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A sourdough starter is a living culture of wild yeast and bacteria maintained in a simple mixture of flour and water, refreshed regularly to keep the microorganisms active and the mixture from developing harmful mold. Unlike commercial yeast, which is a single cultivated strain, a starter hosts a diverse community whose exact composition depends on the flour used, the local environment, and even the baker's hands, meaning no two starters are chemically identical. Bakers often pass starters between generations or friends, and some culinary traditions claim certain starters have been continuously maintained for over a century through repeated feeding. Scientists studying these cultures have found that while the specific microbial populations vary widely, most starters converge on a similar balance between yeast, which produces gas for rising, and bacteria, which contribute the characteristic sour flavor through acid production.