Sparkling wines are made from still wines, such as Chenin Blanc, French Colombard, Chardonnay or Pinot Noir, which serve as a base for the next stage of this unique winemaking process. (Among premium sparkling wines, Chardonnay and Pinot Noir grapes are used almost exclusively.) A "triage"—a blend of the base wine, yeast nutrient and a source of sugar—is added to the base wine and the mixture is fermented a second time in a sealed container which traps carbon dioxide producing the effervescence or bubbles.