Yeast

(yest.) [From A. S. gest.] (Chem.) The barm or froth which rises during the vinous fermentation of vegetable juices. It is a flocculent viscid matter, insoluble in water and alcohol; from its appearance under the microscope, it is now believed to consist of a fungoid growth. It putrefies in a warm atmosphere, and excites fermentation in saccharine and mucilaginous liquids. It is also used in the making of bread, its property being to raise the dough, and by that means render it light and more wholesome. [s. 973]