Levain, the French term for sourdough starter or natural leaven, is a mixture of flour and water that captures wild yeast and lactic acid bacteria from the environment. It serves as a natural leavening agent in bread-making, providing rise and flavor to the dough. The process of creating a levain involves combining equal parts of flour and water and allowing the mixture to ferment over a period of time, usually several days. During fermentation, wild yeast and bacteria naturally present in the environment colonize the mixture, creating a living culture that contributes to the unique taste, texture, and structure of sourdough bread.

A well-maintained levain is an essential element in sourdough baking, as it imparts a distinct tangy flavor and contributes to the development of an open and airy crumb in the bread. Bakers often feed and refresh their levains regularly by adding fresh flour and water, ensuring the health and activity of the microbial culture. The use of levain represents a departure from commercial yeast and is favored by those who appreciate the complexity and depth of flavor that natural fermentation brings to bread.