The winemaker varies the conditions of fermentation according to the particular kind of wine being made. In the case of delicate white wines, the must is fermented for four to six weeks at about 60°F/16°C. With more robust red wines, the must is fermented at a temperature between 65 and 80°F/18–27°C in contact with the skins, to extract pigments, tannins, and flavor. This phase may last for 4 to 14 days (less if heat or a carbon dioxide treatment is applied). Then the must is separated from the skins and fermented again for a total of two to three weeks. One of the most critical variables during fermentation is temperature. The lower the temperature, the slower and longer the fermentation, and the more aromatic molecules accumulate.