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 to produce a specific flavor, such as that associated with the Candida milleri yeast and the Lactobacillus sanfranciscensis bacteria of San Francisco sourdough bread, but it is not necessary. All one needs to make a sourdough culture is flour and water.
The bacteria in a sourdough bread culture is Lactobacillus, or lactic acid bacteria, which is also used in such foods as sauerkraut, kimchi, pickles, cheese, and yogurt. It is this bacteria that gives this bread its characteristic sour flavor, while the yeast allows the bread to rise. The yeast and bacteria naturally present in flour are activated by mixing in water and leaving it in a warm environment. Different types of flour are more or less rich in microorganisms, and sometimes unwashed organic grapes are added to the mixture to increase the natural yeast and help the culture to develop. Diastatic malt may also be added to a young culture to provide extra sugars for the yeast to feed on.
A sourdough culture, also known as a starter or sponge, must be "fed" daily when it is young with fresh flour and water. As the culture matures, it can be refrigerated and fed less often. A part of the culture is used for every new loaf of sourdough bread. A well maintained sourdough culture can serve a baker for many years.
Sourdough is one of the oldest types of bread. The 100% rye bread common in Northern Europe is one example of sourdough bread, and baguettes and other Southern European breads were traditionally made with sourdough before the advent of faster-growing yeast cultures. In the United States, sourdough bread from San Francisco is particularly famous. Some San Francisco sourdough cultures have been in use for 150 years. The sweet Amish Friendship Bread traditionally passed on to friends is another American sourdough bread example.