Sourdough or sourdough bread is a bread made by the fermentation of
dough using wild lactobacillaceae and yeast. Lactic acid from
fermentation imparts a sour taste and improves keeping qualities.
In the Encyclopedia of Food Microbiology, Michael Gaenzle writes: "One
of the oldest sourdough breads dates from 3700 BCE and was excavated in
Switzerland, but the origin of sourdough fermentation likely relates to
the origin of agriculture in the Fertile Crescent and Egypt several
thousand years earlier", which was confirmed a few years later by
archeological evidence.[3] "Bread production relied on the use of
sourdough as a leavening agent for most of human history; the use of
baker's yeast as a leavening agent dates back less than 150 years.