Sourdough bread begins with sourdough starter, which is not merely living but seething. It is a community of organisms comprised of, at minimum, yeast, which is a fungus, and lactobacillus, a bacteria. They eat flour—its sugars—and poop out acid—thus, sour—in addition to carbon dioxide, which, trapped by stretchy, glutenous dough, gives the bread an airy structure, the so-called crumb, at its prettiest a dazzling network of gaps and chambers.
