A sourdough starter is flour and water that has been left long enough for wild yeast and lactic acid bacteria to settle in and multiply. You feed it fresh flour and water, they eat, and they make two things a baker needs: gas, which raises the bread, and acids, which give sourdough its flavour and help it keep.
Those yeasts and bacteria are already on the flour, on your hands and in the air of your kitchen. You don’t add anything to make a starter. You give them food, warmth and time, and keep feeding them until they outnumber everything else in the jar.
Why not just use dried yeast?
Dried yeast is a single strain, bred to work fast. A starter is a whole community of yeasts and bacteria working slowly. That slowness is the point: a long rise gives the flour time to develop flavour, and the acids make a loaf with a chewy crumb and a crust that stays crisp for days.
What you need
- A clean glass jar of about 500 ml, with a loose lid
- Wholemeal or rye flour for the first few days
- Strong white bread flour for later feeds
- Kitchen scales that weigh in single grams
- A spoon or a small spatula, and a rubber band
The rubber band goes round the jar at the level of the starter after each feed. It’s the simplest way to see how much it has risen.
What to expect this week
For the first two or three days not much happens, then there’s a sudden burst of bubbles that dies away again. That false start catches almost everyone out. Keep going. By the end of the week your starter will rise and fall on a rhythm you can plan your baking around.