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Sourdough bread is leavened not by packaged yeast but by a living starter, a mixture of flour and water that captures wild yeasts and bacteria from its surroundings. The bacteria produce mild acids as they feed, giving the loaf its tang and helping it keep longer. Bakers maintain a starter by discarding part of it and feeding the rest with fresh flour, sometimes for years. The slow fermentation breaks down some of the flour's starches, which many people find makes the bread easier to digest than quick-risen alternatives.