Well. . . it is a dough with a sour taste.
Now: why this dough is sour?
Because of fermentation.
There is a bacillus which converts the maltose, present in the dough, in lactic acid. This is the acid of yogurt.
There is another one that converts the alcohol resulting for the action of yeast in acetic acid. This is the acid of vinegar.
Lactic acid is not harmful to our bodies.
The second one: vinegar; is no good to our health.
There is a kind of sourdough made with wheat flour only. The fermentation of wheat is very acetic. The bread made with wheat sourdough is a vinegary one. Some people like it. I cannot take it. Too sour.
Some artisanal bakers reduce the acid level by processing the dough at a low temperature in order to slow down the reproduction of the acidic bacillus. This kind is known as French Sourdough; “Pain au Levain” and San Francisco Sourdough.
There is another kind of sourdough made with rye instead of wheat. This one is very popular in North Europe. Its sourness comes mostly from lactic acid, it is light and causes no harm to our bodies. I like this one.
I do not bake wheat sourdough but only rye sourdough. With a careful control of the fermentation I can reduce its sourness to a delicate tang that completes the complex taste of the resulting rye sourdough bread.
It is possible, as well, to reduce the acidity of rye sourdough to zero by adding a small amount of commercial yeast. In this case we have the best of both worlds. The resulting bread is not acidic at all, with a rich delicious taste and easier to digest. This is the kind I like the most.