Sourdough

Sourdough is a symbiotic culture of lactobacilli and yeasts used to leaven bread. Sourdough bread has a very distinctive taste, due mainly to the lactic acid produced by the lactobacilli.
Sourdough bread is made by using a small amount of so-called starter dough, which has the yeast culture, and mixing it with new flour and water. Part of this resulting dough is then saved to use as the starter next time. It is not uncommon to have a baker's starter dough that has had years of history, from many hundreds of previous batches.

Sourdough was the main bread made in Northern California during the California Gold Rush, and it remains a major part of the culture of San Francisco. The bread became so common that sourdough became a general nickname for the gold prospecters. The nickname remains in Sourdough Sam, the mascot of the San Francisco 49ers.