Types and Sources of Yeast

Bread can be made from a traditional, naturally fermented sourdough starter, called levain in French. Starters are prepared by mixing flour and water and allowing wild yeast and lactic acid bacteria in the flour and air to ferment the mix. Sometimes rye flour, onion, potato, or another source of food for microorganisms is added to the flour and water.

After a week or so of caring and feeding, the starter is ready to use. A portion of it is made into a preferment, then used to leaven a batch of bread. Because different microorganisms and different ways of handling a starter affect flavor, not all sourdough breads taste alike. While San Francisco sourdough bread is noticeably sour, French sourdough bread (pain au levain) is usually milder.