Reading Comprehension

Read the passage and answer the question.

Sourdough bread relies on a living culture called a starter, a mixture of flour and water teeming with wild yeasts and bacteria. Unlike commercial baker's yeast, which produces a fast, uniform rise, the microbes in a starter work slowly and produce acids that give sourdough its characteristic tang. Bakers must feed the starter regularly with fresh flour, otherwise the culture weakens and the bread fails to rise. Each region's wild microbes differ slightly, which is one reason a loaf baked in one city can taste noticeably different from one baked elsewhere using the same recipe.