With an activated culture you are ready to start the first proof, which will last about 12 hours. During this time the lactobacilli produce the unique sourdough flavor that can be obtained in no other way. At the same time the yeast continues to multiply, starts to produce the gases that will leaven the dough, and conditions the gluten to trap those gases. Many bakers refer to the dough at this stage as the “sponge.”
Use a 4-quart mixing bowl, as the dough will at least double in volume. Place the culture in the bowl, and add 2 cups warm water: mix briefly. This will result in a more even dispersion of the culture as the flour is added. Then add 3 cups flour, a cup at a time, Mixing briefly between cups. Again, the mixture need not be lump-free. Cover the bowl with a dry cloth or plastic wrap. Do not use a damp cloth, as the evaporation will cool the culture. Place in a proofing box or other warm location for approximately 12 hours. In general, a shorter fermentation will produce a milder flavor and a longer one a stronger or more sour flavor. Some fast cultures may use all the nutrients and become semidormant in this 12-hour time. If the sponge retracts a bit, there may be some dried crust on the surface and on the sides of the bowl. Stir all of the into the dough before proceeding to the second proof.
Two things are essential. Before you remove some of the sponge to save as the culture for the next time, don’t add anything except flour and water. In particular, never add baker’s yeast or milk. At the end of the first proof, return at least 1 cup of the culture, or whatever is left over from the recipe, to clean culture container before starting the second proof. (If you feed this with a cup each of flour and water and leave it in a warm place for an hour before refrigerating, it will be easier to reactivate the next time.) These precautions will keep your wild culture free of contamination from the other ingredients of bread and will maintain a pure culture for future use.