Traditional presses at Champagne Mumm.
Before pressing even begins, the grapes are crushed under their own weight. The juice released at this stage, called "free-run," is the highest quality juice, since it has had the least amount of contact with the stems.
You may be familiar with the old-fashioned screw presses, but these are rarely used anymore since they tend to mash the grape seeds, which can add bitter flavors to wine. In most wineries today, a much gentler balloon press is used. As the balloon (located in the center of a perforated drum) inflates, it pushes the grapes against the sides of the drum, pressing out the juice. The gentler earlier pressings and the free run yield the highest quality juice used in the best wines. With each successive pressing, the balloon inflates more. The goal is to release as much juice as possible without mashing the seeds.
White grapes are pressed within hours of their arrival at the winery, quickly separating the juice from the skins. Speedy pressing is important with white wines since the bitter tannins contained in the skins can mar white wines' delicate flavors and colors.
At this stage, when they are no longer grapes but not yet wine, the grapes and their juice are called must.
Since it's the skins that give red wines their color and contribute tannins, extended skin contact, called maceration, is necessary before pressing wine off red grapes. Maceration continues throughout the fermentation process (described below the next slide), which can take two days to two weeks. It is only after fermentation that the wine is pressed off the grapes. Winemakers looking for additional extract may let the grapes macerate for another week or two before pressing.