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, due mainly to 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 speciality bakeries. Sourdough bread is made by using a small amount (20-25 percent) of starter dough (sometimes known as "the mother sponge"), which contains the 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, culture and air temperature, humidity, and elevation also makes each batch of sourdough different.