Wild yeasts and lactic acid bacteria in flour start the sourdough fermentation by breaking down starches into gases and acids.

When flour and water mix, wild yeasts and lactic acid bacteria that naturally live in the flour begin to feed on the starches and sugars. The yeasts produce carbon dioxide gas, which makes the dough rise by creating bubbles inside. Meanwhile, the bacteria produce acids like lactic acid and acetic acid, which give sourdough its unique tangy flavor. These acids also help keep the dough fresh by lowering its pH, creating an environment where bad microbes can’t grow.