventions produces closures with a range of well-defined, precise and reproducible oxygen ingress, such as the Green Line range. Wine producers can therefore choose the closure oxygen ingress according to their wine profile and to the aromatic evolution they desire in a given time. The choice must be considered according to the initial wine profile and its sensitivity to reduction and oxidation. For example, for a white wine with a marked thiol profile, if the producer wishes to maintain this profile during bottle storage, closure with low oxygen ingress are recommended. But in the case of a known reduction tendency, a slightly higher oxygen ingress should be favored. Conversely, if the producer wants the white wines to have ripened fruit flavor profiles, closure allowing a moderate oxygen ingress will be preferred. To facilitate this choice and to best help its customers in their decision, the definition of the aromatic profile of the wine and its evolution tendency towards oxygen exposure is a major challenge for Vinventions’ R&D department.
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