Yeast, Bacteria and Sourdough

Yeast and Bacteria don’t really sound like things we want in our food but actually most of them aren’t so bad. They are just tiny little organisms that live all around us, in the air, in the water, on plants, fruits and grains. They also live inside of us. Yeast is a microscopic type of Fungi, mushrooms are also a type of fungi and we like them! Most bacteria and yeast are very helpful to us but some are dangerous and can make us sick. Those are called Pathogens.


A sourdough is dough in which wild yeasts and bacteria have been cultivated. Once water is added to flour it is kept and maintained in a warm place. This creates a perfect environment for the micro-organisms to thrive and multiply. After several days, as they reproduce, the dough will contain a far higher concentration of yeast and bacteria than normal.


 A mature sourdough will take between 5 and 7 days to create but will take over a month before a fuller flavour will be developed. It can then be kept “alive” indefinitely for as long as it is looked after. Some sourdoughs have been known to be over 100 years old, handed down through generations of baker’s.


Lactobacillus is a friendly bacterium which feeds on sugars and excretes lactic acid. The lactic acid gives the sourdough its distinct sour flavour. It also kills off other micro-organisms which tend to spoil food and this is why sourdough bread has such good keeping qualities. 


Wild yeasts within the dough feed on sugars present in the flour and release alcohol and carbon dioxide. This is called fermentation. The carbon dioxide gets trapped and causes the dough to “rise”. In case you were wandering, no you won’t get drunk from eating lots our bread. The minute traces of alcohol present in the dough get vaporised as the bread bakes.


Strains of these micro-organisms vary from region to region, so the flavour and characteristics of the dough will differ slightly from place to place, for instance a San Francisco Sourdough made in London will not taste anything like an original San Francisco Sourdough.