the preparation of sourdough begins with a pre-ferment (the "starter" or "leaven", also known as the "chief", "chef", "head", "mother" or "sponge"), 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.