Yeast is the living single-celled fungus added to bread dough to make it rise.
Baker’s yeast is usually a strain of Saccharomyces cerevisiae, a microscopic fungus used in both baking and alcoholic fermentation. In dough, it consumes fermentable sugars and releases carbon dioxide and ethanol through fermentation. The carbon dioxide becomes trapped in the dough’s gluten network, causing the dough to expand and become lighter.
Fresh, active-dry, and instant yeast are different commercial forms of the same basic leavening organism. Temperature affects its activity: warm conditions generally speed fermentation, while excessive heat can kill the cells. Salt slows yeast activity, and very high sugar concentrations can draw water away from the cells.
Yeast-leavened bread has ancient roots, with early evidence associated with Egypt. Natural yeasts in flour and the environment probably produced some of the first raised doughs. Sourdough still relies on wild yeast, while commercial baker’s yeast provides more predictable rising. Baking kills the yeast, but its fermentation gases and flavour remain important.