Sourdough bread is unique because it does not require any store-bought yeast, as do other rising breads. It features a symbiotic yeast and bacteria culture that arises naturally from microorganisms present in flour. A starter culture can be added, especially if you wish to produce a specific flavor, such as that associated with the Candida milleri yeast and the Lactobacillus sanfranciscensis bacteria of San Francisco bread, but it is not necessary. All you need to begin making the bread is flour and water.
The type of flour that you choose is important. Not only will it affect the flavor of the final product, but it will also make a difference in the ease of growing and maintaining a culture. Organic flours have more natural microorganisms, and wholemeal bran flour has the most. In some cultures, bakers add unwashed organic grapes to the mixture to provide additional natural yeast. Adding a bit of diastatic malt or a starter culture can help push things along, but purists frown upon the use of a starter culture over developing sourdough naturally.
Place 1 cup (125 g) of flour blended with 1 cup (236.5 ml) of warm water into a jar to begin your sourdough culture. Any wide-mouthed jar with plenty of room for your starter will work, but a clear glass jar is nice, since it allows you to see how the culture is developing throughout the mixture. Leave the jar in a warm environment, about 70° to 80°F (about 21° to 27°C). Temperatures exceeding 100°F (38°C) will kill the culture. There is no need to cover the jar, but if it looks dry, placing a cloth or paper towel over the mouth of the jar can help.
The next step in making your sourdough starter is to "feed" it once a day until it is bubbly throughout and has a pleasant, yeasty, beer-like smell. To feed it, throw half away and add another 0.5 cup (62.5 g) of flour blended with 0.5 cup (118.2 ml) water. It should be ready in a few days, but culture growing times vary widely. When the starter looks healthy, put it in the refrigerator with a lid on. There should be a little breathing room, so if you are using a jar with a screw-on lid, poke a hole in the top.
At this point, continue feeding the culture once a week. As it ages, you may not need to feed it as often. Once again, the bubbliness, scent, and consistency of the mixture are indications of how well it is doing. You can stir your culture as often as you like. A dark brown, alcoholic liquid known as hooch may begin to accumulate on the top. Do not drink it; either pour it off or stir it in according to your preference and the moistness of the mixture.
When you are ready to make sourdough bread, prepare a sponge well in advance by pouring your entire starter into a bowl and mixing in 1 cup (125 g) of flour and 1 cup (236.5 ml) warm water. Wash the jar out well, as you will use it to store any leftover sponge to become your new starter. The sponge will take a few hours to become frothy throughout and sour-smelling, at which point it is ready to bake with.
There are many sourdough bread recipes available in cookbooks and on the Internet, and you can experiment using your dough in any recipes you find. For a simple loaf, add 4 teaspoons (16.8 g) sugar and 2 teaspoons (12 g) salt to 2 cups (473 ml) of the sponge, retaining the rest of the sponge in the refrigerator for future baking. You can also add 1 to 2 tablespoons (17.7 to 35.5 ml) of olive oil or softened butter or margarine (14.2 to 28.4 g), if you wish. Knead in flour, 0.5 cup (62.5 g) at a time, until you have a bread dough of a good consistency; you will use approximately 3 cups (375 g) of flour, but use your judgment.
Next, allow the dough to rise until it has doubled in size. This should take at least an hour, but different sourdough cultures perform differently. When the dough is ready, punch it down, knead it a bit more, and make a loaf on a baking sheet covered with cornmeal or grease to prevent the bread from sticking. You can slit the top of the loaf, if you like, then cover it with a paper towel and allow it to rise in a warm environment until it has once again doubled in size.
Finally, bake your bread for 30 to 45 minutes in a 350°F (177°C) oven that has not been preheated. Bake it until the crust is brown and the loaf makes a hollow sound when struck with a wooden spoon. Let it cool for an hour on a rack or towel before eating. A well maintained culture can serve you for several years.