after a couple days of cold soaking, the must is inoculated with yeast and fermentation begins. the carbon dioxide that is produced by the fermentation pushes skins to the top of the tank or bin, making a 'cap' on top of the liquid.this cap is kept in contact with liquid as much as possible because it gives the juice color and tannins. Tannins are the group of chemicals that naturally exist in the skin and seeds of the grapes (and many other plants). they give finished wine varying degrees of astringency.