In a room of refrigerators in Belgium live more than 110 jars of flour, water, and magic.

At least, that’s how you might sometimes think of sourdough starters, the cultures of bacteria and yeast that bakers mix into dough instead of commercial yeast to produce bread of delicious complexity, with a biting acidity or a firm creaminess, depending on the recipe.

Starters are made by leaving an inviting slurry of flour and water out on a counter and waiting until microbes colonise it, turning it bubbly and sour. But most bakers don’t have DNA sequencers on hand to see exactly what is living in their starters, which often look, smell, and taste different. To boot, just where the microbes come from? The air? The flour? The baker’s own microbiome?

Even whether a starter, refreshed with flour and water and passed from person to person over a hundred years, is really the same thing at the end that it was at the beginning, is an open question.

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Luckily, biologists and bakers are starting to delve into the mysteries of sourdough. To get an inside look at a very interesting experiment, Cynthia Graber and Nicola Twilley, the team behind Gastropod, a podcast about food culture and science, went with a pair of microbial ecologists to Belgium, to the sourdough library of Karl de Smedt. (You can listen to the resulting Gastropod episode here.)

Housed in a room at a company called Puratos, where de Smedt is head of the Centre for Bread Flavour, the library began to take its current shape when a Syrian baker of traditional chickpea cookies contacted de Smedt and asked if he would help document and preserve his starter, relates Twilley. The baker’s sons were interested in shifting from the traditional starter leavening to commercial yeast. He hoped that de Smedt would help keep the starter from disappearing.