The final quality or finesse of a wine depends to a great extent on how it is pressed. As a general rule, the harder and longer the pressing, the lower the final quality of the wine. This is because excessive pressing extracts a range of substances from the skins or pomace (aromas, green tannins, etc.) that greatly reduce the quality of the wine. For this reason, all wineries keep the fractions obtained at different degrees of pressing separate and use them for different grades of finished wine. The following press fractions can be distinguished:
- Free-run wine: wine (or must) that separates from the pomace (or skins) by gravity alone, with no pressing. This is the highest-quality wine and is generally used for reserve or limited-production wines.
- Light-press wine: wine obtained by gentle action of the press. Depending on its quality, it may be used in premium wines or in everyday wines.
- Hard-press wine: the last wine obtained, extracted during the final (and hardest) pressing of the grapes. It is generally used to make inexpensive wines or as a by-product of winemaking for producing other kinds of beverages.
Pressing white varieties
White wines are pressed before fermentation. This is because the skins and seeds of white grapes contain tannins and other aromatic compounds that are detrimental to the final quality of the wine. For the same reason, pressing these varieties must be very gentle, preferably using pressure-controlled pneumatic presses. Excessive pressing of white musts damages the skins and seeds too much, passing these unpleasant flavors on to the future wine. Once the must has been separated from the solids, it is ready to begin fermentation.
When pressing white varieties, the turbidity of the must coming from the press is commonly used to separate the different press fractions (press cuts). It is usually measured in NTU (Nephelometric Turbidity Units), either with a turbidimeter or by comparing must samples against a pre-calibrated turbidity kit. The turbidity thresholds for separating the different press fractions are set by the winemaker in charge. Measuring turbidity by comparison.
Pressing red varieties
Unlike white wines, red wines are pressed after alcoholic fermentation. This is because the compounds of interest in a red wine, such as polyphenols (anthocyanins and tannins) and aromas, are concentrated mainly in the skin of the berry and in the seeds. These compounds can only be extracted from the skins and seeds through fermentation, since it is during this process that the action of the yeasts and the temperature conditions are optimal for extraction. Measurement parameters during the pressing of red varieties: since red varieties are pressed after fermentation, when most of the liquid has already separated from the pomace by gravity, it is not common to separate press fractions by quality.