Yeasts
Yeasts consume sugars and release carbon dioxide, helping the dough expand. They also contribute alcohols and aroma compounds that become part of the bread’s flavor.
Flour · Water · Salt
Sourdough is an ecosystem.
A starter is not a single strain of commercial yeast. It is a changing community of yeasts and bacteria that work together to build flavor and make dough rise before it reaches the oven.
The short explanation
What is actually happening?
Flour naturally carries microorganisms, with more present throughout the surrounding environment. When flour and water are mixed together, a community of yeasts and lactic acid bacteria begins to establish itself. Much like yoghurt and other fermented foods.
Within this colony, yeast produces carbon dioxide, which creates air pockets in the dough, while lactic acid bacteria produce acids, aromas, and enzymes that change the dough’s flavor, texture, and shelf life.
Sourdough is therefore both a leavening method and a fermentation process. During a long fermentation, the culture breaks down some of the fructans naturally present in wheat, which may help explain why some people find sourdough breads easier to tolerate than yeasted breads.
My bakery is not exclusive to sourdough, but when I do use sourdough, I don't add any additional commercial yeast.
The microorganisms in a sourdough starter
Lactic acid bacteria
These bacteria produce lactic and acetic acids, lower the dough’s pH, and help create sourdough’s range of flavors—from mellow and yogurty to sharper and more complex.
The partnership
The organisms share and compete for nutrients. Their relationships affect how quickly a starter rises, which aromas it produces, and how it responds to feeding.