What Is Sourdough?

Sourdough is a natural leaven, and a remarkably simple one: you can raise it from nothing but flour — wheat or rye — and water.

Bread is older than metal. By some accounts it goes back to around 10,000 BC. Sourdough is among the oldest ways of leavening it: a culture of wild yeasts and lactic acid bacteria, living together alongside other microbes. Bread raised this way carries the gentle acidity and depth of flavour that those bacteria produce as they ferment.

Which yeasts and bacteria take hold, and in what proportion, depends on where you are — the temperature, the humidity, the altitude. Sourdough from one place does not taste like sourdough from another. And if you keep the same culture going for years, it drifts further still, shaped by the hands that feed it. Every starter ends up with a character of its own.

Getting started with sourdough bread

How a sourdough culture comes to life

1. Wheat flour is about 70% starch; rye flour about 60%. When that starch meets water, amylase — an enzyme already present in the grain — begins breaking it down into sugars, maltose among them.

2. Lactic acid bacteria, living on the surface of the grain and in the air, feed on that maltose and produce further sugars such as glucose and fructose.

3. Wild yeasts feed on the glucose and fructose in turn, growing and multiplying.

4. Carbon dioxide is produced throughout all of this, and that is what lifts the dough.