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 and before primary fermentation. In red wine production, the grapes are also crushed but pressing usually doesn't take place until 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.
When to press and other winemaking decisions
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. Here, the biggest decision will be how much pressure to apply and how much pressed juice the winemakers wants in addition to the free-run juice. Some grape varieties, such as Sémillon and Aurore have very "liquidy" pulps that releases juice easily without needing much pressure that could risk tearing the skins. Other varieties, such as Catawba, have much tougher pulps that will require more pressing. In red wine production the timing of when to press is one of the most important decisions in the wine making process since that will be the moment that maceration and phenolic extraction ceases. Some winemakers use the decreasing sugar level (such as brix measurement) scale and press once the wine has reached complete dryness. Often winemakers will use taste to determine if the wine has extracted enough tannins to produce a balanced wine and may press before complete dryness (such as at 3-8 brix). Though removing the skins by pressing often removes some solids that the wine yeast need to complete fermentation and the benefits of pressing early is often balanced by the risk of potential stuck fermentation. The quality of the vintage year and the overall ripeness of the harvested grapes may also play a role since in cool years when the grapes are often harvested under-ripe, the tannins in the grape are often very "green" and harsh. In these years winemakers might press early (such as at 15 brix), a process that the Australians call "short vatting". In warmer years, the tannins may be full ripe or "sweet" and the winemaker may decide to do a period of extended maceration and not press the grapes for as long as a month after fermentation has completed. Usually the pressed juice will require some additional treatment, which can be done separately to the pressed juice alone or to the entire batch of wine if the pressed juice is blended with the free-run. These treatments may include acid adjustments to lower pH, extended settling periods for clarification and additional racking to remove the extra suspended solids and the use of fining agents to remove extra solids or excess tannins. Grape pulp contains a lot of pectins that create colloid coagulation with these solids that will make the wine difficult to stabilize. Some winemakers will use pectolytic enzymes during the maceration process to help break down the cell walls to allow the release of more juice freely. These enzymes are also used with white wines to assist in clarification. The type of pressing used and the amount of suspended solids plays a particular role in filtering decisions as a high amount of suspended solids (particularly natural gums) can clog and damage expensive filters.
History
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