People ask how I got into sourdough. The short answer is a retirement, a free afternoon, and an unhealthy interest in fermentation science. The longer answer involves fourteen-thousand years of human history and the realisation that the loaf sitting on my counter was, biologically speaking, quite remarkable.

At its simplest, sourdough is bread leavened by a community of wild yeast and bacteria — no commercial yeast involved. But that simplicity hides a considerable amount of chemistry. The panel below is a starting-point reference.

This is the first in a series of twelve science cards. Each one covers a specific aspect of sourdough baking, from microbiology to technique. Start here, then follow the series.