While there is no commercial yeast in this bread machine sourdough recipe, there is wild yeast from your active starter that helps the dough rise and give it a sour flavour. The yeast combines with lactic acid bacteria to create your bubbly starter. A sourdough starter is much like a kombucha SCOBY- it is a symbiotic culture of bacteria and yeast. You can read about the health benefits of sourdough here.