Sourdough breads get their name from the fact that both the dough and the bread are acidic. The acidity, along with other distinctive flavour components, is produced by bacteria that grow in the dough along with various yeasts. The bacteria often include some of the same lactic acid bacteria that make milk into yogurt and buttermilk. The leavening for this kind of bread begins as a “wild” starter, a mixture of whatever microbes happen to be on the grain and the in the air and other ingredients when flour was mixed with water. The mixture of yeasts and bacteria is then perpetuated by saving a portion of the dough to leaven the next batch of bread.
The first breads probably resembled modern sourdoughs, and bread in much of the world is made with sourdough starters that give distinctive regional flavours. The bacteria somehow delay starch retrogradation and staling, and the acids they produce make the bread resistant to spoilage microbes; so sourdough breads are especially flavourful and keep well. Because browning reactions are slowed in acid conditions, sourdough breads tend to be lighter in colour than straight yeasts breads, and their flavour less toasty.
It isn’t easy to make good bread with sourdough cultures. There are two good reasons for this. One is that the bacteria grow faster than the yeasts, almost always outnumbering them by factors of a hundred of a thousand, and inhibit the yeasts’ gas production: so sourdough breads often don’t rise well. The other is that acid conditions and bacterial protein-digesting enzymes weaken the dough gluten, which makes it less elastic and the resulting bread more dense.
“On Food and Cooking; The Science and Lore of the Kitchen