Sourdough is a dough containing a lactobacillus culture, usually in symbiotic combination with yeasts. It is one of two principal means of leavening in bread baking, along with the use of cultivated forms of yeast (Saccharomyces). It is of particular importance in baking rye-based breads, where yeast does not produce comparable results. In comparison with yeast-based breads, it produces a distinctively tangy or sour taste, mainly because of the lactic acid produced by the lactobacilli; the actual medium, known as “starter” or levain, is essentially an ancestral form of pre-ferment. In English-speaking countries, where wheat-based breads predominate, sourdough is no longer the standard method for bread leavening. It was gradually replaced, first by the use of barm from beermaking, then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts. However, some form of natural leaven is still used by many speciality bakeries.

Sourdough bread is made by using a small amount (20-25 percent) of starter dough (sometimes known as “the mother sponge”), which contains the culture, and mixing it with new flour and water. Part of this resulting dough is then saved to use as the starter for the next batch. As long as the starter dough is fed flour and water weekly, the sourdough mixture can stay in room temperature indefinitely and remain healthy and usable. It is not uncommon for a baker’s starter dough to have years of history, from many hundreds of previous batches. As a result, each bakery’s sourdough has a distinct taste. The combination of starter, culture and air temperature, humidity, and elevation also makes each batch of sourdough different.

Sourdough likely originated in Ancient Egyptian times around 1500 BC, and was likely the first form of leavening available to bakers. Sourdough remained the usual form of leavening down into the European Middle Ages until being replaced by barm from the beer brewing process, and then later purpose-cultured yeast.

Bread made from 100 percent rye flour, which is very popular in the northern half of Europe, is usually leavened with sourdough. Baker’s yeast is not useful as a leavening agent for rye bread, as rye does not contain enough gluten. The structure of rye bread is based primarily on the starch in the flour, as well as other carbohydrates known as pentosans; however, rye amylase is active at substantially higher temperatures than wheat amylase, causing the structure of the bread to disintegrate as the starches are broken down during cooking. The lowered pH of a sourdough starter therefore inactivates the amylases when heat cannot, allowing the carbohydrates in the bread to gel and set properly. In the southern part of Europe, where baguette and even panettone were originally made with wheat flour and rye flour, sourdough has become less common as the standard of living has risen; it has been replaced by the faster growing baker’s yeast, sometimes supplemented with longer fermentation rests to allow for some bacterial activity to build flavor.

Sourdough was the main bread made in Northern California during the California Gold Rush, and it remains a part of the culture of San Francisco today. The bread became so common that “sourdough” became a general nickname for the gold prospectors. The nickname remains in “Sourdough Sam”, the mascot of the San Francisco 49ers.

The sourdough tradition was carried into Alaska and the western Canadian territories during the Klondike Gold Rush. Conventional leavenings such as yeast and baking soda were much less reliable in the conditions faced by the prospectors. Experienced miners and other settlers frequently carried a pouch of starter either around their neck or on a belt; these were fiercely guarded to keep from freezing. Ironically, freezing does not kill a sourdough starter; excessive heat does. Old hands came to be called “sourdoughs”, a term that is still applied to any Alaskan old-timer.

San Francisco sourdough is the most famous sourdough bread made in the U.S. today. In contrast to sourdough production in other areas of the country, the San Francisco variety has remained in continuous production for nearly 150 years, with some bakeries (e.g., Boudin Bakery among others) able to trace their starters back to California’s territorial period. It is a white bread characterized by a pronounced sourness (not all varieties are as sour as San Francisco sourdough), so much so that the dominant strain of lactobacillus in sourdough starters was named Lactobacillus sanfranciscensis. Sourdough also became popular because of its ability to combine well with seafoods and soups such as cioppino, clam chowder and chili.

Sourdough has not enjoyed the popularity it once had since bread became mass-produced. However, many restaurant chains, such as Cracker Barrel, keep it as a menu staple. Manufacturers make up for the lack of yeast and bacteria culture by introducing an artificially made mix known as bread improver into their dough.