Why This Recipe Works
By nurturing a culture of naturally occurring bacteria and yeast, a healthy sourdough starter will leaven bread while also lending it the trademark sour flavor. For a simple, foolproof approach, we began by mixing all-purpose flour with whole-wheat flour, which provided extra nutrition for the developing bacteria and yeasts. We then added enough water to form a wet dough and let it sit at room temperature. After a few days, when it showed signs of life in the form of gas bubbles and a pungent aroma, we began a routine of daily feedings, mixing some of the culture with fresh flour and water to refresh the food supply. After 10 to 14 days, we found that the starter smelled pleasantly yeasty and doubled in volume 8 to 12 hours after the last feeding, a sign that it was ready to use to bake bread.
We also came up with an easy way to maintain the starter between uses. We found that we could refresh the food supply just once a week by letting the culture sit for 5 hours at room temperature after feeding it and then moving it to the refrigerator for storage.