While most people are familiar with wine, they may not be so familiar with how it is made. In fact, it is a fairly simple process of fermenting grapes that produces wine, but the difference between red and white wine is due to the presence of the skins during the fermentation process of red wine. It is the skins of red or black grapes that give red wine its colour.
When the whole grapes are crushed the skin tends to rise to the surface. They are pushed back down several times a day during the fermentation process so that they remain in contact with the liquid.
The sugar content
Grapes must be picked when ripe so that the sugar content is stable. Traditionally, grapes were picked by hand; these days many larger vineyards have their grapes picked by machine. In warmer climates they are often picked during the night. They then have to go through a process to remove the stems and any leaves or sticks. This too, is done by a machine that also lightly crushes the grapes.