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Sourdough bread relies on a living culture rather than packaged yeast. A sourdough starter is a mixture of flour and water that, when left at room temperature, attracts wild yeasts and bacteria from the surrounding air and the flour itself. These microbes feed on the flour's sugars and release gases that make the dough rise, along with acids that give the bread its characteristic tang. Bakers keep a starter alive by regularly discarding part of it and feeding the rest with fresh flour and water. A well-tended starter can last for many years, and some bakeries proudly maintain cultures passed down across generations.