Sourdough

Sourdough is a leavening agent made from naturally occurring yeast and lactobacillus bacteria. The term refers to bread baked with this agent, while the agent itself is called a starter. The fermentation process creates lactic acid, which gives the bread its characteristic sour flavor and helps it stay fresh longer.

One of the oldest known sourdough breads, dating to 3700 BC, was found in Switzerland, though the practice likely began with the rise of agriculture in the Fertile Crescent and Egypt thousands of years earlier. For most of human history, sourdough was the standard method for leavening bread. It was eventually replaced in the Middle Ages by barm from beer brewing, and then in the late 19th and early 20th centuries by industrially produced baker’s yeast. Pliny the Elder recorded several sourdough leavening methods. After 1871, purpose-cultured yeast became common.

French bakers brought sourdough to Northern California during the Gold Rush, and it remains part of San Francisco’s culture—the city’s NFL mascot is named Sourdough Sam. Although often linked to 1849 prospectors, they more frequently used commercial yeast or baking soda. The famous San Francisco sourdough is a white bread with a strong sour taste; its starter contains the bacterium *Fructilactobacillus sanfranciscensis* and the yeast *Kasachstania humilis*. During the Klondike Gold Rush of 1898, sourdough starters were carried into Alaska and the Yukon, as conventional leavenings were unreliable in those conditions. Miners kept pouches of starter on their person to prevent freezing—though freezing does not kill a starter; excessive heat does. Experienced settlers became known as “sourdoughs,” a term still used for Alaskan or Klondike old-timers, immortalized in Robert Service’s *Songs of a Sourdough*.

In English-speaking countries, sourdough is no longer the standard leavening method for wheat breads. It was gradually replaced by barm, then by cultured yeasts after Louis Pasteur’s work on germ theory. Though commercial bakeries largely abandoned sourdough in the 20th century, it has seen a revival among artisan bakers and, more recently, in industrial bakeries. In places without a legal definition of sourdough, some marketed breads use baker’s yeast or chemical leaveners instead of, or alongside, a live starter. The Real Bread Campaign calls such products “sourfaux” and advises against most modern breads due to these techniques and additives like baking soda and powder, which may be linked to celiac disease.

To make sourdough, a pre-ferment called a starter is needed—a live mixture of flour and water containing wild yeast and lactobacilli. The starter’s purpose is to create vigorous leavening and develop flavor. Starters vary in hydration, from liquid batter to stiff dough. To create one, flour and water are mixed and left in a warm place for a week or two. The enzyme amylase in wheat breaks down starch into sugars like glucose and maltose, which the natural yeast can use. With time, warmth, and regular refreshments, a stable culture forms that can leaven dough. The bacteria ferment starches the yeast cannot, producing maltose that the yeast then converts into carbon dioxide, raising the dough.

The starter is maintained by periodic additions of flour and water, called refreshments or feedings. As long as it is fed regularly, it stays active. The ratio of fermented starter to fresh flour and water—the refreshment ratio—is critical for stability. Higher ratios improve microbial stability. In San Francisco sourdough, this ratio is 40% of total weight, roughly 67% of the new dough’s weight, keeping the pH low.