Carbon dioxide (CO2) plays a vital role in winemaking, both as a byproduct of fermentation and in creating sparkling wines. During fermentation, yeast converts sugars into alcohol and CO2. In still wines, most CO2 escapes, but in sparkling wines, CO2 is retained, creating the bubbles characteristic of these wines. CO2 can also be used in winemaking techniques to preserve freshness and protect against oxidation.