Today, you can find a huge variety of breads on supermarket shelves, only a few of which are called “sourdough.” For most of human history, though, any bread that wasn’t flat was sourdough—that is, it was leavened with a wild community of microbes.

And yet we know surprisingly little about the microbes responsible for raising sourdough bread, not to mention making it more nutritious and delicious than bread made with commercial yeast.

For starters, where do the fungi and bacteria in a sourdough starter come from? Are they in the water or the flour? Do they come from the baker’s hands? Or perhaps they’re just floating around in the foggy air, as the bakers of San Francisco firmly believe?

This episode, Cynthia and Nicky go to Belgium with two researchers, fifteen bakers, and quite a few microbes for a three-day science experiment designed to answer this question once and for all. Listen in for our exclusive scoop on the secrets of sourdough.