Sourdough is a leavening agent containing naturally occurring yeast and lactobacillus bacteria. The term is used to describe bread baked with sourdough, while the sourdough itself is referred to as a starter. In addition to leavening the bread, the fermentation process produces lactic acid, which gives the bread its distinctive sour taste and improves its keeping qualities.
History Sourdough is one of the most ancient and popular forms of leavening bread. It was the standard method of breadmaking for most of human history until the Middle Ages, when it was replaced by barm. Barm, in turn, was replaced in the late 19th and early 20th centuries by industrially produced baker's yeast. The Encyclopedia of Food Microbiology states: "One of the oldest sourdough breads dates from 3700 BC and was excavated in Switzerland, but the origin of sourdough fermentation likely relates to the origin of agriculture in the Fertile Crescent and Egypt several thousand years earlier", and "Bread production relied on the use of sourdough as a leavening agent for most of human history; the use of baker's yeast as a leavening agent dates back less than 150 years from the year of 2014". Pliny the Elder described several methods of sourdough leavening to make bread. Sourdough remained the usual form of leavening down into the European Middle Ages until being replaced by barm from the beer-brewing process, and after 1871 by purpose-cultured yeast. French bakers brought sourdough techniques to Northern California during the California Gold Rush, and it remains a part of the culture of San Francisco today. (The nickname remains in "Sourdough Sam", the mascot of the San Francisco 49ers.) Sourdough has long been associated with the 1849 gold prospectors, though they were more likely to make bread with commercial yeast or baking soda. The "celebrated" San Francisco sourdough is a white bread characterized by a pronounced sourness, and indeed the strain of Lactobacillus in sourdough starters is named Fructilactobacillus sanfranciscensis (previously Lactobacillus sanfranciscensis), alongside the sourdough yeast Kasachstania humilis (previously Candida milleri) found in the same cultures. The sourdough tradition was carried into the Department of Alaska in the United States and the Yukon Territory in Canada during the Klondike Gold Rush of 1898. 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 them from freezing. However, 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 or Klondike old-timer. The significance of the nickname's association with Yukon culture was immortalized in the writings of Robert Service, particularly his collection of "Songs of a Sourdough". 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 beer making, then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts. Although sourdough bread was superseded in commercial bakeries in the 20th century, it has undergone a revival among artisan bakers and, more recently, in industrial bakeries. In countries where there is no legal definition of sourdough bread, the dough for some products named or marketed as such is leavened using baker's yeast or chemical raising agents as well as, or instead of, a live sourdough starter culture. The term sourfaux has been applied to such breads by the Real Bread Campaign, an initiative that has advised against the consumption of most modern breads (sourdough and otherwise) because of such preparation techniques plus the inclusion of leavening agents, such as baking soda and baking powder, which may be linked to celiac disease.
Preparation
Starter Sourdough leavening requires a pre-ferment (the starter or leaven, variously known as the chief, chef, head, mother, or sponge), a live mixture of flour and water, containing a colony of microorganisms including wild yeast and lactobacilli. The purpose of the starter is to produce a vigorous leavening and to develop the flavour of the bread. In practice there are several kinds of starters, as the ratio of water to flour in the starter (hydration) varies. A starter may be a liquid batter or a stiff dough. To initially create the pre-ferment, water and flour are mixed and left in a warm place for a week or two. When wheat flour comes into contact with water, the naturally occurring enzyme amylase breaks down the starch into the sugars glucose and maltose, which sourdough's natural yeast can metabolize. With sufficient time, temperature, and refreshments with new or fresh dough, the mixture develops a stable culture. This culture will cause a dough to rise. The bacteria ferment starches that the yeast cannot metabolise, and the by-products, chiefly maltose, are metabolised by the yeast, which produces carbon dioxide gas, leavening the dough.
Refreshment or feeding of the starter
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