Sourdough is a dough containing a Lactobacillus culture in symbiotic combination with yeasts. It is one of the principal means of biological leavening in bread baking, the others using cultivated forms of yeast (Saccharomyces). It is important in baking rye-based breads, where yeast does not produce comparable results. Compared to breads made with baker's yeast, it produces a mildlysour taste because of the lactic acid produced by the lactobacilli.
The preparation of sourdough begins with a pre-ferment, (the "starter" or "levain", also known as the "chief", "chef" or "head"), made of flour and water. The purpose of the starter is to produce a vigorous leaven and to develop the flavour of the bread. In practice there are several kinds. The ratio of water to flour in the starter (the "hydration") varies and a starter may be a fluid batter or a stiff dough.
When wheat flour comes into contact with water, naturally occurring amylase enzymes break down the starch into maltose; the enzyme maltase converts the maltose sugar into glucose, which yeast can metabolize. Flour naturally contains a variety of yeasts and bacterial spores. With sufficient time, temperature, and refreshments with new or fresh dough, the mixture develops a balanced, symbiotic or stable culture. This culture will cause a dough to rise if the gluten has been developed sufficiently. The bacteria ferment sugars that the yeast cannot metabolise and their by-products are metabolised by yeast, which produces carbon dioxide gas, which leavens the dough.[note 1] Obtaining a satisfactory rise from sourdough takes longer than in a dough leavened with packaged yeast because the yeast in a sourdough is less vigorous. In the presence of lactic acid bacteria, however, some sourdough yeasts have been observed to produce twice the gas of baker's or packaged yeast. The acidic conditions in sourdough, along with the bacteria also producing enzymes that break down proteins, result in weaker gluten and may produce a denser finished product.