There is a wide variation from year to year in the quality of the grapes, and hence the wine, that are produced because the vineyards are so far north and the weather is not consistent. The best Champagnes are 'vintage', i.e. made almost entirely from wines of one good year; non vintage Champagnes are a blend of good and not so good wines.
After they have been harvested the grapes are pressed. Whole grapes are loaded gently into enormous presses and, slowly, the juice is extracted. This occurs three or four times and each time more tannin and colour comes out of the skins, although the actual skins and pulp have to be removed.
Fermentation takes place in stainless steel cylinders or barrels; where yeast is added to the grape juice so fermentation can occur. Separate blocks are kept for each vineyard. The yeast feeds on the sugar in the grape juice and releases alcohol (as ethanol), heat and carbon dioxide. In this first fermentation, the CO2 is not important, so it is released via valves in the tanks.