King Arthur Flour offers a good recipe for sourdough starter. This sourdough starter does not use commercial yeast. The flour and water attract wild yeast from the air to start the culture.
On the first day, combine 1 cup of rye, pumpernickel or whole wheat flour with 1/2 cup distilled water. Mix until all the flour is wet. Whole grain flour provides the nutrients to encourage the growth of wild yeast. Chlorinated water interferes with yeast reproduction. Cover the container loosely and place in an area where temperatures are approximately 70 degrees Fahrenheit for 24 hours.
Remove one-half of the starter and discard it or save it for another purpose. Add 1/2 cup of distilled water and another cup of flour. Stir to mix the ingredients.
By day three, the starter should form bubbles and develop a fruity flavor. Begin weighing out 4 ounces of starter, discarding any excess and adding the same amount of water and flour as before, repeating the process every 12 hours for up to a week until the starter is very active.
For the first loaf of bread, remove approximately 8 ounces of starter. Weigh out 4 ounces of the remaining starter, transfer it to a crock and feed it. Refrigerate the starter for future use. On baking days, remove it from the refrigerator, feed it, place it in a warm spot and wait 12 hours before removing the starter for the loaf.