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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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Winepress
A winepress is a device used to extract juice from crushed grapes during winemaking. There are a number of different styles of presses that are used by wine makers but their overall functionality is the same. Each style of press exerts controlled pressure in order to free the juice from the fruit (most often grapes). The pressure must be controlled, especially with grapes, in order to avoid crushing the seeds and releasing a great deal of undesirable tannins into the wine.
Oeil de perdrix
Oeil de Perdrix (œj də pɛʁdʁi) is a rosé wine produced in Switzerland. The history of the wine style dates back to the Middle Ages in the Champagne region of France and from there spread to the Canton of Neuchâtel in Switzerland where it would become a popular dry rosé made from pinot noir. The name œil de perdrix means "partridge's eye" in French, a reference to the pale pink colour of the eye of a partridge in death throes. Until about a century ago, it was common for vineyards to have the red and white grapes unseparated unlike today where each vineyard has a unique grape.
Yeast in winemaking
The role of yeast in winemaking is the most important element that distinguishes wine from fruit juice. In the absence of oxygen, yeast converts the sugars of the fruit into alcohol and carbon dioxide through the process of fermentation. The more sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to dryness. Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with dessert wines.