Sourdough breads use a fermented batter-like dough starter to make them rise and enhance their flavour. A portion of the sourdough starter is mixed with the bread's ingredients, while the remainder is kept and 'fed' with more flour and water to use in future batches.
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Before commercial yeast became available, bakers would use a bubbling mixture of yeasts and bacteria (a leaven) to both aerate the crumb and give a sharper flavour to the wheat. You can make your own bread improver by creating a natural leaven: in a clean jar mix four teaspoons of wholemeal flour with four teaspoons of water, then add the same quantity of each every day for three days. After the initial three days, remove four-fifths of the mixture each day and replace with a half-and-half mixture of flour and water. Do this until the mixture bubbles and smells pleasantly sour. Then add 150g/5oz of this mixture to every 500g/1lb 2oz of flour in your recipe to help the loaf stay moist and give the crumb more flavour.
If you want to use this leaven to make bread without yeast, you will need to allow for longer proving times.