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As soon as you mix together flour and water, an enzyme naturally present in the flour (called amylase) is activated and goes to work breaking down the complex flour starches into sugars. Wild yeast and bacteria both feed on these sugars during fermentation.

Wild yeast (saccharomyces exiguus) is a different strain of yeast than commercial, packaged yeast. It lives all around us in the air and on plants, grains, and fruits. There is some naturally present in the flour and more gets incorporated when you stir the flour and water together. The wild yeast is here as the leavening agent (the same function as commercial yeast in regular bread dough), and also produces alcohol and carbon dioxide as a by-product.

Also in residence in your starter is a friendly, non-harmful bacteria called lactobacillus. This is the same bacteria present in yogurt and many cheeses. Lactobacillus also feed on sugar and produce two kinds of acid as a by-product: lactic acid that gives the sourdough its mellow rich flavor and acetic acid that gives its tang and punch.

Depending on the conditions of your starter, one or the other of these acids is produced in greater abundance. Liquidy starter (near equal balance of flour and water) makes more acetic acid and your bread will have a more distinct sour flavor. A stiff starter (higher percentage of flour to water, about 2:1) makes more lactic acid, giving your final bread a more mellow, rather sweet taste.

Wild yeast doesn't actually contribute to the flavor of the sourdough at all! It's necessary because it's made of sterner stuff than commercial yeast, which would die in the acidic environment created by the bacteria. This same acidic environment protects your starter from the strains of bacteria that would make us sick.

The science of sourdough can get pretty fuzzy, and one of the best resources we've found is The Bread Baker's Apprentice by Peter Reinhart.

So how is your starter coming along?

Image of sourdough starter by Emma Christensen for the Kitchn.
Image of lactobacillus courtesy of National Center for Complementary and Alternative Medicine.
Image of saccharomyces exiguus courtesy of Wikipedia.