What is a sourdough starter?

To make a starter, flour and water are mixed and 'fed' with fresh flour regularly over several days. Wild yeasts and bacteria from the flour and environment multiply, producing carbon dioxide gas that makes dough rise. Lactic acid bacteria create lactic and acetic acids, giving sourdough its tangy flavour and helping it keep longer. A healthy starter can be kept alive indefinitely, and some bakeries claim to use starters passed down for many decades. Using wild fermentation to raise bread is thousands of years old, long before commercial baker's yeast was available.