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If you mix flour and water into a paste and leave it in a warm place for some time, it will go "sour" (this generally takes about 48 hours). What actually happens is that the wild yeasts present in the wheat, rye or the environment are usually the strain Saccharomyces exiguous, which thrives in surroundings more acid than normal bread dough.
The sour taste of this dough comes from lactic and acetic acid produced by bacteria which feed on the maltose in the flour starch. In normal bakers" bread, the maltose would be rapidly consumed by Saccharomyces cervisiae (common bakers yeast), leaving little food for S. exiguous, which therefore wouldn"t thrive.
Bakers yeast, used in most factory-made British breads, permits rapid fermentation, and with chemical "improvers" this can be speeded up further - such bread has little chance of developing much flavour.
Rye bread, in particular, benefits from being made with sourdough instead of (or in addition to) bakers yeast - otherwise it can taste bland.
As well as a flavour boost, sour-doughs" have the further advantage of having much better keeping qualities than their yeast-made cousins.