4.1.8.a Pressing management

Should the harvest be destemmed or whole during pressing?

Stems are beneficial for pressing quality, they have a self-filtering effect and help to avoid having to carry out multiple further pressings. They can therefore be particularly advantageous in horizontal pressing.
However, in the case of a botrytized harvest, it is better to destem the grapes to ensure a thorough sorting.
Lastly, if the stalks are kept in cases of long pressing and/or high pressure, it is possible to extract more phenols, which are oxidation vectors for white wine aromas.

Should the harvest be crushed?

Crushing prior to pressing increases the quantity of grapes that are placed in the press, since there is a dynamic draining system in place that enables juice to be collected in the must collection trough, thereby leaving more room for grapes in the press.
Pressing times are potentially shorter when the grapes have been crushed. However, the must obtained is always more cloudy.
Moreover, any crushing of grapes that are verging on being under-ripe can lead to greater extraction of herbaceous aromas such as hexanol.

Is pre-fermentation maceration appropriate?

Pre-fermentation cold maceration enables the extraction of more aromatic compounds, thus revealing the grape variety's characteristics in more depth. However, it can be accompanied by a significant quantity of phenols which are potentially oxidative. It is only suitable for certain grape varieties. The grapes should then be lightly crushed and macerated for a few hours. This method is appropriate for grape varieties whose main aromas are not sensitive to oxidation.

Should a vertical or horizontal press be used?

There is no wrong choice here. For a vertical press, the self-filtering effect of the marc, whether it is destemmed or not, is substantial. It can sometimes make a settling period unnecessary. However, care must be taken with the gap between the cage's blades. If it is too large, grapes can escape from the press.
Horizontal presses are faster and can process a larger volume. They also enable good self-filtering with whole bunches. However, settling of the must remains necessary.

What are the principles of good quality pressing?

• A low pressing temperature (below 15°C)

• Slow, gradually increased pressure, for example, a 3 to 4 hour pressing programme.

• Relatively little turning of the marc for additional pressing.

• Moderate maximum pressure, the goal being to obtain the maximum amount of juice at low pressure.

Turning of the marc/high pressure is likely to result in the extraction of more phenols or bitter, vegetal compounds, which is to be avoided.

Should the different pressure levels be separated?

This is the case for the majority of wines except in the case of major logistical issues (lack of critical volume). The higher the pressure, the more aromatic elements, phenols and plant compounds are extracted. Similarly, cations are extracted such as potassium. The must's pH increases as the pressing pressure increases.
To monitor pressing and must separation, three options are often used:

• pH monitoring: for example as soon as the must has a pH above 3.6, it is separated from the free-run wine. The pH value depends on the must's initial pH. The separation is usually done at a pH jump of 0.2. The total acidity can also be monitored along with the pH to provide additional information on the quantity of acids present in the environment.

• Conductivity monitoring, which gives an indication of the proportion of free cations in the must.

• Tasting: a major criterion when separating batches based on aromatic evolution and the appearance of tannins.

Is it possible to filter the lees?

It may be possible if the volumes involved are worthwhile. A press filter can then be used. The filtered wine can then be reincorporated into the vat. It is necessary to have a must with a turbidity lower than 20 NTU.

How should oxygen be managed?

Most of these reactions are due to enzymatic activities of the polyphenol oxidase type (laccase and tyrosinase) with the phenolic acids. This leads to browning and the formation of polymers that will precipitate. At this stage, the risk is that oxygen will react with the aromatic molecules that are characteristic of the grape variety.
The most classic example is Sauvignon Blanc and its thiol-type aromas that are highly sensitive to oxygen; however, Viognier, whose aromas are characterized by terpenes, is also at risk. In the event that protection is necessary, several methods are possible:

• Sulphiting

• Inerting with dry ice

• Nitrogen or CO2 inerting of the press during pressing

On the other hand, grape varieties such as Chardonnay are not vulnerable to such issues. Hyperoxygenation can then be used to stabilize the must prior to fermentation. Browning occurs along with phenol precipitation. Afterwards, the must is ready for fermentation. At this point, the wine must be protected from oxygen.
It is also possible to consider controlled oxygenation to eliminate the phenols without attacking the aromatic compounds. In this case, it is a precise procedure requiring measurement of the phenols to be eliminated in order to know the necessary dose of oxygen.