An Interim Sourdough Lesson

Kay, so after "Our Sourdough, Ourselves: Part I" I started getting really curious about the science behind sourdough, and really, bread baking in general. I've had some awareness of the science just from all the recipes I've read and what I've picked up talking to people, but I've never sat down to put it all together. Much of the 'science' some recipe authors give as background seems to be the stuff of legend, and there's a lot of garbled pseudo-science rumbling around out there.

Here is a little fact/fiction break down for ya, gleaned from my bread books and a bit of web research. Please feel free to correct/add/subtract/tell me I'm wrong. There's so little good info out there, I'd hate to add to the muddle. Also let me know if you have a question or come across another baking science myth--I'd love to look into it. Keep an eye on this page; I'll add new info as I go:
----------
Fiction: Adding a pinch of yeast as you begin cultivating your starter will help collect wild yeast from the air.

Fact: MEEEH! Try again! Though it sounds like fun, yeast does not happily float through the air in search of a good party. Yeast is a kind of fungus. Fungi absorb their food and reproduce asexually by division of cells (also called 'budding' or 'fission'). Yeast cells are also not mobile organisms able to seek out their food (or kinfolk); in other words, yeast has to live in or on its food in order to eat and survive. Adding a pinch of yeast to your initial starter is more like planting a seed or using a match to start a fire. Honestly, I don't think you even really need the pinch of yeast to get things going--the yeast will eventually make a home in your starter on its own. Certainly you should never need to add any commercial yeast after you've gotten your initial starter going.
----------
Fiction: Yeast eats flour.

Fact: Yeast eats sugar. Sugar comes, eventually, from the flour. Flour is made up of starch molecules (complex carbohydrates). Starch molecules are composed of many many threads of simple sugar molecules wrapped very tightly together. Something is needed to break down the starch molecules and release the sugar molecules. That "something" is an enzyme called amylase. Once the amylase breaks down the starch molecules into simple sugar, the yeast begins feeding on the sugar.
----------
Fiction: The sour taste in sourdough comes from dead yeast cells.

Fact: Honestly, I'm a little shaky on this one. Here's one definite fact: There's more than just wild yeast in them thar waters. Also in residence in your little pot 'o starter is a friendly, nonharmful bacteria called lactobacillus. The fuzziness here is that I can't figure out if the lactobacillus breaks down starch molecules into sugars for the yeast, or if enzymes still break down the starch and both the yeast and the lactobacillus feast on it. In any case, lactobacillus produce two kinds of acid as a by-product: lactic acid gives the sourdough its mellow, rich flavor and acetic acid gives sourdough its tang and punch. Another spot of fuzziness: I *think* one or the other of these acids is produced in greater abundance depending on the conditions in your starter, like whether you keep your starter in more of a liquid state or in a stiff dough. Liquidy starter (near equal balance of flour and water) makes more acetic acid and makes your sourdough more sour. A stiff starter (higher percentage of flour to water, about 2:1) makes more lactic acid and gives your final bread a more mellow, rather sweet taste.

Wild yeast doesn't actually contribute to the flavor of the sourdough at all. Wild yeast is necessary because it's made of sterner stuff than commercial yeast, which would die in the acidic environment created by the bacteria. The wild yeast is in there as the leavening agent (in the same way that commercial yeast leavens regular bread dough), and also produces alcohol and carbon dioxide as a by-product. (The carbon dioxide is what leavens the dough--carbon dioxide molecules get trapped in the web of gluten strands and lifts the structure up.)
----------
Fiction: Wild yeast is commercial yeast, only it lives in the air instead of a little paper packet.

Fact: Actually, wild yeast and commercial (or domesticated) yeast are two different strains of yeast entirely. Wild yeast is Saccharomyces exiguus, and commercial yeast is Saccharomyces cerevisae. Who'd a thunk it. Commercial yeast likes a near-neutral pH environment in which to do it's groove thang. Wild yeast likes the acidic environment of the sourdough. Wild yeast is also longer lived and can stand up to the longer, more rigorous process of sourdough bread baking.