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.

Sourdough is among the oldest and most widespread forms of bread leavening. For most of human history, it was the standard method, until barm from beer brewing replaced it in the Middle Ages. Barm was later supplanted in the late 19th and early 20th centuries by industrially produced baker’s yeast. According to the *Encyclopedia of Food Microbiology*, one of the oldest known sourdough breads dates to 3700 BC and was found in Switzerland, but the practice likely began with agriculture in the Fertile Crescent and Egypt thousands of years earlier. Pliny the Elder described several sourdough leavening methods. Sourdough remained common in Europe until barm took over, and after 1871, purpose-cultured yeast became widespread.

French bakers brought sourdough techniques 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 sourdough is often linked to 1849 prospectors, they more commonly used commercial yeast or baking soda. The famous San Francisco sourdough is a white bread with a strong sour taste. Its starter contains a specific strain of *Lactobacillus*, now called *Fructilactobacillus sanfranciscensis*, along with the yeast *Kasachstania humilis*.

The sourdough tradition also traveled to Alaska and the Yukon during the Klondike Gold Rush of 1898. Conventional leavening agents were unreliable in those harsh conditions. Miners and settlers often carried a pouch of starter around their neck or on a belt, guarding it from freezing. Freezing does not kill a starter, but excessive heat does. Experienced old-timers became known as “sourdoughs,” a term still used for Alaskan or Klondike veterans. This nickname was immortalized in Robert Service’s *Songs of a Sourdough*.

In English-speaking countries, where wheat bread is common, sourdough is no longer the standard leavening method. It was gradually replaced by barm and later by cultured yeasts after Louis Pasteur’s germ theory was accepted. 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 bread, some products marketed as sourdough are leavened with baker’s yeast or chemical agents instead of, or in addition to, a live starter. The Real Bread Campaign calls such breads “sourfaux” and advises against most modern breads due to these techniques and additives like baking soda and baking powder, which may be linked to celiac disease.

To make sourdough, a pre-ferment called a starter (also known as the chief, chef, head, mother, or sponge) is used. This is a live mixture of flour and water containing wild yeast and lactobacilli. The starter’s purpose is to produce vigorous leavening and develop flavor. Starters vary in hydration—the ratio of water to flour—and can be a liquid batter or a stiff dough. To create a starter, flour and water are mixed and left in a warm place for a week or two. When wheat flour meets water, the enzyme amylase breaks down starch into glucose and maltose, which the natural yeast can metabolize. With time, warmth, and regular refreshments (adding fresh flour and water), a stable culture develops. This culture causes dough to rise. The bacteria ferment starches the yeast cannot, producing by-products like maltose, which the yeast then uses to generate carbon dioxide gas, leavening the dough.

As the starter ferments, sometimes for days, its volume is increased by periodic additions of flour and water—called refreshments or feedings. If fed regularly, the starter remains active. The ratio of fermented starter to fresh flour and water, known as the refreshment ratio, is critical for development and maintenance. Higher ratios lead to greater microbial stability. In San Francisco sourdough, this ratio is 40% of the total weight, roughly equivalent to 67% of the new dough’s weight. A high refreshment ratio keeps the pH of the refreshed dough relatively low.