Yeast is a tiny, microscopic plant and like any other plant it needs light, heat and moisture for its growth. The temperature most favorable is from 70° to 75° F. If colder than this, its growth may be retarded altogether. Above 90° F. the conditions are favorable for the growth of lactic acid bacteria and the bread "sours." Sugar feeds the yeast; so when added to the sponge it hastens the growth. The starch of the flour is, to some extent, converted into sugar and thereby serves the same purpose. The soluble carbohydrates are acted upon by the yeast and converted into alcohol and carbon-dioxide. The carbon-dioxide gas becomes entangled in the gluten, and by expansion when heated, raises the bread.

Compressed yeast is commercially made from grain in factories equipped with highly specialized and complicated machinery. The grains most used are corn, rye and barley malt. The grain is ground in a mill, mashed with water and the mash, cooked and allowed to cool, and finally fermented with yeast of a previous making. The result is the growth and multiplication of yeast cells.

When the fermenting process has been carried to the proper stage, the yeast is separated from the fluid containing it, thoroughly washed with water, filtered, pressed, cut into cakes and wrapped. Every yeast cake contains millions of tiny yeast plants.

Baking powder, like yeast, is a leavening agent which causes dough to rise by the expansion of imprisoned carbonic acid gas. The two principal ingredients are an acid and an alkali, usually cream of tartar and bicarbonate of soda. These, when mixed together and moistened, react on each other and form gas bubbles which rise through the mixture, making it light and porous.