Sourdough starter is a fermented mixture of flour and water, containing a colony of microorganisms including wild yeast and lactobacilli. 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 of starters, as the ratio of water to flour in the starter (hydration) varies. A starter may be a liquid batter or a stiff dough. It can be used in a number of preparations that would normally use other commercial yeasts or some other type of leavening agent, such as cakes, cookies, pizza dough, etc.