Sourdough is a dough containing 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 specialty bakeries.

Sources: From Wikipedia, the free encyclopedia.

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 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. 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. 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.

When wheat flour contacts water, naturally occurring amylase enzymes break down the starch into disaccharides (sucrose and maltose); maltase converts these 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. Some 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 because of 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 100:1 and 1000:1, 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.

Sources: From Wikipedia, the free encyclopedia.