Yeast, the common name of species of Saccharomyces, a genus of low plants (fungi) probably belonging to the Ascomycetes. The yeast-plant is a single, simple cell, which produces new cells by a peculiar budding-process. Usually the cells thus produced cling together in short chains. The most generally known of the 25 described species is S. cerevisiæ, the yeast used by brewers and bakers. The use of yeast is due to its great activity in the process of fermentation. When placed in sugar-containing solutions and exposed to a proper temperature, the cells multiply with great rapidity, and in their living cause the sugary solution to “sour.” In this “souring” process the gas — carbon dioxide — is given off, and if it is held by some tenacious substance, as dough, it will puff it up, that is, make it “light.” The process of adding yeast to dough is simply sowing yeast-plants that they may grow. See Ascomycetes.