Sourdough is bread leavened not by packaged commercial yeast but by a wild, living culture of yeast and bacteria that lives in flour and water. This culture, called a sourdough starter, is a symbiotic ecosystem often referred to as a SCOBY, or Symbiotic Culture Of Bacteria and Yeast. The yeasts found in sourdough belong to various species, most commonly Saccharomyces and Kazachstania, while the bacteria are usually different kinds of lactobacilli, with Lactobacillus sanfranciscensis being especially common in traditional sourdough cultures. Together, these microorganisms transform a simple mixture of flour and water into bread that is flavorful, naturally leavened, and shaped by the activity of a wild microbial community.
The yeast and bacteria each play distinct but complementary roles. The yeast consumes sugars in the dough and produces carbon dioxide gas, which leavens the bread by inflating bubbles that lift the dough; sourdough yeasts also contribute volatile aromatic compounds that build complex flavor. The bacteria produce lactic and acetic acids, which give sourdough its characteristic tang, help preserve the bread naturally, and contribute to the long keeping quality that has historically made sourdough valuable. The balance between yeast and bacterial activity shapes the bread's flavor profile, and bakers learn to read this balance through practice. Because sourdough relies on wild fermentation rather than a packet of dried yeast, fermentation times are considerably longer than for conventional bread, typically ranging from four to twenty-four hours or more depending on temperature, hydration, and recipe.
The fundamental difference between sourdough and conventional yeasted bread comes down to the source of the leavening. Yeasted bread uses a single species of commercial yeast added in measured amounts, while sourdough uses a diverse, unpredictable community of wild microbes that must be fed, kept alive, and allowed to ferment over time. This wild fermentation is the foundation of every technique that follows, from starter care to shaping to baking.