Sponge de John Spottiswood

A starter is a mixture of flour, water, and other ingredients that's been colonized by wild airborne yeast and friendly bacteria. These one-celled immigrants lend the starter--and the breads made with it--a special character. Sourdough starter, for example, contains a strain of yeast that's tolerant of the lactic and acetic acids produced by the lactobacilli. Those acids give sourdough bread its characteristic tang. The French use a soupy starter called a poolish to make their breads, while the Italians use a thicker one called a biga. Up until the late 19th century, all yeast breads were leavened with starters, and keeping a starter alive in its crock was a routine household chore. To keep your own starter alive, wait until it's established, then store it in an airtight container in the refrigerator. To keep it healthy, bring it to room temperature once a week and remove all but about 25% of it (either make bread with it or discard it). Replace what you've taken with a mixture of equal parts warm water and flour, stir, then return it to the refrigerator. Properly maintained, a starter can last for decades, developing an ever more distinctive character as it ages. To use a starter to make bread, remove some of it (usually about 2 cups), and use it in place of other forms of yeast. Replace the amount you took with a mixture of equal parts flour and warm water. Discard your starter if it becomes orange or pink, or if it develops an unpleasant odor. It's easy to make starters from scratch, but even easier to borrow some from a friend. Since sourdough starters must be colonized by strains of yeast and lactobacilli that are particular to certain regions (like San Francisco), a homemade starter might not yield sour bread. Your best bet is to get a powdered sourdough starter mix from your supermarket or a mail order supplier.