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Technical article

The separation of white and rosé juices during pressing enables a vinificaction process adapted to fractions that are both poor and rich in polyphenols, in order to prevent the risks associated with the premature oxidation of wines.

Conductivity testing has been used since the late 1990s for the online monitoring of extraction (J-L. Favarel, Protection de la vendange et extraction des jus en vendange blanc, colloque Cinquantenaire ITV France, 1998). This robust technology involves the placing of a sensor on the production line, although it does not measure the polyphenols directly. The increased strength of the signal is linked to the extraction of K+ ions from the grape skin. Increased conductivity during pressing indicates that gradual pressure increases lead to the extraction of grape skin compounds. Most of the tannins in white grapes are found in the skins, hence an estimate can be made of their extraction when the level of conductivity increases.

Monitoring of pH levels provides similar information to that of conductivity monitoring. The measurement of pH poses a problem related to the robustness of the sensor: it is difficult to use online.

No direct measurement of polyphenols is made in the two cases. The measurement of pH and conductivity does not, therefore, provide information on the polyphenol content of the juices. These are simply indicators of the potential moment in which the polyphenols may have a higher concentration.

In contrast, the voltametric measurement using the PolyScan provides a direct assessment of polyphenol content, which is influenced by the extraction of compounds from the skin and also by the polyphenol oxidation process.

In certain cases, decision-making based on conductivity or voltammetry is similar in terms of the moment of separation of the juice, as explained in example 1 below. There may be a difference in other cases, mainly linked to a higher or lower level of oxidation of the polyphenols during pressing, as in example 2.

Finally, measurement by voltammetry means that polyphenol content can be evaluated. The oenological process can then be adapted, which pH and conductivity measurements do not allow.

Example 1: Pressing Chardonnay

The pressing process was monitored by measuring conductivity, pH and electrochemical response (voltammetry).

Initially, increases in conducivity, pH and PhenOx (an index obtained by voltammetry and representing total polyphenols) are observied during the pressing process. For readability reasons, the evolution of the pH has not been included in Figure 1, although Figure 2 shows that pH and conductivity are linearly correlated.

The separation of the juice is done in the traditional manner after the first sequence at 1600 mbar. At that moment, more than 90% of the juice has been extracted. However, whether by considering the levels of pH, conductivity or PhenOx, it is clear at that moment that juices of different quality have been mixed in terms of phenolic composition. The three indicators show a significant increase in their values. When considering the increase in conductivity, a separation at around sequence 11 (800 mbar) is recommended, after an increase in conductivity of around 20%. With the level of PhenOx the separation seems optimal at around sequence 10 (600 mbar), after an increase of 80 to 100 units of PhenOx following a plateau at around 700. The level of pressing obtained at that time is higher than the mean value observed in Chardonnay juices at pressing (700). In both cases, separation at the moment of pressing allows you to obtain more than 80% of the volume of juice extracted and to preserve a relativly low level of polyphenols in the top-quality juices.

At the end of the pressing process, one sample showed a PhenOx value that stays the same while the level of conductivity increases. This leads to a lower number of polyphenols while the K+ concentration continues to rise (a point below the correlation in Figure 3).

In this example, the decision taken on conductivity or the PhenOx level obtained by voltammetry means that correct decisions can be taken, recommending an earlier separation of the juice than is done traditionally, to avoid mixing juices with a high concentration of polyphenols with those of lower concentration.

Exemple 2: pressing of Chardonnay

In contrast, in the second example of a Chardonnay press, an increase in conductivity was observed during pressing, as in the previous example. Nevertheless, voltammetry monitoring shows a pressing phase here with a very high polyphenol content. This is linked to stronger extraction and lesser oxygenation in this phase of the pressing process. The end of the pressing is therefore more oxidative and leads to a lower PhenOx value. Monitoring by voltammetry enables a more fine-tuned analysis than by measuring conductivity. It provides access to the real level of polyphenols in the juice that arises from simultaneous extraction and oxidation. This effect is linked to re-draining the wine press at the time of pressing, which leads to a considerable increase in the flow rate of the juice (Figure 5) and less oxidation.

The decision on the optinmal fraction after considering the voltametric data should be taken at 1600 mbar, as soon as an increase of 100 units is observed. The following measurements confirm a strong concentration of press juice in the vat, and, therefore, the implementation of a treatment strategy for the juice to bring down the phenolic load (fining or oxygenation).

In conclusion, the voltametric method enables a fast and accurate analysis of the quantities of polyphenols extracted during pressing. Voltammetry can, as a minimum, give the same results as measuring conductivity However, this technique also allows more precise monitoring, which helps to detect oxidation that may occur during the process. The data obtained by voltammetry allow the fine-tuning of press separations and the consideration of treatments before reassembling the wines with top-quality juice.

Extraction of polyphenols during pressing of Chardonnay grapes in Burgundy, impact of pre-pressing operations and consequences in settling tanks.
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