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. It is important in baking rye-based breads, where yeast does not produce comparable results. Sourdough bread has a mildly sour taste not present in most breads made with baker's yeast, and better inherent keeping qualities than other breads due to the lactic acid produced by the lactobacilli. In the presence of lactic acid bacteria some sourdough yeasts have been observed to produce twice the gas of baker's 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.