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Concept
Pressing (wine)
Summary
In winemaking, pressing is the process where juice is extracted from the grapes with the aid of a wine-press, by hand, or even by the weight of the grape berries and clusters. Historically, intact grape clusters were trodden by feet but in most wineries today the grapes are sent through a crusher/destemmer, which removes the individual grape berries from the stems and breaks the skins, releasing some juice, prior to being pressed. There are exceptions, such as the case of sparkling wine production in regions such as Champagne where grapes are traditionally whole-cluster pressed with stems included to produce a lighter must that is low in phenolics.
In white wine production, pressing usually takes place immediately after crushing or/and before primary fermentation. In red wine production, the grapes are also crushed but pressing usually doesn't take place till after or near the end of fermentation with the time of skin contact between the juice and grapes leaching color, tannins and other phenolics from the skin. Approximately 60-70% of the available juice within the grape berry, the free-run juice, can be released by the crushing process and doesn't require the use of the press. The remaining 30-40% that comes from pressing can have higher pH levels, lower titratable acidity, potentially higher volatile acidity and higher phenolics than the free-run juice depending on the amount of pressure and tearing of the skins and will produce more astringent, bitter wine.
Winemakers often keep their free-run juice and pressed wine separate (and perhaps even further isolate the wine produced by different pressure levels/stages of pressing) during much of the winemaking process to either bottle separately or later blend portions of each to make a more complete, balanced wine. In practice the volume of many wines are made from 85 to 90% of free-run juice and 10-15% pressed juice.
The timing of pressing and the methods used will influence other decisions in the winemaking process. In white wine making, pressing usually happens immediately after harvest and crushing.
Official source
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