Starter

A sourdough starter is a living mixture of flour and water that has captured wild yeast and lactic acid bacteria from the flour, the air, and your kitchen. Once established, it is fed on a regular schedule so the yeast and bacteria stay active, and a portion of it is used in place of commercial yeast to leaven bread. A healthy starter smells pleasantly sour or yeasty, doubles or more after a feeding, and is full of bubbles at its peak.

The wild yeast produces carbon dioxide that gets trapped in the dough's gluten network, making the loaf rise, while the bacteria produce lactic and acetic acid that give sourdough its tang and help preserve the crumb. Because the culture is unique to your flour and environment, no two starters behave exactly alike, which is part of why timing and temperature matter more than any fixed recipe. A starter that is well fed and used near its peak gives the most reliable rise and the best flavor balance.

When choosing a jar to keep a starter in, look for a wide mouth for easy stirring and feeding, clear sides so you can see the rise, and enough headroom for it to double or triple without overflowing. A common mistake is baking with a starter that has already peaked and started to collapse, which leaves less lifting power for the dough. Another is keeping it too cold or too warm without adjusting the feeding schedule, which throws off how predictable your bakes are.