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Technical article

Wines do not all behave in the same way when exposed to oxygen and do not develop in the same way following such exposure.

The terms “reactivity with oxygen”, “sensitivity to oxygen”, “oxidizability”, etc., are widely used in the oenological sector without having a specific definition. Furthermore, today, there is no analytical method that makes it possible to evaluate the capacity of a wine to build better resistance or, on the contrary, undergo oxidative damage due to oxygen exposure. This capacity has to date been evaluated empirically by winemakers via direct tasting or through tasting after putting the wine into contact with the air (air resistance test).

The Vinventions Enology team defines sensitivity to oxidation as the tendency for a wine to acquire oxidative organoleptic characteristics during exposure to oxygen. Generally speaking, a wine’s sensitivity to oxidation evolves over time and depends on the operations which the wine undergoes during winemaking and ageing.

We have developed a predictive test for the tendencies of a wine to develop based on electro-chemical analysis of wine carried out using the NomaSense PolyScan P200 analyser. This test, called Tendency of Evolution (TE), provides winemakers with decision aids throughout the elaboration of their wines, by indicating the “sensitivity” of the wine with regard to oxygen, i.e. the risk of emergence of oxidative characteristics during exposure to oxygen. This test is not a snapshot of the wine’s oxidative state at the moment of the test. For example, a wine that displays an oxidised profile may not necessarily be described as sensitive because the extra oxidation will not necessarily lead to any further evolution of the wine’s structure.

At the time of the electro-chemical measurement, the result of TE allows determination of whether the wine is sensitive or insensitive, enabling the winemaker to adapt his or her winemaking choices – such as using inerting gas, or sulphites, blending, choosing a specific type of ageing, or closure, etc.,.

Background and experimental device

To evaluate the sensitivity of a wine, some winemakers achieve a “air resistance test”, involving leaving the wine in contact with oxygen for several hours. The wine is tasted before and after this contact with the air and its profile evolution is evaluated, in particular regarding the emergence of oxidative notes. The difficulty with this test lies in the objectivity of tasting samples with a time lapse between two tastings. Evaluations by other analytical methods, such as spectrophotometry, provide few results on wine, because changes in colour generally take more time than is allowed for the test.

The Vinventions Enology team set up an experimental device to assess by electrochemistry the effect of oxygen on wine during air resistance test, using the NomaSense PolyScan P200. Samples of wine were collected from end of alcoholic fermentation to bottling and the opinions of winemakers regarding the sensitivity of these wines with regard to oxygen were recorded. These wines were then subjected to air resistance test and the evolution of the voltamograms was studied.

More than 800 samples of red, white and rosé wines were tested. As the results were obtained, the electro-chemical analysis method was optimised in order to improve the test performance time, the electro-chemical method parameters and the wine sampling method.

The effect of the presence of SO2 on the test results was also studied. In order to do this, sulphite free wines were stimulated with increasing oenological doses of SO2 then subjected to the Tendency of Evolution test. The evolution of the voltamograms remained similar for all the samples tested, suggesting that implementation of the anti-oxidant effect of SO2 requires a longer time than the duration of the test. We were able to conclude that this test highlights the very sensitivity of the wine’s matrix.

We also evaluated the evolution of results from Tendency of Evolution test during storage, before and after bottling of different wines, principally through a cooperative project with the IFV and Domaine de Donadille, funded by the Occitanie region.

During this project, red wines were micro-oxygenated and compared to the control sample. In particular, it was possible to highlight that micro-oxygenation made the wines less sensitive to oxidation than the control sample and that during storage in tanks then in bottles, this sensitivity increased for all the methods, constantly remaining higher in the non micro-oxygenated control sample. These observations were in keeping with the empirically observed evolutions of the wines.

Results and observations

The experiments made it possible to demonstrate that evolution of voltametric signals differed according to the wines’ sensitivity to oxidation. Two different types of evolution were observed.

In the case of wines deemed to be less sensitive by winemakers, an increase in the voltametric signal was observed over time (figure 1), corresponding with the formation of extra oxidizable substances during contact of the wine with air. Figuratively speaking, this could be described as a “construction” of the wine’s structure via consumption of oxygen.

Figure 1: example of a wine with a voltammetric signal increasing between first and second measurement (2 hours interval), signifying that the wine has a low sensitivity to oxidation.

On the contrary, wines sensitive to oxidation according to winemakers displayed a decrease in such signals following contact with air. Figuratively speaking, this could be described as a “deterioration” of the wine’s structure via the consumption of oxygen.

Figure 2: example of a wine with a voltammetric signal decreasing between first and second measurement (2 hours interval), signifying that the wine is highly sensitive to oxidation.

The evolutions of voltamograms observed were higher than the uncertainty inherent to the measurement.

Tendency of evolution testing

Thanks to the experimental results obtained, it was possible to set up a Tendency of Evolution test and integrate it into the NomaSense PolyScan P200. To carry out this test, a precise protocol must be followed.

Test protocol

To evaluate the tendency of evolution of a sample, it is necessary to:

  • Carry out electro-chemical analysis on the sample in question with no prior preparation.
  • Leave the sample in contact with the air for 2 hours.
  • Carry out further electro-chemical analysis at the end of testing.

Several precautions must be taken in order to perform the test in the best conditions.

  • Carry out the first electro-chemical analysis (T0) using the PolyScan P200, in the “tendency of evolution” menu immediately after sampling from the tank / barrel or after opening of the bottle, without waiting for the wine to enter into contact with the air. For example, do not take all the samples in the cellars then transport them to the laboratory to carry out the analysis. Aeration of the sample may compromise the test results.
  • Sample 50 ml of wine using a pipette and carefully place it in a 125-ml flask with a septum. Do not shake. Leave the sample at the temperature of the wine when it was sampled (in the cellar, for example) for two hours.
  • Two hours after sampling (the countdown is shown on the screen of the PolyScan P200), carry out further electro-chemical analysis (T 2 hrs) on the sample.
  • The PolyScan then indicates the category of the wine: high sensitivity or low sensitivity, as well as the EasyOx and PhenOx values of the initial sample. These values complete the analysis: if they are particularly low, it is possible that the sample is already oxidised.

Remarks:

Two tests performed on a same wine may have a different result if the second test is performed on the wine that has remained in the open air. The wine that remains in the open air oxidises and therefore the result will not reflect the actual oxidative potential of the wine found in the bottle or the vat.

An “oxidised” wine may sometimes not become oxidised any further. Therefore, it is possible that the test no longer deems the wine to be sensitive, simply because the mechanism of oxidation is already advanced.

The addition of SO2 does not compromise the result of the wine evolution test.

Test results and decision aids

The Tendency of Evolution test provides an indication of a wine’s sensitivity to oxidation at a given moment, i.e. the moment when the test is performed.

It may change throughout the winemaking process and its interpretation should be corroborated with the stage of the wine’s elaboration. Two results are possible: either the wine is deemed to be sensitive or it is classed as not very sensitive.

Depending on the test result, the winemaker can adapt his or her winemaking, ageing, blending and storage choices.

For example, to optimise the shelf-life of a wine sensitive to oxidation, a winemaker can:

  • Avoid long and oxidative ageing processes (micro-oxygenation, barrel-ageing) or carry out blending beforehand with a wine that is not very sensitive.
  • Maintain a certain level of fine lees in the storage vat in order to benefit from their anti-oxidative effect.
  • Avoid selecting these wines for sulphite-free conservation and commercialisation.
  • Avoid oxygen intake during transfers, filtering and bottling.
  • Choose closures that have low oxygen permeability and prefer short consumption circuits.

On the contrary, for a wine that is not very sensitive to oxidation, it is possible to:

  • Carry out long ageing whilst implementing monitoring via the Tendency of Evolution test to avoid carrying on the ageing process for too long.
  • Limit sulphite addition if the preparation and bottling stages are well controlled.
  • Choose a closure and its permeability to oxygen depending on the desired shelf-life for the wine.
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