Sourdough bread is a bread made by allowing the dough to ferment using
naturally occurring lactobacillaceae and yeast before baking. The
fermentation process produces lactic acid, which gives the bread a sour
taste and improves its keeping-qualities.
Sourdough bread is one of the most ancient forms of bread, and it was
the standard method of breadmaking for most of human history. It was
replaced by barm during the Middle Ages, which was then replaced by
industrially produced baker's yeast in the late 19th and early 20th
centuries. Sourdough can be used to make a wide variety of baked goods.