Sparkling wine
The alcohol in wine is formed by the fermentation of the sugars in grape juice, with carbon dioxide as a by-product. Yeast is naturally present on grapeskins, and this alone can be sufficient for the fermentation of sugars to alcohol to occur. A pure yeast culture, most often "S. cerevisiae", is usually added to ensure the fermentation is reliable. Other yeast cultures like "Pichia", "Torulaspora" and "Kluyveromyces" are naturally present or added to create special flavours in the wine. Sparkling wine, including champagne, is made by adding further yeast to the wine when it is bottled. The carbon dioxide formed in this second fermentation is trapped as bubbles.