Sourdough is a bread product made by a long fermentation of dough using naturally occurring yeasts and lactobacilli. In comparison with breads made quickly with cultivated yeast, it usually has a mildly sour taste because of the lactic acid produced by the lactobacilli. 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 mildly sour taste because of the lactic acid produced by the lactobacilli.