Yeast is manufactured commercially by combining cultivated pure yeast cells with molasses. Because yeast is a “living organism,” making it is not a chemical procedure. Instead, yeast quite literally needs to be grown to be manufactured. Yeast also exists in the natural world and the fermentation process (i.e.: the process whereby yeast is converted into carbon dioxide and alcohol) can happen, for instance when a ripe piece of fruit falls from a branch, splitting open and exposing its sugar-filled flesh. Yeast in the air feed off the fruit’s innards and fermentation occurs. The yeast necessary to leaven a loaf of bread can also be made in one’s very own kitchen (a.k.a a sourdough starter). When water and flour are combined in a warm kitchen, the natural yeast in the air feed off of the flour in the starter, creating the carbon dioxide that will eventually make your boule gloriously round and tall.