Sourdough is probably the first agent to be used for bread fermentation. Egyptian written records show that it was used approximately 1500 BC – and we know that cereals have been used for 10,000 already. Egyptians milled grains, mixed them with water and the resulting paste was dried over a fire. It seems that sourdough was discovered quite by accident, actually the yeasts and bacteria present in the environment made it into the milled grains and caused it to ferment.
Until the discovery of the microscope and L. Pasteur studying microorganisms it was not known what was the cause of dough fermentation. Sometimes the bread fermented, sometimes not! The bread experts always kept a part of a good sourdough for the next kneading of bread.
Which microorganisms are actually in sourdough?
Sourdough occurs spontaneously – the source of the microflora of sourdough are the grains (but also the environment in which the dough is made!) As an example we can mention the grain of wheat that holds up to 70 species of bacteria and 10 species of yeasts on its surface.
This are lactic acid bacteria which consume sugars from flour and the products of their metabolism are lactic acid and some aromatic compounds. Some of these bacteria are Lactobacillus casei and Lactobacillus plantarum. Other lactic bacteria present in grains produce less lactic acid and more acetic acid and aromatic compounds. An example of these bacteria are Lactobacillus brevis and Luconostoc mesenteroides.
The omnipresent yeast is Saccharomyces cerevisiae, but we can find other like Candida humilis, Candida krusei, Pichia saitoi and Turulopsis Holmii.
Sourdough has a low pH, it possesses active protease enzymes that hydrolyze the network of gluten which therefore makes the dough more elastic. The resulting amino acids allow Maillard reaction to take place on the surface of bread when baking. The result id the brown coloring and flavor of bread crust.
The addition of sourdough affects the quality of the bread so that the crust is harder, but at the same time more elastic and less crumbly. The baked bread contains more humidity and the shelf-life increases. Sourdough serves as a natural preservative in bread (lactic, acetic and propionic acids inhibit the growth of molds and yeasts).
It seems that the bread manufactured with sourdough has a lower glycemic index compared to bread prepared with yeast. The acids present partially degrade phytates, compounds that bind different minerals, and thus their bioavailability increases (calcium, iron).
Jerko Penić: Aktivno kiselo tijesto, Novi pekar, Prosinac 2010.