Red wine is made from the must (pulp, including the juice) of red or black grapes and
fermentation
occurstogether with the grape skins, which give the wine its color. White wine is made by
fermenting juice which is made by pressing crushed grapes to extract a juice; the skins are removed and play no
further role.
To start primary fermentation, yeast may be added to the must for red wine, or may occur
naturally as ambientyeast on
the grapes (or in the air). For white wine, yeast may be added to the juice. During this
fermentation , which often takes
between one and two weeks, the yeast converts most of the sugars in the grape juice into ethanol (alcohol)
and
carbon dioxide (which is lost to the atmosphere).
This process is also known as "alcoholic fermentation"
and is a vital step in the winemaking process, as it
helps to shape the wine's flavor, aroma, and appearance.