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You can actually vinify red grapes into white wine by separating the pomace from the must early enough after pressing the grapes. The red pigments are located in the skins of the grapes, so if you do this early, they won’t tranfer into the juice. A white wine made in this fashion is called blanc de noirs („white from blacks“).

If you let must macerate without removing the pomace the red color and a lot of tannins will transfer over and you get red wine.

As you usually don’t want a lot of tannins in white wine you separate the pomace from the must as well, even if you use white grapes (but of course you could let it macerate on the skins, but then you end up with orange wine).

White wine made from white grapes without maceration on the skins is called blanc de blancs.