yeast Many species of unicellular fungi, most of which belong to the Ascomycetes and reproduce by budding. The genus Saccharomyces is used in brewing and winemaking because in low oxygen concentration it produces zymase, an enzyme system that breaks down sugars to alcohol and carbon dioxide. Saccharomyces is also used in bread-making. Some yeasts are used as a source of protein and of vitamins of the B group