Bread develops its light, airy texture thanks to fermentation and the development of a protein in flour called gluten. When yeast, a microscopic plant in the fungus family, meets a warm moist environment -- say, a bowl of tepid water -- it begins to reproduce rapidly. The hungry little yeasties feed off the sugars and complex carbohydrates in flour and release alcohol and carbon dioxide. The CO2 gets trapped within the strands of gluten, giving the dough its characteristic rise. When the risen dough is baked, the heat of the oven kills the yeast and burns off the alcohol, and the fluffy porous structure that we know and love remains.
The earliest bread-bakers didn't have access to packets of standardized yeast; they relied on capturing wild yeast. Various strains of yeast have been cultivated for their consistent behavior and are now readily available to both commercial and home bakers. But sourdough breads still rely on wild airborne yeast or cultures that have been passed down through generations. Fortunately, we don't have to relearn ancient hunting skills to stalk the elusive wild yeast, nor do we need any exotic equipment. All it takes is some flour and water and a little bit of patience.
Marty's Spokane Sourdough Starter
Mix two cups of unbleached all-purpose flour with two cups of warm (but not hot) water. Stir vigorously, then let the bowl sit uncovered on the kitchen counter. After a few hours -- or maybe a few days, depending on how much yeast is in the air -- you should begin to see bubbles, a sign of fermentation. Next, begin feeding the culture three times daily, stirring in equal amounts of flour and warm water each time. In the morning, dump out all but a cup or two of the starter mix, then begin the feeding schedule again. After about three to four days, your starter should be ready to use in baking.
To keep starter dormant but alive, place it in the refrigerator. Revive it by bringing it to room temperature and feeding it again for a day until it is frothy and lively once again.