The gas is trapped in the dough, forms larger and larger bubbles, and makes the dough rise.
Wild yeast spores are almost always present in the air and will land naturally on the dough. The first people to discover the value of yeast were the Egyptians. They tried baking with fermented dough and liked the lighter, tastier bread. Bread that rises with the aid of wild yeast, however, may turn out differently each time. This is because different kinds of yeast may fall on it.
The Egyptians discovered a way to control this. Each time they baked they set aside some of the leavened dough to mix with the next batch. In this way, they could be sure of using the same kind of yeast.
Around 1000 b.c. Phoenician traders carried the art of making leavened bread to the Greeks, who became the master bakers of antiquity. The Greeks had over 70 different recipes for bread.
The Romans turned baking into a large-scale industry and passed many laws governing the quality of bread. The bakers were so proud of the superior taste of their bread that each baker marked his loaves with his name, just as bakeries put their brand name on the wrappers today.
During the Middle Ages, only rich people ate white bread. Dark, often sour, rye bread was what most of the people ate.
