With origins in ancient Egypt, sourdough is the oldest form of leavened bread, and although the bread is made around the world, it’s most closely tied with San Francisco. Sourdough from San Francisco is special, and researchers have discovered specific bacteria in the foggy air that lends the loaves an extra-sour flavor, but in the early days, it was pretty unappetizing.

Sourdough came to California with the influx of prospectors during the Gold Rush as a cheap food source that required minimal upkeep and could last indefinitely. The loaf was so pervasive that prospectors were known as “sourdoughs,” but their loaves were often cooked over the fire in the same pan they used to pan for gold, resulting in dense loaves with a strong sour taste.

It wasn’t until European immigrants arrived in San Francisco that the bread’s reputation began to change. French and Italian immigrants began producing sourdough that appealed to the masses, and the city’s Boudin Bakery, which was founded by French immigrants in 1849, has been in continuous operation ever since and using the same sourdough starter the whole time.