Sourdough (kvas, kvásek)

Under Czech Decree No. 18/2020 sourdough (kvas or kvásek) is a fermented semi-product of flour, water and a starter culture (rye flour for rye sourdough) whose microorganisms are active enough to acidify the dough. Mature sourdough can be propagated repeatedly, is made without additives or enzymes, and its acids arise from fermentation alone. So-called stable sourdough, by contrast, is dried or concentrated and may be additionally acidified.

Sourdough is a community of lactic acid bacteria and yeasts in which the bacteria dominate and yeasts are one to two orders of magnitude fewer. The bacteria produce mainly lactic and acetic acid, the yeasts produce the carbon dioxide that raises the dough, and both groups shape aroma and flavour. It is kept going by inoculating fresh flour and water with part of the previous batch.

In Bohemia traditional sourdough was made of water and mostly rye flour and built up in stages until it formed 50–60% of the recipe. Longer fermentation gives a fuller but sourer bread. The French counterpart is levain, the Italian lievito madre. Czech home recipes also use kvásek for a yeast sponge, which is not sourdough.