Sourdough starter
A sourdough starter is a living culture of flour and water in which wild yeast and lactic acid bacteria coexist. The yeast provides lift, the bacteria provide acidity and flavour. That symbiosis is what separates sourdough from commercially yeasted bread: it rises more slowly, keeps longer and tastes more complex.
A starter is built from scratch over one to two weeks of regular feeding with flour and water, then maintained indefinitely. A mature starter rises predictably after feeding, stays domed rather than collapsed and smells tangy and fruity, without a sharp vinegar or acetone note.
How fast it works is governed by temperature and feeding ratio: in a warm kitchen it peaks within hours, in a cold one it can take a day or more. Rye starters are usually more active than wheat ones because rye flour offers the culture more to feed on.