This term is used in two ways by meadmakers. The first defini- tion involves sealing the bottles against outside air once fermenta- tion is complete. A machine is used to apply the caps. Many meadmakers prefer corks, which can be applied with ease. The second use of the term "cap" refers to the somewhat firm layer of fruit that rises to the surface of the must during the primary fermentation. Some recipes call for "punching a hole in the cap" to admit oxygen. The cap, as well as any sediment in the bottom of the fermentation vessel, is left behind at the first racking.
The layer of skins, pulp, stems, and seeds that floats on top of fermenting grape juice during the first phase of winemaking. The cap must be either punched down or somewhow kept wet with fermenting juice in order to avoid drying out, which can cause bacteria to grow. As well, keeping the cap in contact with the juice ensures that proper extraction of flavor, color, aroma, and tannin can take place.* capsule: The material used to cover the top of the wine bottle and ensure that the cork does not dry out or become contaminated. Usually in the form of foil or wax.
When red wine ferments, carbon dioxide is produced which rises to the top of the tank. As it does, it pushes up the skins and seeds to the top, forming a "cap".