A sourdough starter is a living culture of wild yeast and friendly bacteria, grown from nothing more than flour and water. It's what makes sourdough bread rise and gives it its signature tangy flavour — used in place of commercial baker's yeast.
When you mix flour and water and leave it, you capture wild yeasts (naturally present in flour and the environment) and lactic acid bacteria. Fed regularly over a week or two, these multiply into a stable, active culture. The yeast produces the gas that makes bread rise; the bacteria produce the acids that give sourdough its tang and help preserve the loaf. It's natural fermentation, the way bread was made for thousands of years before packaged yeast.
Starters are hardy — a grey liquid on top ('hooch') just means it's hungry, and a sluggish one usually revives with a few feeds. Many bakers keep the same starter going for years. Our free sourdough bake log and beginner's sourdough guide help you start baking. Follow a trusted recipe and basic food hygiene.