Sourdough bread is made by using a small amount of “starter” dough, which has a 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 carries years of history, from many hundreds of previous batches. As a result each bakery’s sourdough has a distinct taste. 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 prospectors. The nickname remains in Sourdough Sam, the mascot of the San Francisco 49ers.