White wines are pressed prior to fermentation. There are a variety of presses used today, but the most advanced technology is an enclosed bladder press in which the grapes and juice are flushed with nitrogen to prevent oxidation. The grapes are run through a crusher and destemmer and then loaded into the press. The bladder slowly inflates pressing the grapes against a grill to extract the juice.
At the opposite end of the technology spectrum is the vertical or basket press. This type of press has been used since medieval times. Whole clusters of grapes are loaded into the press and pressure is evenly applied to grapes. Although some basket presses are now enclosed, they were traditionally open vats, leaving the must open to oxidation and contamination.
Whether the design is bladder, membrane or vertical press, today's grape presses are highly sophisticated, with computerized programs that can be customized to vary time and pressure over the entire length of pressing. Remarkably, today's cutting edge presses can complete an extremely slow, gentle process of crush without breaking the skin of the pressed grapes.
Once the grapes are pressed, the unfermented juice (must) is allowed to settle in order to remove skins, stems, leaves and any other debris. Some winemakers use pectolytic enzymes to break down the pectin in the grape juice and speed up the settling. The must is also sometimes allowed to cold settle, filtered, fined or run through a centrifuge.
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