Raise a starter in seven days

A starter is a culture of wild yeast and lactic acid bacteria, kept alive in a jar of flour and water. You do not need a packet, a special flour or a warm cupboard that never changes. You need a jar, a scale and the habit of showing up.

Begin with equal weights. Stir 50 g of whole-wheat or rye flour into 50 g of water, cover the jar loosely, and leave it somewhere near 22–26 °C. The first day often looks like nothing. That is fine. The organisms you want are already on the grain; they are simply not numerous yet.

On the second day, throw away half the mixture and feed what remains with another 50 g of flour and 50 g of water. Repeat that once a day. By the third or fourth day you may see bubbles and a smell that is sharp, then sweeter. A false rise sometimes happens early, driven by bacteria that will not stay. Keep feeding. The culture that lasts is the one that rises again after the smell settles.

Around day five to seven the starter should double within four to eight hours of a feeding, smell pleasantly sour, and fall back with a few streaks on the glass. That is ripe enough to leaven the classic boule. Save a spoonful before you bake, feed it, and it becomes tomorrow’s starter.

Two details matter more than the brand of flour. Use weight, not cups, so the hydration of the culture stays near 100%. And use water that tastes clean; if yours is heavily chlorinated, let it sit in an open jug for an hour first. Heat above 30 °C hurries the bacteria and can make the jar smell of solvent. Cooler than 20 °C only asks for patience.

When the jar doubles on a predictable schedule, it is no longer an experiment. It is an ingredient. How to keep it from here is the next note, on feeding and storing.