Sourdough bread begins with sourdough starter—a homemade culture of wild yeast and bacteria that flavors and leavens bread and changes the dough's pH, allowing it to absorb more water, resulting in a chewier final texture. But some bakers occasionally add a little yeast with the starter to a sourdough loaf to give a boost to fermentation. Sourdough bread baking is a slow process, and even a small amount of yeast can speed it up significantly. An assist from yeast is especially helpful in the winter, when temperatures in the kitchen can dip below 70 degrees and warmth-loving sourdoughs can be especially sluggish. (In fact, in France, bakeries are allowed to add up to 0.2 percent yeast relative to the weight of the flour as a “seasonal amendment” to their doughs and still legally call the bread sourdough.) Furthermore, there's overlap between wild yeast and the commercial kind: A recent study performed by the Wolfe lab in the Department of Biology at Tufts University, which uses fermented foods to study microbiomes, showed that the dominant yeast species in most sourdough cultures is Saccharomyces cerevisiae—the very same species used in store-bought yeast.
When we added just ½ teaspoon of yeast to our recipe for the classic sourdough loaf pain au levain, it made a big difference: Instead of taking 8 hours to proof (4 hours before shaping and 4 hours after), it was ready to bake in less than half that time, just like most nonsourdough bread formulas containing yeast alone. And despite this, the loaf still retained some of the tang and chew of straight sourdough. To help proofing go faster with your own sourdough bread, try adding ⅛ teaspoon of instant yeast per cup of flour.
