The leaven for bread is a mixture of flour and water. After proper fermentation, it becomes a natural means of loosening and acidifying the dough. There are several dozen different bacteria and yeasts in sourdough, and the most important are wild yeasts and bacteria responsible for the production of lactic acid and bacteria that produce acetic acid.
Bread baked with natural leaven (yeast) has a specific taste and aroma. It is fresh longer, does not mold and, what can I say, is healthier, especially bread with rye flour.