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A sourdough starter is a live culture of wild yeast and bacteria maintained in a simple mixture of flour and water, which bakers use to leaven bread instead of commercial yeast. Unlike commercial yeast, which is a single cultivated strain, a starter typically hosts a diverse community of microorganisms drawn partly from the surrounding environment and partly from the flour itself, and this diversity contributes to the tangy flavour associated with sourdough bread. Maintaining a starter requires regular feeding with fresh flour and water, usually daily, because the microbial community will otherwise exhaust its food supply and become sluggish or die off. Many bakers report that starters kept for years can develop distinctive local characteristics, though controlled studies suggest that flour type influences flavour more consistently than a starter's geographic origin.