Sourdough is a type of bread made using wild yeast and bacteria that live naturally in flour and the air around us. Instead of adding packets of store-bought yeast, bakers create a sourdough starter: a mixture of flour and water that they feed regularly, letting wild microorganisms grow and bubble inside it. This starter becomes a living culture that bakers keep alive for months or even years, using a bit of it each time they make bread.

The wild yeast makes the dough rise slowly, and the bacteria produce acids that give sourdough its characteristic tangy, slightly sour flavor. The longer, slower fermentation also creates that crispy, golden crust and chewy texture that makes sourdough special.

Sourdough is one of humanity's oldest bread-making methods, used for thousands of years before anyone understood the science behind it. During the California Gold Rush of 1849, prospectors became so associated with sourdough bread (which they could make at remote mining camps) that they were nicknamed sourdoughs. Today, many bakers treasure their starters like living companions, feeding them regularly and even giving them names. Some bakeries use starters that have been kept alive and passed down for generations.