Sourdough is one of the oldest forms of grain fermentation. It's believed to have originated in ancient Egypt around 1,500 BC and remained the customary form of bread leavening until baker's yeast replaced it a few centuries ago (1Trusted Source). A leavened bread is a bread whose dough rises during the bread-making process as a result of gas being produced as the grain ferments. Most leavened breads use commercial baker's yeast to help the dough rise. However, traditional sourdough fermentation relies on "wild yeast" and lactic acid bacteria that are naturally present in flour to leaven the bread. Wild yeast is more resistant to acidic conditions than baker's yeast. This is what allows it to work together with lactic acid-producing bacteria to help the dough rise. Lactic acid bacteria can be found in several other fermented foots, including yogurt, kefir, pickles, sauerkraut and kimchi.