A starter made with the traditional sourdough method is often created simply by combining flour and water, which is allowed to ferment by airborne yeast. Some sourdough starters may include some commercial yeast to speed the fermentation process. Many sourdough starters require periods of fermentation followed by feeding with additional flour. Several days may elapse before they are ready to use. The recipe for a sourdough starter will differ depending on the type of sourdough bread being prepared, so it is best to follow the instructions on the recipe for the correct proportions of ingredients and specific procedures. Once the sourdough starter has been prepared, a small piece of the dough can be reserved to be used later as the starter for a new batch of bread. It can be stored for several days in the refrigerator or it can be frozen if it cannot be used until later.