[AS.] The froth that rises on the top of liquors in the process of fermentation; or the substance used for raising dough to be baked into bread (q. v.). Although yeast looks like a liquid to the naked eye, yet under the microscope a drop of yeast is found to contain thousands of extremely small rounded bodies, which are tiny plants of the Fungus kind. The yeast-plant feeds on part of the starch in the flour, and the result is that this starch is changed into grape-sugar. Leaven is only a little flour and water which has been left exposed to the air until some of the "spores" 0r "seeds" of the yeast-plant (which are always floating about in the air) have settled in it and begun to grow, which they do with wonderful rapidity.
YEAST FUNGUS, MAGNIFIED 4OO TIMES.