Hmm, the high educational value of this topic might make it a good post for Lilting House.
Jane and I are exploring the science behind the loaves we're baking. Found this fun site: The Science of Cooking. An excerpt from the sourdough page:
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 sourdoughits sour taste.
Actually, all doughs contain at least some bacteria. So why aren’t all breads sour? In doughs made with bakers’ yeast (the kind you buy in the store), the yeast outnumber the bacteria. Since both compete for the same sugars, the yeast win out, and the bacteria don’t have a chance to produce their acidic by-products. In sourdough, yeast and bacteria are more closely balanced, so the bacteria have a chance to add their flavors to the bread.
Sourdoughs and other raised breads also differ from one another because of the eating habits of the yeasts that make them rise. The predominant yeast in sourdough, Saccharomyces exiguus, cannot metabolize maltose, one of the sugars present in flour. Baker's yeast, on the other hand, has no trouble feeding on this sugar. Since the bacteria that give sourdough its taste need maltose to live, they do much better in the company of sourdough’s yeast because they don’t have to compete for this sugar.
Sourdough culture is a yeast living symbiotically with a friendly lacto-bacteria. We need to start with enough of the right organisms so that they can become the dominant culture, food and water and the right temperature.
Given the right organisms, the optimum temperature is just over 80F/27C. Much hotter and the activity of the yeast declines. Above 95F/35C the yeast is effectively dormant or dead. The bacterial activity peaks at 93F/34C, so some bakers choose to ferment at 90F/32C to get a sourer bread. At 70F/21C the activity of the yeast has roughly halved, so the fermentation will take twice as long.