Dissolve 2 tbsp/15 g fresh yeast or 1 tbsp/7 g active dry yeast in ¼ cup/60 ml water and 1 tsp sugar. Stir in 2 cups/500 ml warm water and about 2 cups/250 g unbleached all-purpose flour—enough to obtain a thick but pourable batter. Cover the starter with a damp cloth and set it in a warm, draft-free place. Let it ferment at room temperature for three to five days before using. Commercial sourdough starters are also available.
To use a sourdough starter, add about ½ cup/125 ml starter for every 2 tbsp/15 g of yeast called for in the recipe. (For reliable results, replace only half, not all, the fresh yeast.) Whenever you use a starter, reserve at least 1 cup/250 ml and replenish it with as much flour and water (in equal amounts) as you have taken out. Sourdough starters should be nourished every four to five days by adding ¼ cup/30 g flour and ¼ cup/60 ml water.
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