Sourdough baking starts with… a starter! Wild yeast and lactobacilli are harvested from fresh air and grow in a mixture of flour and water. This is achieved simply by leaving the flour and water mixture exposed to air for a few days. Once activity can be seen in the mixture (air bubbles forming), the starter is maintained by decanting a small amount into a new container and feeding with more flour and water. Refreshed like this every day, the starter can be maintained indefinitely at room temperature or if not required for imminent baking, can be stored in the fridge for a week or two before needing a refresh.
In the fermentation process, yeast is used to breakdown glucose (from the flour) into carbon dioxide and ethanol. The carbon dioxide creates bubbles in the dough, producing the nice air pockets in the finished loaf. Lactobacilli also feed on sugars and produce lactic acid – putting the ‘sour’ into sourdough.
…carbon dioxide creates bubbles in the dough, producing the nice air pockets in the finished loaf. Lactobacilli also feed on sugars and produce lactic acid – putting the ‘sour’ into sourdough.
This starter is used to create a ‘levain’ or preferment – a small mixture of starter, flour and water allowed to rise a few hours ahead of the full dough mix which incorporates the levain with more flour, water and salt.
See the Sourdough Starter article for more information.
During the last hour of the levain rise, flour and water for the full dough is mixed together and allowed to sit for an hour. In this phase, gluten development begins – forming a network of proteins that gives the dough it’s elasticity and smooth texture. Also at this time starch begins being broken down into glucose.
Bulk fermentation occurs when wild yeast and lactobacilli from the levain get to work on the whole batch of flour and water.
The dough is placed in the fridge for 12-24 hours to ‘retard’ wild yeast activity and allow lactobacilli to proliferate – this is when sour flavour is developed.
If the dough production has gone well, we’ll see some good ‘oven spring’ as the bread fills out into a lovely voluminous loaf.