In natural-wine making the grapes are gathered when fully ripe (sometimes, as for Sauternes, when overripe). Mechanical extraction of the juice, called must, has almost entirely replaced treading, the traditional method. For red wines the must is fermented with the skins and pips, from which the newly formed alcohol extracts coloring matter and tannin. Fermentation starts when wine yeasts ( Saccharomyces ellipsoideus ), existing on the skins of ripe grapes, come in contact with the must. It may take from a few days to several weeks, according to the temperature and the amount of yeast present or introduced. When the new wine has become still and fairly clear, it is run off into large casks, where it undergoes a complicated series of chemical processes including oxidation, precipitation of proteids, and formation of esters that create a characteristic bouquet. The wine is periodically fined (clarified), then racked into smaller casks. After some months, or for certain wines several years, the wine is ripe for bottling.
The very rare, superfine natural wines made in good vintage years from perfect grapes of the better varieties and possessing the unaccountable quality that vintners call breed are produced in the Bordeaux, Burgundy, and Rhône regions of France, in the Rhine valley of Germany, in California's Napa and Sonoma valleys and other parts of the United States, and in other regions of the world. The fine sherry of Spain and port of Portugal are superior fortified wines. Champagne is the best-known fine sparkling wine, but superior sparkling wines are also produced elsewhere in the world.