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The Sourdough Library

Join the sourdough librarian as he presents his collection of 133 starters from 25 countries kept in mason jars to be preserved for future generations and research.

Published onMay 20, 2021
The Sourdough Library

Editors note: This publication contains the video of the talk from the Fermentology webinar series, as well as a lightly edited transcript of the lecture.


The sourdough library is unique in the world. Right now, 133 starters from 25 countries are kept in mason jars to be preserved for the future. Their biodiversity is kept for the following generations and all subjects of study. Studies conducted with different universities from around the world. This amazing collection of wheat, rye, rice, wholegrain, durum wheat etc. is maintained following the original protocol and the original flour.  Join the sourdough librarian, Karl De Smedt, for this exciting talk!

Header image: Studies on bread and bread making at the University of Minnesota in 1899 and 1900 by Harry Snyder (public domain).

Watch the talk

The Sourdough Library with Karl De Smedt


I'm pleased to introduce you to the one and only Sourdough Library, located in the eastern part of Belgium, very close to the border with Germany. This is probably one of the most unique places within the baking world, that we inaugurated back in 2013. So we have a couple of years behind us, and hopefully many, many more beautiful years ahead of us. We created the Library because we had a broad collection of sourdoughs already in-house after completing a couple of studies on sourdough.

Sourdough is one of these very traditional ingredients that are used in the baking business that have, well, disappeared. Sourdough has been around for more than 5,000 years, some sources say 14,000. I recently heard from people in Australia that they found baked breads that date to 18,000 years ago. But what we believe is that all these generations of bakers that have baked breads wherever on Earth, whatever recipes, whatever ingredients or raw materials they had available, they probably always tried to bake the best breads that they could. And what we are doing today is still that-—trying to make better bread than what we did yesterday.

Therefore, we believe that the Sourdough Library will help us to reincorporate that ingredient that has been around for so many years back into the baker's life, because we have to be honest: sourdough is an ingredient that has disappeared over the last 150 years.

Revitalizing sourdough in the baking industry

When commercial yeast made its entrance within the bakery world, many bakers stopped making and maintaining sourdough, and they switched en masse to the use of yeast because they were able to make bread fast. They were able to make more breads in the same amount of time, and feed more people. It was a kind of miracle product that has been widely adopted. And as such, the knowledge of making sourdough has not been transmitted the way it could have, or it should have.

So with this library we try to acquire knowledge that we can disperse or distribute back into the bakeries so that we start using that ingredient again. Because we see, and we know, from recent studies—and there are a lot of studies ongoing—that sourdough is really a very important ingredient in increasing the nutritional value of a bread, or just to make a bread more digestible for people who suffer from bad digestion when they eat bread. With sourdough, we see this indigestion happening less—unless you have celiac disease. Of course then we cannot help you, even with sourdough in the recipe.

Origins of the Sourdough Library

So we started studying sourdoughs. The very first sample we collected was back in 1989 within the company I work for, Puratos. We are a worldwide ingredient supplier for bakeries, patisserie, and chocolate, in existence since 1919. And in 1989, we collected the first sample of sourdough from San Francisco that was brought back by a colleague who retired last year. He worked all these years for us in the research department. When I started to work for the company in 1994, it was the very first sourdough I saw in my life. I was a trained baker, patissier, and chocolatier here in Belgium. I worked for six years in a patisserie in Brussels. And in school, or even in that bakery or patisserie, we never saw a sourdough. We never learned what it was. We never used it. So I discovered it here, and that sample is still standing right here in the Library. It's jar number 43.

When I started, one of my very first tasks was to refresh that sourdough—to feed it with flour and water, to ferment it. It was a new world opening to me because it smelled funky. It was nice. It was something I'd never smelled before. I baked my very first sourdough bread with that starter, so I have a special relationship with it.

People ask me, “Which one is your favorite?” I do not have favorites because it's like asking a parent, which child is your favorite? But with this one, I have a special relationship because it was the first one. And they say you’ll never forget the first one.

Italian sourdoughs & biodiversity

So that was the first one we collected. And over the years, we started to acquire a sourdough from a region in the South of Italy called Altamura that was already renowned 2000 years ago, described by the Roman poet Horace. And he said, whenever you are in the region of Apulia, you should take bread with you for your onward journey, because it's the best bread you can find within the Roman Empire. We were intrigued by that story, and we've been in that town and we collected sourdough. We were able to duplicate that and turn it into one of our commercial sourdough products. We analyzed what was inside of that sourdough, and that study was so interesting that we said, “Well, let's have a look on the biodiversity of what you find in Italian sourdoughs.” And together with Professor Marco Gobbetti, who was working in Bari in those days—today he's also active in Bolzano in the North of Italy—we started up a research project. We had, in total, 37 Italian sourdoughs from mainland Italy, Sicily, and Sardinia, and we discovered 400 different strains of microorganisms, which we did not really expect.

We had no idea of this biodiversity. And little by little, we started looking deeper. At the same moment we had a colleague in Greece telling us, “But my mother, she has a sourdough based on basil leaves.” So we looked into those sourdoughs. And then we had some examples from Hungary. We ended up with this collection of 43 sourdoughs. We still hadn’t had the idea of creating a library. Now the Library itself is a non-for-profit initiative of our company because we can do it, and we believe we should do it in order to acquire that knowledge again.

Mission of the Sourdough Library

Now the three reasons why what we do here in this Library is:

  1. First of all, to preserve the biodiversity of sourdough

  2. To protect the tradition and heritage of sourdough

  3. And last but not least, for the sourdough that we have here to serve as a backup in case something goes wrong in the original bakery the sourdough comes from.

Preserving biodiversity

And so we have these 133 sourdough starters right now. Our latest arrival is number 133. That's a sourdough from Panera Breads, a well-known chain bakery in the United States. There are also some recent additions that have yet to arrive. We know who they are from and where they come from, but they still haven't gotten here due to all these COVID issues.

We keep the sourdoughs in the fridge at 4° C, which is just a normal fridge temperature. They have different colors and different consistencies. Some are very liquid, some are stiff. Number 59 is a stiff brown sourdough from France, and next to it is the sourdough from La Brea from Nancy Silverton. We also have two sourdoughs from Maison Kayser, which is a bakery in Paris but also a well-known bakery all over the world.

And so this collection of Italian sourdoughs that we started with, as I mentioned, gave us over 400 different strains of microorganisms. Today we can say that we have, out of these 133 sourdoughs, over 1,500 different strains.

Maintaining the starters

The way to preserve the starters here in the library is that every two months we feed them with the original flour that the owners send—and I say owners because these sourdoughs do not belong to us. We do not use them for commercial purposes. As I said in the beginning, it is a not-for-profit initiative. Although number 1, number 43, number 69 and number 101 are starters that belong to us, and that we can eventually use for commercial purposes because they are ours. But all the rest do not belong to us and we can only use them to study them and to have that backup here in case something goes wrong.

We feed them every two months with the original flour. Now when someone does not send the flour at a certain moment, I cannot maintain the starter. We know that it's dead. We had an issue with the supply of that flour for one or another reason, and we had to let it go.

Collecting the starters

Fortunately, from every starter that we get into the library, we collect three jars. So when I go to a country, or when it's too far, we have this nice collector box with cooling elements inside that guarantees conservation of 96 hours, which is more than time enough from wherever it comes to get here in Belgium.

One jar is analyzed in one of our microbiological labs. We measure the TTA, or the total amount of organic acids. We measure the pH. We measure the amount of lactic acid and the amount of acetic acid. These kind of things we can do in-house.

Then a second jar is sent to a university in Italy where they do the main part of the work. It takes almost 45 working days, more or less three months, on Petri dishes with different substrates to grow yeasts and to grow lactic acid bacteria. The Petri dishes are inoculated with sourdough that has been diluted and then they start growing the colonies. Then they isolate them on different Petri dishes, and they build up an amount of Petri dishes until they are certain that all these colonies that are present in the sourdough are isolated. Once they are isolated, we put them into little tubes and store them in boxes, and they go in a freezer at minus 80°, which is very cold. 80° Celsius or Fahrenheit is almost the same. It's very cold. That’s how we preserve the biodiversity.

So luckily, from starters, we always have the microorganisms. So if I could get that flour back here, I would be able to resurrect a sourdough.

Jar number 51 is an empty jar. And that's because it's a sourdough from France, from the region of Toulouse, and the problem was that it was a spelt sourdough. And the farmer actually stopped growing the spelt, so the baker did not have the flour anymore to send to me. So these are two exceptions of flours that did not get here on time in order to refresh the sourdough. But we do have the microorganisms that were present in there.

Feeding the starters

So every two months we feed them with the original flour, following the original recipe, and that's why you see that some are stiff, some are very liquid, some are in between. And that guarantees that the microorganisms that are present in the sourdough survive.

We do not bake with those sourdoughs. Well, not during these refreshments, at least, because it's already very labor intensive when we have to feed these 133 starters. So we split that up into two groups, so one month we do one half and he next month we do the other half. The week of the 3rd of June number 61 up to number 133 will be refreshed. And then in July it will be back from 1 up to 60. And so it goes on and on and on.

Now that is not a system we invented. That's the protocol that we got from Professor Marco Gobbetti in order to maintain the starters here in the library. Using the original flour is a kind of a guarantee, and only doing it once every two months is a guarantee that these starters are not evolving tremendously. We see some changes. We see that some families might be less dominant, or become more dominant.

Sometimes, I get questions or remarks on social media where people say, “Yes, but as soon as you open a jar of sourdough in another environment, it's contaminated and it's changing.” Well, that's not the case. It doesn't go that fast because if you have a mature starter that is well fed, that is in good health, it contains billions or trillions of these microorganisms that have created their own environment in which they thrive optimally. And so it's very difficult for intruders to get in there to survive and to take over the place.

I often compare mature sourdough to a very nice city that has already existed for thousands of years: London was London 1,000 years ago and will be London 2,000 years from now. The walls will be there. You will recognize that it is London. Some families might still be there, but some families might have gone, or might be replaced, or might be in less quantities or more quantities. That's a bit like how a sourdough can evolve.

Last year we wanted to do an experiment where we would invite two bakers who deposited their starters in 2014, in 2015, 2016, 2017, 2018, and 2019 to compare what we have today in the Sourdough Library with what they have today in their bakery, and what we received in those respective years. We couldn't complete the experiment due to COVID, but it's still something that is on our list. We are curious to see, are these starters in a bakery the same? Have they evolved? And have they evolved differently than what we have here? It's the thing you can do with a library, you have a place where you can study all these different things. So that's how we preserve the biodiversity of sourdough.

Protecting heritage

Secondly, I mentioned that we protect the heritage. The best example of protecting the heritage of sourdough is one of our jars here. It is not a sourdough, but it's chickpeas. This whole idea of creating a library was inspired by chickpeas. As I mentioned before, we had a collection of 43 starters in the company. And we were called by a baker in Lebanon who had a process of fermenting chickpeas.

Fermenting chickpeas for sourdough

Now fermenting chickpeas—if you haven’t done it yet, you should try it and see what it does—is you take this kind of Erlenmeyer flask, and you fill it with with chickpeas. You add hot water, 60o C. And you put it someplace warm, 40o C, and you keep it like that for eight hours.

What happens is that the microorganisms that are on the outside of the chickpeas start to ferment. It's mainly yeasts that are created. You will see the chickpeas start to swell, and then you will see little bubbles of CO2 appear. They come to the surface and they create a kind of white foam.

This white foam looks like a bit of a meringue-— like whipped egg whites. You can remove that after eight hours, and then you strain the water because you don’t really need the chickpeas. Some bakers only use the water. Others, they mix it and they use everything. And that is used to ferment.

Now the smell of these chickpeas is awful. It's a very complex and very unpredictable fermentation. Sometimes it works. Sometimes it doesn't work.

In the summer of 2019 I went to Crete and to Turkey in order to make a movie about this fermentation. I was supposed to go to that third bakery in Lebanon, but again I had to cancel because of COVID. But sooner or later we will release that movie showing how it is done.

The baker in Crete always had three jars prepared with these chickpeas: one in his house, one in one bakery-—he and two bakeries-—and one in the other bakery because one or two of them was not certain to produce what he required. So it's very unpredictable and it's very complex to understand what is going on.

The two sons of that bakery in Lebanon visited an exhibition in Paris, and they said to their father, “When we take over the bakery, we will switch to instant dry yeast instead of your chickpea fermentation.” And that baker got a bit disappointed. He was upset because he was sure that he was not going to convince his sons to continue with that process, so he called us and he said, “Look, you are a company. You’ve existed for almost 100 years. I am going to give you my recipe. And when my sons realize that what they will do with yeast is not the same as what I and my ancestors have done with the chickpeas, they can come back to you and you will have that recipe stored. You can explain them how this is done.”

So that was protecting his heritage. And that actually triggered us to protect the heritage of more than just the chickpea, but trying to protect sourdough heritage for the future, because we believe sourdough is a very important ingredient that should be reintroduced in bread baking all over the world.

Providing backup

Last, but not least, we provide a backup. Now the Sourdough Library has existed since 2013, and I have always said we guarantee that we have a backup here. But I never had to use it until two months ago.

In the beginning of March I got a phone call from a baker in Denmark. His name is Hendrik. And he calls me one Friday or Saturday morning and he says, “Karl, I have an issue. My employee has used all my mother dough in the final dough. He made bread out of it. And he didn't get a piece of the mother aside to feed it, to refresh it for tomorrow.”

So I came to the library, refreshed this sourdough, and sent it back to Denmark. And the next day he had his mother culture back. So that's how we guarantee a backup.

Now for Henrik, his bakery is very near to Belgium. I could have driven there by car, if needed. For some countries—if I have to send back a sample to Australia, for instance—it might take a bit longer. But we do guarantee that we can send it back to their respective owners.

Tour of the Library

So I will continue a little bit where I left off. In jars number 64 and 65 I have starters from China, Australia, Spain, Portugal, Italy. There is actually a pannetone starter that has been around since 1923.

Here I have the starters from San Francisco. Number 71 is from the San Francisco Baking Institute, and 72 is a starter from the city of Guadalajara in Mexico to make a famous birote bread. Number 73 is from the biggest pannetone producer in Peru.

Numbers 74 and 75 are from Switzerland. Number 76, as I said earlier, is from Denmark. Numbers 77 up to 84 are all from the United States. Here we have three starters from Brazil. Spain, Germany, the Netherlands. There’s one from my friend in the UK, Vanessa Kimball, and another from a friend in Singapore, William Wu. Number 95 is from Spain, and 94 is empty. It should come from the Philippines, but it hasn't reached the Sourdough Library yet.

We have a sourdough from Austria, from São Paulo in Brazil, then another couple from the United States. This one, number 99, is from Michael Kalanty in San Francisco. Starter number 100 is not made with flour, but with cooked rice. It comes from Tokyo, Japan.

Number 101 is a traveling sourdough that comes from Germany. It has even a name, and a book is written about it. It is called Vitus. And then 102 is Levain James. For those of you who know the famous book of Modernist Bread, well, Levain James is the sourdough that has been used by Nathan and Francisco in order to do the trials with sourdough breads.

I have another German sourdough, and then 104 is a starter dating back to the Klondike Gold Rush, so going back to 1896 from the region of Seattle. Today it belongs to Will Grant, the owner of That’s A Some Pizza, and in 2017 he won the Caputo Cup with pizzas made with this sourdough.

There is one from Toronto from Blackbird Bakery, and another one from the Klondike Gold Rush. It's from the great-great-grandfather of Ione Christensen, one of the first pioneers during the Klondike Gold Rush.

We have one from Alaska, a sourdough that has been traveling to Chile, to New Zealand, but has always come back to Canada. Number 180 is from a very nice pizzeria in Verona, and it's actually started with cow dung. I might come back to that later.

We have sourdoughs from Spain and Turkey. Number 125 is from Anita Sumer from Sourdough Mania. People who are on Instagram or Facebook might know her. Then 128 comes from Dubai, and 130 is a gluten-free starter from Germany made of rice flour. There is one from Canada, from a retailer where they make amazing cinnamon rolls with it. And then, as I mentioned, 132 and 133 are starters from Panera Bread in the US.

Finding & documenting starters

This collection is getting broader and broader. We now have a network of bakeries that we know from all around the world through which we can collect sourdoughs. But in order to detect sourdoughs that we don’t know exist, that are living in a remote area, in a household or a school, or in a restaurant outside our network, we have developed a website called The Quest for Sourdough.

On that website everybody who has a starter at home can register it in our online database, sharing the name of the sourdough-—because it's important to give a name to your sourdough-—the ingredients you use in there, and a little bit of history. You can describe your sourdough, and you can even create the flavor profile of how it tastes. You can upload pictures of what you bake with it. It's a place where you can share, or make, your little inventory of sourdough, or sourdoughs if you have more than one.

We try to motivate people to share their sourdough and create that community feeling of showing others what you are doing in order to exchange ideas and inspire each other. For us, it is a source of potential candidates to become part of this collection and therefore part of the study of their microorganisms.

Right now several of these sourdoughs are part of scientific studies that are going on in Italy, in Spain, in Switzerland, in Belgium, in the UK. In order to find the impact of sourdough bread on digestibility, gut health, nutritional values, and so on, because that will give us a lot of information that we can share with bakeries all around the world in order to promote a positive message about bread.

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