Meet Karl De Smedt, the manager of Belgium's Puratos World Sourdough Library—home to 125 sourdough starters from around the world. De Smedt has traveled the globe searching for rare and old sourdough starters with varying flavors and histories, from sake sourdough in Japan to a sourdough fed with beer in Mexico.

On a recent episode of Milk Street Radio, the sourdough librarian spoke with Christopher Kimball about everything from the oldest starter he’s encountered to the role of yeast in bread baking. Get an extended look at their conversation from the excerpts below, then listen to the full interview here on our website or via Apple Podcasts.

On success with sourdough

People give up too fast. You will notice that when you start a sourdough, the first couple of days it stinks. It can really stink, and you think that you did something wrong, but you just have to keep going. A sourdough starter is actually very simple to make. It’s a blend of flour and water that comes to life by feeding it for six or seven days daily with fresh water and flour.

On sourdough’s lifespan

The oldest starter we have in our possession, we don’t know how old it is, but we have had it in our possession since 1989. You can have starters that go on for generations and generations. Two years ago when I was in Whitehorse, Canada, this woman has a sourdough starter that came from her great, great grandfather from the Klondike Gold Rush dating back to 1896.

On yeast

Yeast is a living single-celled organism that is capable of converting sugars into alcohol and carbon dioxide. Its responsibility in bread baking is to make the bread rise and in beer it’s creating carbon dioxide to make the beer bubbly. It’s a gas producer.

On sake sourdough

There’s a bakery in Tokyo, Kimuraya Bakery, and they have this starter made from cooked rice and malted rice. It originates from the moment in Japan in 1874 that the Emperor was taking all the power and the samurai suddenly were all out of the job. A lot of them became independent and Mr. Kimura became a baker and learned how to make sourdough, only he was not happy with the flavor of it. He had a friend who was doing sake and so they started to convert the flour to rice and they ended up with this amazing sake sourdough.

On getting attached

People compare having a starter to having a baby or a good friend. Everybody takes good care of it because it’s something you brought to life by yourself.

Quotes have been edited for clarity.

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