Bacterial Fermentation

The second important point is that sourdough starters contain bacteria as well as yeast. The most important of these bacteria belong to a group called Lactobacilli (singular form: Lactobacillus). Like the yeast, these bacteria ferment some of the sugars in the dough and create carbon dioxide gas. In addition, they create acids. These acids give sourdough its sourness. As in the case of wild yeasts, the exact strains of bacteria present vary from starter to starter, so each starter has unique characteristics.