How is sourdough used to leaven rye bread?

Sourdough leavens rye bread through the process of fermentation, utilizing a sourdough starter culture composed of flour, water, and wild yeasts.

While most commercial breads rely on baker’s yeast (Saccharomyces cerevisiae), some rye breads are specifically leavened using sourdough. To leaven the bread, the fermented sourdough starter is added to a mixture of rye flour and water. During this stage, the wild yeasts and bacteria within the starter consume available sugars, producing carbon dioxide and ethanol. The carbon dioxide gas becomes trapped within the dough, causing it to rise, while the fermentation process simultaneously develops the bread's characteristic flavor and texture.12

Because rye flour has different structural properties than wheat, the use of a sourdough culture is particularly effective for achieving the desired leavening and crumb structure in these loaves.21