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Sourdough bread relies on a living culture of wild yeasts and bacteria rather than commercial baker's yeast. When flour and water are mixed and left at room temperature, naturally occurring microbes begin to feed on the starches, producing the gases and acids that give the dough its characteristic rise and tangy flavour. Bakers maintain this culture by regularly discarding part of it and feeding the remainder with fresh flour and water.

The dough rises because the microbes in the culture —fermentfreezefilter sugars in the flour and release gas, while the sour taste comes from the —acidsmineralsfibres they produce during the same process.