Sourdough (or, more formally, natural leaven or levain) refers to the process of leavening bread by capturing wild yeasts in a dough or batter, as opposed to using a domestic, purpose-cultured yeast such as Saccharomyces cerevisiae. Sourdough more specifically refers to a symbiotic culture of lactobacilli and yeasts, giving a distinctively tangy or sour taste (hence its name), due mainly to the lactic acid and acetic acid produced by the lactobacilli. Sourdough is no longer the standard method for bread leavening in most developed countries, as 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 (20-25percent) of "starter" dough (sometimes known as "the mother sponge"), which contains the yeast 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 daily, 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, yeast culture and air temperature, humidity, and elevation also makes each batch of sourdough different.
A sourdough starter is a stable symbiotic culture of bacteria and yeast present in a mixture of flour and water. The yeasts Candida milleri or Saccharomyces exiguus usually populate sourdough cultures symbiotically with Lactobacillus sanfranciscensis. . Lactobacillus sanfranciscensis (bacteria) was named for its discovery in San Francisco sourdough starters.
Often a starter will consist of basic items such as: water, bread flour, rye flour and a sourdough starter which can be purchased at certain grocery stores. Once the starter is made water and flour must be added in time increments over a period of days. Depending on the locale of the bakery and the type of bread being made, the starter can be either a relatively fluid batter or a stiffer dough; as a general rule, more sour breads are made with a liquid starter. Firm starters (such as the Flemish Desem starter) are often more resource-intensive, traditionally being buried in a large container of flour to prevent drying out.
A fresh culture begins with a mixture of flour and water. Fresh flour naturally contains a wide variety of yeast and bacteria spores. When wheat flour contacts water, naturally-occurring amylase enzymes break down the starch into complex sugars (sucrose and maltose); maltase converts the sugars into glucose and fructose that yeast can metabolize. The lactobacteria feed mostly on the metabolism products from the yeast. The mixture develops a balanced, symbiotic culture after repeated feedings.
There are several ways to increase the chances of creating a stable culture. Unbleached, unbromated flour contains more microorganisms than more processed flours. Bran-containing (wholemeal) flour provides the greatest variety of organisms and additional minerals, though some cultures use an initial mixture of white flour and rye flour or "seed" the culture using unwashed organic grapes (for the wild yeasts on their skins). Using water from boiled potatoes also increases the leavening power of the bacteria, by providing additional starch. Bakers recommend un-chlorinated water for feeding cultures. Adding a small quantity of diastatic malt provides maltase and simple sugars to support the yeasts initially.
The flour-water mixture can also be inoculated from a previously maintained culture. The culture is stable due to its ability to prevent colonization by other yeasts and bacteria as a result of its acidity and other anti-bacterial agents. As a result, many sourdough bread varieties tend to be relatively resistant to spoilage and mold.
The yeast and bacteria in the culture will cause a wheat-based dough, whose gluten has been developed sufficiently to retain gas, to leaven or rise. Obtaining a satisfactory rise from sourdough, however, is more difficult than with packaged yeast, because the lactobacteria almost always outnumber the yeasts by a factor of between 100 and 1000, and the acidity of the bacteria inhibit the yeasts' gas production. The acidic conditions, along with the fact that the bacteria also produce enzymes which break down proteins, result in weaker gluten, and a denser finished product.
The other manner of using sourdough starter is common for making quick breads or foods like pancakes. It involves using baking soda (and sometimes baking powder) to neutralize some or all of the acid in the starter, with the acid-base reaction generating carbon dioxide to provide lift to the dough or batter in a manner very similar to Irish soda bread. This technique is particularly common in kitchens where the starter is intentionally kept off-balance, with a substantially high acid level, and is particularly associated with areas such as Alaska.
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 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. The sourdough starter, however, had to be kept warm to survive. Experienced miners and other settlers frequently carried a pouch of starter either around their neck or on a belt and were often fiercely guarded. 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 US. In contrast to the majority of the country, it has remained in continuous production for nearly 150 years, with some bakeries 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 the 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.
Other recipes use starters that aren't actually truly natural leavens. The Italian Biga and French Poolish add sourdough-like flavors to breads by allowing the yeast a lengthy half-day or longer fermentation. Unlike a true sourdough, these recipes usually start with commercial yeast, and cultivation of lactobacillus bacteria is generally an incidental effect.