Baker Karl De Smedt is obsessed with bread—especially sourdough. But De Smedt is allergic to flour. So he needed a way other than baking to turn his doughy passion into a profession. De Smedt rose to the occasion, becoming the world’s only sourdough librarian. His research traces bread’s past—and studies how to make better bread in the future.

Fizzing, bubbling jars line the shelves of the Puratos Sourdough Library in St. Vith, Belgium, where De Smedt works. Unlike most libraries, you can’t borrow anything here. Instead, each jar holds a different starter, a fermented flour-water mixture that gives sourdough its tangy taste and helps it rise.

Some starters in De Smedt’s sourdough vault are very old. Some are new but unusual for reasons such as origin or rarity. #100 is from Japan. It is made from rice. #43 is from a famous San Francisco sourdough batch.

De Smedt started out as a baker at the Puratos Bakery. After several years, his allergy became a problem. De Smedt asked to begin displaying the various starters he and others were collecting. The sourdough library began officially in 2013. Today, there are 105 foaming specimens.

To begin a starter, bread makers place flour and water in a jar and set it aside. But it must be “fed” regularly. That means half the mixture gets scooped out and discarded (or used in another recipe), and then new flour and water is added. Over time, those simple ingredients undergo an amazing change. They’ll become bubbly and double in size—over and over. This is fermentation.

Fermentation happens because of two different living single-cell organisms: bacteria and “wild” yeast. That’s right—starters are alive! That’s why they must be fed regularly.

Both bacteria and yeast exist naturally in flour, air, and even on human hands. Mixed with water, they break down sugars and starches in the flour. This produces carbon dioxide. The gas makes bread rise and also affects its shape, color, and density.

The research has found connections among sourdoughs from around the globe. Two starters, one from Switzerland and one from Mexico, share a wild yeast that none of the others has, perhaps because both came from high altitudes.

Last year, the Puratos Sourdough Library sent identical flour and recipes to 16 bakers from 16 countries. Microbiologist Anne Madden found that bacteria from the bakers’ hands made each starter a bit different.

Remember the gospel song “He’s Got the Whole World in His Hands”? It’s true. God’s hands control everything, including the microscopic particles on the whole world’s hands.