What actually lives in a sourdough starter?

A sourdough starter is a living community of two kinds of microbes: lactic acid bacteria and wild yeasts. It is nothing but flour and water left to sit, yet it becomes a stable culture in which the yeast produces carbon dioxide gas that leavens the dough, and the bacteria produce the lactic acid that gives sourdough its tang.

Where do they come from? Nobody adds them: they are introduced through the environment, the water and the flour. When flour meets water, an enzyme in the wheat called amylase breaks starch into sugars such as glucose and maltose, and that is the food the microbes start with. The two teams then share the work: the bacteria eat sugars that the yeast cannot, and the yeast eats what the bacteria leave behind. This partnership is stable enough that bakers keep an unbroken line of starter from the first batch, and the original culture may be many years old.

The famous San Francisco sourdough is named after a bacterium, Fructilactobacillus sanfranciscensis, which was found in starters there. But despite its name it is not endemic to San Francisco. A starter's character depends on the flour, the temperature and how wet it is. A drier, cooler starter makes more acetic acid, the sharp, vinegary kind of sour, while a wetter, warmer one makes more lactic acid, the milder kind.

Starters are old companions. One of the oldest sourdough breads dates from 3700 BC and was found in Switzerland, while baker's yeast as a leavening agent dates back less than 150 years (a figure given in 2014). During the Klondike Gold Rush, miners carried a pouch of starter around the neck or on a belt, fiercely guarded to keep it from freezing: freezing does not kill a starter, but excessive heat does. The old hands came to be called "sourdoughs".