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.
"The origins of bread-making are so ancient that everything said about
them must be pure speculation. 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"