Press wines represent about 15% of overall wine volume and are generally richer in tannins and anthocyanins as well as residual sugars because some of the berries may have been left uncrushed in the marc. The pH is higher (lower acidity) due to potassium ion extraction from the skins, but the total acidity is higher.
Only the alcoholic content is lower compared to the free-run wine. As with the sugars, this is due to the unfermented berries in the marc. However, if the amount of residual sugar is significant, it is possible to have press wines with a higher alcohol content than the free-run wine.
Pressing the marc increases the final volume of wine and therefore optimizes the use of the raw material. Pressed wines with the best tannins and good density can be included in the final blend.
In addition to the economic advantage, press wines can contribute significant tannic richness to the blend.
However, special care must be taken at this stage in order to obtain satisfactory results, as the marc is fragile after alcoholic fermentation and very sensitive to oxidation.
In general, crushing the marc should be avoided as this can lead to its oxidation and non-selective extraction of compounds. This may produce herbaceous aromas mainly due to the hexanol, and bitter tannins. These molecules are also extracted at high pressures. At this stage, it is possible to extract MND-type compounds, which are markers of cooked fruit aromas.
On the other hand, if there was over-extraction during alcoholic fermentation, it is possible that the marc has nothing desirable to offer, with juices that are poor in tannins and anthocyanins that can only be obtained through distillation.