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Bread yeast. Baker's or brewer's yeast, Saccharomyces cerevisiae, is used to help bread to rise during baking and to convert sugar into alcohol during brewing. It is added to beers and lagers to ferment the sugar into alcohol and carbon dioxide. When added to bread, the production of carbon dioxide causes the bread to rise. Yeast is a kind of single-celled fungus. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE PERCIVAL)