The short physics of sourdough

A sourdough starter is a shared apartment for wild yeast and lactic acid bacteria, fed on flour and water. The loaf rises because the yeast makes carbon dioxide. A lot of the flavor, and some of the keeping quality, comes from the bacteria.

Flour already carries microorganisms, plus enzymes that break starch into sugars. Mix it with water and wait, and the organisms that like that paste begin to win. In a stable starter the winners are usually yeasts, which produce carbon dioxide and a little alcohol, and lactic acid bacteria, which produce the acids you taste as sour. The famous bacterium of San Francisco sourdough, Fructilactobacillus sanfranciscensis, is one of those tenants. It is not magic and it is not confined to one city. It is a bacterium that does well in the particular conditions of a repeatedly fed dough.

Refreshing the starter — discarding most of it and feeding what remains with new flour and water — keeps those organisms growing instead of starving. A neglected jar turns hooch and smells of solvent because the easy sugars are gone and the balance has tipped. A refreshment is not a ritual for its own sake. It is a reset of the food supply, and it dilutes the acids that would otherwise slow everyone down. How often you feed, and how much, decides whether you bake with a culture at full speed or with one that is sleepy and sharp.

Hydration, the water relative to the flour, changes the speed and the feel. Bakers talk in percentages: a starter at 100 percent hydration has equal weights of flour and water and looks like a thick batter. A stiffer culture, with less water, usually ferments more slowly and can lean toward a sharper, more acetic taste. A wetter one moves faster and is easier to stir into a dough. Neither is more authentic. They are different ways of setting the clock. The recipe that says “a cup of starter” without saying how wet that starter was is already missing a variable.

In the dough, two clocks run at once. Wheat proteins, glutenin and gliadin, form a network, gluten, once flour is wet and moved. That network can hold gas. Fermentation makes the gas and, more slowly, the acids and the enzymes that begin to cut proteins and starches into smaller pieces. Shape too early and the loaf cannot hold itself: the network is there, the gas is not, and the flavor has not developed. Wait too long and the network weakens, the gas leaks, and the dough spreads instead of rising. Bakers call the useful window proof. It is a judgment of volume, jiggle, and smell. Your kitchen is not the kitchen in the recipe, so the minute mark is a rumor about someone else’s temperature.