Sourdough is the oldest form of leavened bread. Last year Seamus Blackley, father of the Xbox, baked a loaf using what was purportedly 4,500-year-old yeast scraped off ancient Egyptian pottery. All modern recipes begin with a starter, basically a flour-and-water slurry colonized by bacteria and wild, airborne yeasts that eat, breed and exhale carbon dioxide, which helps the bread rise. The tangy taste and brightly acidic smell derive from lactobacilli, cousins of the bacteria that curdle milk into cheese and yogurt. Starters are bespoke to the environments in which they were created; no two sourdoughs taste exactly alike.

Mr. De Smedt travels the world for new specimens. He prioritizes renown, unusual origins, the type of flour used, and the starter’s approximate age. “Most importantly, the sourdough must come from a spontaneous fermentation, and not inoculated with a commercial starter culture,” he said. He adds up to two dozen new sourdoughs to the library every year, from cooking schools, home-bakers, pizzerias, and artisan and industrial bakeries. “Sourdough is the soul of many bakeries,” he said. “When bakers entrust you with their souls, you’d better take care to it.”

He has harvested starters from 25 countries, including Slovenia, Peru and Singapore. Starter No. 1 is from Altamura, Italy. The bread is traditionally made of semolina flour, the ground form of durum wheat, and dates at least to 37 B.C., when the Roman poet Horace praised it as the best he had ever eaten.

“Number 100 is special because it’s Japanese and made with cooked sake rice,” Mr. De Smedt said. “Number 72 is from Mexico and has to be refreshed with eggs, lime and beer.” Number 43 is a sentimental favorite. “It’s a San Francisco starter, and was my first one I ever saw,” he recalled. “When I became a test baker for Puratos in 1994, one of my tasks was to refresh 43.” Indeed, he baked his first loaf of sourdough with it.

He has no way of knowing which of the 125 starters is the oldest. “We can’t carbon-date them,” he said. “The microbial colonies of a starter can change entirely, depending on how it is fed and maintained. If someone insisted she had a 500-year-old sourdough, I’d have to believe her.”

Two years ago Mr. De Smedt and a film crew tracked the path of the Klondike Gold Rush’s starter-packing prospectors, starting in Seattle, then Alaska, and ending the expedition in Dawson City, Yukon, in northern Canada. “At the turn of the 20th century, stampeders had to show mounted police at the Canadian border that they had enough provisions to survive a year in the Yukon,” he said. “In addition to potatoes and canned goods, the stampeders would bring starters, often in linen bags tied around their necks, so that they always had dough ready to make flapjacks.”

Mr. De Smedt hit the mother lode in Whitehorse, the Yukon capital, where he met up with Ione Christensen, the 86-year-old former mayor. Her starter was passed down from her great-grandfather, Wesley David Ballentine. “It’s a family pet, if you will,” she said. Back in 1897, Ballentine stowed the starter in a flour sack and trekked over the Chilkoot Pass on his way to the Klondike gold fields. On cold nights, he and his fellow stampeders would cuddle with the sacks to keep their contents warm and alive.