What is sourdough bread? Sourdough is a leavened bread, which means the dough naturally rises as a result of gas which is produced as the grain ferments. While most commercial breads use baker’s yeast as the raising agent, sourdough is made using a sourdough starter. A sourdough starter is a culture of yeasts and beneficial bacteria which naturally occur and are allowed to ferment over the course of approximately five days. A portion of the starter, known as the levain, is mixed with bread flour to make sourdough.