A loaf of bread isn't fluffed up by chemistry alone. It's slowly built up by colonies of yeast, which you heartlessly extinguish when you bake the dough in the oven. There's an ecosystem in every loaf, and a special kind of loaf, sourdough, is built by a special kind of ecology.

If you make bread from time to time, you probably make it with yeast, which you leave in warm water for about five minutes or so before mixing in the flour. Baker's yeast is meant to be efficient, not sturdy. It's also the most expensive part of the bread, which is why habitual cooks have starters — a colony of yeasts kept in a clean bowl with some flour and warm water. The baker just adds flour to it every now and again so the colony has something to eat, and covers it to keep it from getting contaminated. It's like having a bunch of pet sea monkeys which you slaughter every time you want a sandwich.

Multiple things can contaminate a yeast colony, including mold, which makes the thing disgusting and impossible to bake with, and certain kinds of bacteria, which should make the thing impossible to bake with. Bacteria, when they take over a starter, out-number the yeast to the tune of one-hundred-to-one. The bacteria that are most at home in the flour-and-water environment are Lactobacillus, which take in sugar and churn out lactic acid, among other things. They kill off baker's yeast by out-competing them for sugars and turning their environment into an acid bath.

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Wild yeasts overcome that problem. Yeasts like Candida milleri and Candida krusei can survive in an acid-rich environment. When baked, the yeast makes the dough rise, and the acid from the bacteria give the bread a tartness.

That only works for a while. You've still got two consumers competing for the same food - glucose. It's not an ideal system. Lactobacillus sanfranciscensis makes it ideal. The bacteria is named after the world's most famous sourdough-bread-producing city. It was originally thought to be a wild yeast exclusive to San Francisco, but has showed up around the world. When L sanfranciscensis invades, it does something different from most yeasts. It creates lactic and acetic acid, sure, but it also creates glucose, a sugar that yeast eats. It produces the glucose from maltose, a sugar that the yeast can't eat.

Essentially, this creates an ecology in which the bacteria eat the maltose that the yeast can't eat and provide glucose to keep the yeast thriving. The bacteria also keeps competition for dextrose down by acidifying the environment and providing any other creature in the starter alternate sugar to eat. If that sounds a bit dirty to you, the bacteria squirts out an antibiotic, cycloheximide, which kills off nastier things that might contaminate the colony. In essence, sourdough bread is a big commune full of cooperating creatures working toward an environment in which everyone thrives. Then it gets baked.

Image: Jarkko Laine.

[Sources: Culinary Reactions, Lactic Acid Fermentation in Sourdough.]