The crusty layer, up to two feet or deeper, of grape skins, pulp, stems, and seeds that rises and floats on top of the juice during a red wine’s fermentation. In parts of the northern hemisphere, red wines are fermenting right now. The cap must be kept in contact with the juice by one of several methods. It may be frequently pushed down into the juice, called “a punch down,” or the juice can be pumped over, that is, drawn up from the bottom of the tank and then showered over the cap. As a result of being punched down or pumped over, the alcohol and heat from the fermenting juice can extract color, aroma, flavor, and tannin from the cap. If the cap is not broken up and kept wet with the juice, it dries out and becomes a haven for acetic acid bacteria which will ultimately mar the wine, turning it to vinegar.