Natural leaven

A natural leaven is a mixture of flour and water in which naturally occurring yeast and bacteria have begun to ferment and reproduce. After the initial mixture is prepared, more flour and water is added every 24 hours to provide these microorganisms with fresh nutrients. After several days, the leaven should be frothing due to the carbon dioxide being released by the fermenting yeast. A quantity of the leaven, which is often called a starter, can then be used to prepare the bread dough.

All leavened bread was made with natural yeast cultures up until compressed baker's yeast became available in the late 19th century. Because commercially produced baker's yeast is concentrated, it acts much faster than a starter does; it can take many hours for naturally leavened bread to rise. The longer fermentation time allows lactobacilli to start fermenting and producing acids that give naturally leavened bread a tangy flavour. Because of its distinctive taste, the American-English term for it is sourdough bread.

Want to find out more? Look at the chapter on pre-ferments in the book flour and water.

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