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4.3.2.c Choosing the correct pressure and duration

Best practices during pressing

Traditionally, the maximum pressure for a horizontal press is 2 bars. The presses are then separated according to various criteria detailed in the following section (link). Generally, extraction is considered to be quite high over 1.6 bars on a pneumatic press. For a vertical press, the pressure values are different. The maximum pressure is around 5 bars, although some vertical presses can potentially reach 14 bars.

To obtain a good-quality pressing, it is necessary to:

  • Have a steady flow of wine

  • Avoid crushing the grapes during filling and during the pressing itself. This is achieved by minimizing the number of rotations of the marc. For this reason, chain presses are not recommended.

  • Avoid rapid pressure build-up, as this does not favour a steady flow rate and increases the risk of extracting bitter and vegetal compounds.

Pressing times are longer during the early stages because there is more juice flowing out. On the other hand, at high pressures, there is very little juice and in order to maintain a constant flow rate, the time required at a given pressure level is considerably shorter.

New pressing systems now make it possible to have a customized programme for each pressing. The press itself is equipped with a sensor measuring the flow. As long as the flow is constant, the pressure is maintained. If it decreases, the pressure increases.
There is thus a pressing programme for each vat. Its only limit is the final pressing time, which remains variable. Working on a just-in-time basis can sometimes be complex.

What is the condition of the raw material?

The choice of programme is critical. There are some general guidelines but it is often necessary to adapt depending on the raw material. A thorough understanding of the characteristics of the harvest, the terroir and past vinifications is therefore crucial.

How ripe are the grapes?

The grape variety's maturity will have a direct impact on the consequences of the pressing programme. Pressing under-ripe grapes too hard can lead to a significant extraction of vegetal compounds. The same applies to over-ripe grapes where there is a risk of extracting dry tannins. Moreover, when high pressure levels are reached, it is possible to extract compounds that are markers of premature oxidation, such as MNDs, which can lead to prune-like odours. This is especially true when the grapes are very ripe.

**What level of extraction was applied in the vat during AF?

In the event that there was intense extraction during alcoholic fermentation, the marc is often depleted. The wine from the final pressing stages is likely to be deficient and difficult to use. Consequently, it is not always necessary to continue the pressing programme to high pressure levels.

**How healthy are the grapes to be pressed?

With a botrytized harvest for example, it is better not to prolong or intensify the extraction excessively.
For a harvest affected by mildew with grapes suffering from brown rot, high pressure will lead to the extraction of cooked/herbaceous compounds in greater quantities.
A healthy, good-quality harvest enables more flexibility in the pressing programme.

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