There are many different ways of making Sourdough bread. But basically:
In bread making yeast is normally used to ferment the ingredients. With sourdough the ingredients (usually just flour and water) are mixed together without yeast and allowed to stand for several days or weeks.
The fermentation happens due to the friendly bacteria and natural enzymes in the flour and the air. (In San Francisco USA the atmosphere is thought to harbour natural bacteria and natural yeasts giving a distinct flavour to bread.)
Every day the baker takes some of the fermenting sour dough to use to make bread by adding further ingredients. It is essential that the left over sour dough mixture is remixed with fresh ingredients from one day to the next. In some bakeries this process has been repeated for years and even centuries - from the same dough.
There are even companies that specialise in making “starter cultures” to help bakers make their sourdough bread.
Sourdough bread has a distinctive “richer” flavour and a firmer chewier texture compared with regular bread.
At Fosters Bakery we have many different sour doughs perpetually fermenting in large insulated fermentation vats. Every hour we stir the vat and every day we use some of the sour dough and then replenish it for the next day.
We make 100% sour dough bread and also breads where we add sour dough to conventional bread.