Which gas is primarily produced by yeast during fermentation?

Carbon dioxide is the gas primarily produced by yeast during fermentation. In alcoholic fermentation, yeast breaks down sugars and releases carbon dioxide and ethanol as waste products.

The process begins when yeast uses enzymes to convert glucose into pyruvate and then into ethanol. The overall reaction converts one glucose molecule into two molecules each of ethanol and carbon dioxide, while also producing a small amount of usable cellular energy. The carbon dioxide is the gaseous product; ethanol remains dissolved in drinks or evaporates during baking.

This gas explains two familiar results of fermentation. In bread dough, carbon dioxide becomes trapped in the gluten network, forming bubbles that make the dough expand and rise. In beer, wine, and cider, most carbon dioxide escapes during fermentation, although it can be retained to create carbonation in sparkling beverages.

Oxygen changes yeast metabolism, but it does not make nitrogen or hydrogen the normal gas produced by alcoholic fermentation. Common brewing and baking yeast, including Saccharomyces cerevisiae, is especially associated with this sugar-to-ethanol-and-carbon-dioxide pathway.