Breadmaking is probably one of the oldest technologies of humankind. Likewise, the sourdough process as a fermentation medium is one of the most ancient biotechnological mechanisms for food production using cereals such as wheat and rye.
Sourdough bread is prepared from a mixture of flour and water that ferments spontaneously due to lactic acid bacteria naturally found in flour. This process produces a more gaseous mass and a more aerated bread that does not require yeast. It has a pleasant acid taste, firm consistency, and crispy crust.
Ref. Chavan, R. S., & Chavan, S. R. (2011). Sourdough Technology-A Traditional Way for Wholesome Foods: A Review. Comprehensive Reviews in Food Science and Food Safety, 10(3), 169–182. https://doi.org/10.1111/j.1541-4337.2011.00148.x