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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 specialty bakeries. Sourdough starter is made with a small amount of old dough saved from a prior batch, and is sometimes called mother dough or chef. This small amount of old-dough starter contains the culture, and its weight is increased by additions of new dough and mixing or kneading followed by rest or leavening periods. A small amount of the resulting dough is then saved to use as old-dough starter for the next batch. As long as this starter culture is fed flour and water weekly, it can stay at room temperature indefinitely. Sourdough bread is made by combining the increased amount of starter with another new-dough addition, along with any other desired ingredients to make the final dough. The starter comprises about 20-25% of the final dough, though particular formulas vary and that ratio may be higher. This final dough may be divided and shaped, then is allowed to rise, and is followed by baking. 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 processes, refreshment ratios and rest times, culture and air temperature, humidity, and elevation also makes each batch of sourdough different.

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