Yeasts ferment the sugar in the dough, producing ethanol and carbon dioxide.
CO 2 is the leavening agent and the alcohol evaporates off during baking. Yeast cakes contain, in addition to yeast cells, small amounts of starch, vegetable oils, and some lactic acid bacteria. These yeast cells are dried carefully at low temperatures so the cells will survive. You will be studying the difference between the active dry yeast and the yeast cakes. Boyd, Robert F. 1988.