Sourough (or, more formally, natural leaven or levain) refers to the process of leavening bread by capturing wild yeasts in a dough or batter, as opposed to using a domestic, purpose-cultured yeast such as Saccharomyces cerevisiae. Sourdough more specifically refers to a symbiotic culture of lactobacilli and yeasts, giving a distinctively tangy or sour taste (hence its name), due mainly to the lactic acid and acetic acid produced by the lactobacilli. Though no longer the standard method for bread leavening in most developed countries (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), some form of natural leaven is used in many specialty bakeries.