Sourdough: Bread That Is Alive

A Sourdough is the oldest form of leavened bread, and also, in a sense, the most alive. What raises it is not the packet of yeast a modern baker buys from a shop but a starter: a soft, living culture of flour and water in which wild yeasts and particular bacteria have made their home. Mixed into fresh dough, this culture ferments it, producing the gas that lifts the loaf and the acids that give sourdough its name. A bread is called sourdough precisely because it is raised by such a natural culture rather than by manufactured yeast. Making one from scratch requires nothing but flour, water and patience: left to stand and fed with fresh flour each day, a simple paste will, within a week or two, begin to bubble and turn sour of its own accord, as wild microbes drift in from the flour and the surrounding air and settle down to work. No two starters turn out quite the same, which is part of their fascination.

B The starter is really a tiny partnership between two kinds of microbe. The wild yeasts feed on the sugars in the flour and give off carbon dioxide, the gas that inflates thousands of little bubbles and makes the dough rise. Alongside them live lactic acid bacteria, or lactobacilli, which produce the sour-tasting acids that flavour the bread and, by making the dough acidic, help to keep the finished loaf from going mouldy as quickly as ordinary bread does. Yeast and bacteria live together in a stable balance, each thriving on what the flour, and the other, provides. The acids the bacteria produce do more than flavour the loaf. They also change the very texture of the dough, strengthening it and giving a good sourdough its characteristic chewy crumb, its open holes and its crisp, blistered crust, so that the microbes shape not only the taste of the bread but its whole character.

C For almost all of human history, every raised bread was a sourdough of one kind or another, for there was simply no other way to make dough rise. The practice is ancient, and is usually traced back to Egypt thousands of years ago. This long reign ended only in the nineteenth century, when scientists learned to isolate and grow a single fast-acting species of baker's yeast. This purified yeast raised dough quickly and predictably, and without any sourness, and it swept through industrial bakeries, pushing the slow old cultures to the margins. For much of the twentieth century sourdough survived mainly in out-of-the-way places and in a few stubborn local traditions, such as the famous sour loaves of San Francisco, rather than in the ordinary bakery, where speed and predictability had become everything and the long wait of a natural culture was simply bad business.

D In recent years sourdough has been prized not only for its flavour but for possible benefits to health. During the long, slow fermentation, the bacteria and yeasts break down some of the components of the flour that can be hard to handle, including a portion of the gluten and a substance called phytic acid that hinders the body's absorption of minerals. Partly for this reason, many people report finding sourdough easier to digest than quickly risen bread, and some studies suggest it may raise blood sugar more gently, though the research is still going on. None of this makes sourdough a miracle food, and a great deal depends on how long the dough is left to ferment and on the kind of flour that is used. Even so, the slow method clearly does something to bread that the fast one does not, and that difference is now the subject of serious scientific attention.

E Perhaps the most charming feature of sourdough is that a starter is a living heirloom. Because the particular yeasts and bacteria in each one depend on the flour, the kitchen and the very hands that tend it, no two starters are exactly alike, and each is in effect a small local ecosystem. Fed regularly with fresh flour and water, a starter can be kept alive indefinitely, and some bakeries treasure cultures that have been handed from baker to baker for decades, or even longer. This blend of science, craft and inheritance lies behind the worldwide revival of artisan sourdough today. In recent years, when many people found themselves at home with time on their hands, the keeping of a starter became a small craze, and countless kitchens acquired a bubbling jar on the shelf, fed and watched over as carefully as a household pet. This mixture of the very old and the very new has given the oldest of breads an unexpectedly bright future.