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Sourdough bread relies on a living partnership rather than packaged yeast. A baker keeps a starter, a mixture of flour and water in which wild yeasts and bacteria thrive together. The yeasts produce gas that makes the dough rise, while the bacteria release acids that give the bread its tang and, importantly, slow the growth of moulds. This sourness is not just flavour; the acidic environment helps the loaf resist spoilage for longer than ordinary bread. Because the microbes vary from kitchen to kitchen, no two starters behave exactly alike, and bakers often guard a starter for years.