The goal of this experiment is to catch some wild yeast and try to keep them happy so that they will multiply. By harnessing the power of wild yeast in a sourdough starter, we’ll eventually be able to bake bread without using any commercial yeast at all. That’s what traditional sourdough is – a bread leavened by wild yeast. The process can be very technical and scientific, but I’m not going to delve into all those boring details right now. Science of Bread: Basic Sourdough Starter Recipe.
In addition to flour, water, and yeast, your starter also contains bacteria.
When these bacteria feed on the sugars in flour, they produce acidic by-products. This is what gives sourdough its sour taste. TMF: baking bread / Living Below Your Means.