I have started and killed more than half a dozen sourdough starters in my baking life, countertop vessels of bubbly, khaki-colored goo that have inspired curiosity and revulsion from a series of roommates. “What is … this?” they ask, gingerly holding what looks like long-abandoned oatmeal at arm’s length across the threshold to my room. An explanation and an exhortation to “smell it!” have not often been met with equal enthusiasm. Sourdough starter smells like the soul of bread—a cool, yeasty aroma that can make a centuries-old Parisian bakery of even the most meager outer-borough galley kitchen—but what is it, exactly? 

Every loaf of sourdough begins with sourdough starter, a culture of wild yeast in a mixture of flour and water, an ideal environment (warm, moist) for yeast (a single-cell fungus) to thrive. Yeast breaks down starch in the flour, producing sugar, ethanol, and carbon dioxide, which allows yeast to act as a leavening agent in bread. Most bread is baked with commercial yeast, which is highly refined and specialized, often sold in dehydrated form—active dry yeast —and needs to be rehydrated before use. Wild yeast remains hydrated within the starter, but is far less predictable, both in its flavor and its behavior. Because starters are cultivated from yeast existing ambiently in a given environment, no two starters are quite the same.

Any starter, also called a “mother” (in the kombucha sense), can be kept alive, theoretically, forever. Sourdough starter is the eternal bread flame. For $8.95 plus shipping, King Arthur Flour will send you one ounce of its century-old sourdough starter, which you can nurture into your own personal progeny, bequeath to your children, grandchildren, great-great-grandchildren. But unless all of you live in or around Norwich, VT, where King Arthur Flour is based, your starters will diverge into wholly new cultures of yeast and bacteria.

In terms of kitchen acumen required, maintaining a sourdough starter ranks somewhere between hand-churning butter and keeping basil alive—a combination of knowing what you’re doing and submitting to the fickle will of nature. It's like a pet, or a plant. Starter can turn if not maintained properly with regular, carefully balanced feedings; some bakers insist on using only wooden or silicone utensils to stir their starter when feeding it, as certain metals can impart a bad taste.

There is a romance to even the science of sourdough, in the knowledge that San Francisco sourdough is molecularly different from sourdough in Austin, or Montreal, or Provence. Sourdough isn’t baked any differently in San Francisco, but, just as New York City bagels are mythically unlike bagels anywhere else in the world, ambient wild yeast, bacteria, and microorganisms that contribute to the unique environment of San Francisco won’t be found anywhere else in the world. 

Most of my bygone starters have been neglected to death, forgotten behind a gallon of apple cider in the fridge only to be rescued two weeks later, thick with wet, gray mold, scraped defeatedly into the trash. (More questions from the roommate.) The disappointment of losing a starter is like that of losing a houseplant—another victim of my parenting—but the costs of starting over are significantly lower.

To make your own sourdough starter, combine equal quantities of unbleached all-purpose flour and water in a bowl, mixed together to form a paste. Cover with plastic wrap or a clean kitchen towel and leave somewhere more or less “room temperature” for 24 hours. The next day, throw out half of the starter. Add equal portions of flour and water to the remaining half, stir, recover, and leave for another 24 hours. This constitutes one “feeding.”

Repeat this process until the starter is bubbly and smells strongly of yeast. This may take only a day or two, or it may take longer depending on the temperature, elevation, and humidity of your kitchen, as well as the general character of your local wild yeast. Once your starter is “going,” it can be kept in the fridge and fed about once a week. One or two days before you want to bake bread, feed your starter on a 24-hour cycle so it’s really fired up. Discarded starter (the half taken out during feeding) can be an excuse to make other bready things with tangy, tasty wild yeast, including pancakes, biscuits, popovers, waffles, pretzels, and even pizza crust.