Secrets of Sourdough: TRANSCRIPT

This is a transcript of the Gastropod episode Secrets of Sourdough, first released on December 19, 2017. It is provided as a courtesy and may contain errors.

CYNTHIA GRABER: That’s really good.

NICOLA TWILLEY: Really good. That’s good.

GRABER: One more—I know, I just need one more little bit.

TWILLEY: Just one more piece.

GRABER: I’ll join you in that.

TWILLEY: How can I not? It’s so good.

GRABER: It’s so warm and yummy. I’m going to taste some of this. Mmmm, Nicky—hot pita with garlic butter?

TWILLEY: Welcome to an episode of carb lovers anonymous!

GRABER: Not so anonymous. Nicky, they know who we are. I’m Cynthia Graber—

TWILLEY: And I’m Nicola Twilley, and this is actually Gastropod, the podcast that looks at food through the lens of science and history. And Cynthia and I are the not-so-anonymous carb lovers.

GRABER: We spent three days in Belgium with two scientists and more than a dozen bakers. We were in theory investigating a deep scientific question about bread—but actually

TWILLEY: We were eating our body weight in bread. And Belgian waffles.

GRABER: Nicky, I still am not sure I can forgive you for encouraging me to eat that second hot Liege waffle—I felt a little sick afterwards—but it was frigging amazing.

TWILLEY: Listeners, I ask you: was that a bad thing that I did? No. When in Belgium, eat the Liege waffles.

GRABER: But you’re not here to find out how many pieces and what types of bread we gorged ourselves on in a 72-hour period. You want to know what we wanted to know: all about sourdough. In fact, many of you have written us emails asking us to do this very episode. For instance, listener Alex Freedman, who lives nearby in Somerville but grew up in San Francisco, wanted to know about the history of sourdough. Alex, we’re on it.

TWILLEY: Listener Danae Garriga is northern Illinois requested an episode devoted to sourdough starters. As a baker, she’d read about wild yeasts and how the environment the starter is made in affects the microbes in it. And she wanted to know, if she gave some of her sourdough starter to a friend, would the microbes in that starter change? Danae: exciting news, that is exactly what we went to Belgium to figure out, in the world’s most delicious science experiment. In fact, we have the world exclusive scoop on this brand new research!

GRABER: It’s true, we tagged along with scientists at the cutting edge of sourdough. The question they were trying to answer is: those microbes that make up your wild sourdough starter, where do they come from?

TWILLEY: Is it from the water, like so many people—especially in San Francisco—believe? Is it from the baker or the bakery?

GRABER: Or is it from the flour?

TWILLEY: This was a gigantic scientific mystery. Up till now.

GRABER: We are going to take you along to Belgium with us on this path of scientific sourdough discovery. But a quick note, if you’re a regular listener, you know we have a Gastropod drinking game: we say microbes, you yell, “drink!” and then, you know, do so. If you do that this episode, you’ll be drunk. Really fast.



GRABER: This summer, Nicky and I traveled to a remote corner of Belgium. We were visiting the headquarters of Puratos, one of the world’s biggest bakery ingredients companies. They’d invited more than a dozen bakers from more than a dozen different countries to participate in a science experiment.

PAUL BARKER: Hi, my name’s Paul Barker and I’m from the U.K.

CHRISTOPH VÖCKING: My name is Christoph Vöcking, I’m from Germany.

JOSEY BAKER: My name is Josey Baker, and I’m from America.

STAVROS EVANGELOU: Hello, my name is Stavros, I speak English not good.

HAKAN DOGAN: I am Hakan, I’m from Turkey.

LETICIA VILCHIS: I am from Mexico. I am a baker too.

TWILLEY: And then there were also two scientists: Anne Madden and Rob Dunn. They work together in Rob’s lab at North Carolina State University. And they were meeting all these bakers for the first time too—to introduce the experiment.

ROB DUNN: We know that when you make a sourdough, the species and strains of microbes in that starter, they influence the nutrition of that bread, they influence the flavor of that bread. They influence every part of the bread. And yet it’s still pretty mysterious what determines which of those microbes are originally in your starter.

GRABER: Rob and Ann are microbiologists. They’ve been studying communities of microbes in all sorts of places—your bellybutton, your showerhead…

DUNN: And we’ve worked on microbes for a long time and often the responses is repulsion, like oh gross, there are microbes in my house.

ANNE MADDEN: When you talk to people about bacteria that might be in their bathroom it’s ugh, ugh, please stop talking, please don’t tell me any more. I don’t want to know. But when you talk to people about the microorganisms in their sourdough, it’s like, what did my children do? This is lovely. Like, can we put it on the refrigerator? Are there pictures? I love the response.

DUNN: And this was this one little niche where people seemed to gather around the idea that this was a beautiful kind of microbe, that there was something wondrous about them.

TWILLEY: And there really is something wondrous about a sourdough starter. It’s a community of wild microbes that somehow, miraculously, makes bread rise.

GRABER: And you need something to make the bread rise, because otherwise, if you mix flour and water and bake it together, you get matzah. Or, you know, a cracker. Hard and flat.

TWILLEY: Today, if I’m a baker and I want to make my bread rise, I can just go to the store and buy some baker’s yeast. Baker’s yeast is precisely one microbe, Saccharomyces cerevisiae, but it does the trick.

GRABER: But bakers have been making leavened bread in an oven—bread that puffed up and got soft like ours does today—people have been baking that for thousands of years. The ancient Egyptians made bread.

KARL DE SMEDT: So our question was okay, so where did the Egyptians bought their yeast? Because to make bread you need flour, water, salt, yeast. So where did they bought their yeast? They didn’t. It was there.

TWILLEY: This is Karl de Smedt. He’s the communications and training manager at Puratos, and, for this experiment, he was the one in charge of wrangling the bakers. And before we got started on the science, he dropped some sourdough history on the group.

GRABER: Nobody knows exactly where and when sourdough bread was first invented. The earliest evidence we have for making bread comes from a site in Africa. Archaeologists have dated the remains of that bread to about a hundred thousand years ago. It was probably made from pounded sorghum and water and baked on a hot stone.

TWILLEY: We’re not sure whether that was a sourdough or not—but it may have been something like the injera that Ethiopians still eat today. That’s sort of spongy and bubbly, and those bubbles are created by a community of wild microbes, just like today’s sourdough.

GRABER: Basically, if you combine ground up grains—something like wheat—with water, and you forget about it and leave it alone, eventually it starts bubbling. And that’s because a bunch of different microbes, usually a combination of fungi like yeast and bacteria like Lactobacillus, they colonize the mixture and feed on the flour and that is both the start of beer, and a sourdough starter!

TWILLEY: There’s hot debate among historians about whether humans first figured this out because they were making booze, or making bread. I am on team beer, to be honest, but short of Cynthia finally inventing her time machine, we will probably never know. Either way, humans figured that this wild bubbly mix made their flatbreads into breads—the non-flat kind. These loaves of bread would all have been sourdoughs. There was no other way to make bread rise.

DE SMEDT: So for thousands of years sourdough was being used by each and every baker or person that would bake bread.

GRABER: And even before people knew what microbes were, they were already caring for these wild communities of bubbling beige gloop, feeding them with more flour and water to keep them alive and happy. They figured out that you only need to add a dollop of starter to your dough to leaven it, which means you can keep the same starter going for years and years—decades even—just by feeding it with flour and water and using a little bit of it every time you bake. It becomes like your own personalized wild leavening mix that you can keep alive and use it again and again and again.

TWILLEY: Other people developed variations on this approach. In ancient Greece, for example, Pliny the Elder describes people saving a piece of their dough from the previous day to raise their bread the next day.

GRABER: Pliny also reported that people in Gaul and Iberia, otherwise known as France and Spain, they would use the foam they’d skimmed from beer to produce what he called “a lighter kind of bread than other peoples.” It’s the beer/bread question again—either way, it’s communities of microbes that grow on mashed-up grain-and-water mixes, and that have the power to both leaven bread and ferment sugar into alcohol.

TWILLEY: Over time, we figured out how to curate and stabilize these communities, so that they worked as expected, most of the time. Still, they were all a little different and a little finicky—my sour culture might make bread rise faster, yours might produce a better crumb, mine might all the sudden stop working.

GRABER: But these sour cultures were the only tool we had to bake leavened bread. And then everything changed.

DE SMEDT: And with the discovery of the microscope, with some research done by scientists, actually with Louis Pasteur, who wrote this Memoire sur la Fermentation Alcoolique, who opened actually the production of commercial baker’s yeast.

TWILLEY: It was two Hungarian born brothers, Charles and Max Fleischman, who first commercialized Pasteur’s insight. They started selling baker’s yeast—fresh yeast, sold in little cakes.

DE SMEDT: And it was such a convenient product that bakers embraced it with open arms. They all started to switch from that very inconsistent, complicated, long process that is sourdough towards something that is very precise, very accurate, very fast, very reliable, that’s called yeast. And so, in 150 years, bakers switched completely.

TWILLEY: Like I said, commercial baker’s yeast is just one microbe, not a community. Which has both pros and cons.

DUNN: So, commercial yeast is super boring, right. So nobody ever thought Saccharomyces cerevisiae, this baker’s yeast, was the most flavorful, that it had the best effect on the bread. We just thought you could make a ton of bread really quickly.

GRABER: Because not only is it a single yeast that you can buy whenever you need some, and that doesn’t need feeding or watering or loving care, but it also makes your dough rise a lot faster than that sourdough starter you’ve been keeping alive. By the 1960s, boring commercial baker’s yeast was available as shelf-stable granules in little packets. And, by then, bakers had also invented industrial processes that sped up the whole rising and baking process to just over three hours.

TWILLEY: This  bread—the bread of 1960s, the bread of our parents—this was not good bread. Karl says the 1960s was bread’s nadir. Sourdough all but disappeared.

GRABER: The 1960s sucked for bread, commercially. But it was also the time of good bread’s rebirth. The country’s first Zen Buddhist monastery was created in California in the late 60s. It was called Tassajara. The monks there baked bread slowly as part of their spirituality. They saw bread as being alive.

TWILLEY: And a young Zen student named Edward Espe Brown, who lived and worked at Tassajara—he published a book collecting the monks’ recipes in 1970. It was super homemade and hippie—the cover is made of brown paper, it was published in a tiny edition by Shambhala Press, and Edward received the princely sum of $100. But it sold out immediately, and went into second and third and fourth printings. Making your own sourdough bread at home became part of the counterculture—and a way to eat healthier.

GRABER: At the same time, there was another group of people who thought that commercial bread kind of tasted like crap. They weren’t inspired by spirituality or health, but by flavor. Between them, these two groups helped create the sourdough revolution.

TWILLEY: This revolution took a while to spread. During Karl’s own training as a baker, he never set eyes on a sourdough. It wasn’t till he started working at Puratos, in 1994, that he first encountered it.

DE SMEDT: I’d been to one of the better bakery schools in Belgium and we never learned how to make sourdough. It’s just not part of the educational program. So it was a discovery. I had to take out a bucket of the fridge. It looked strange. It smelled strange. It was funny when you touched it—it was a bit sticky.

GRABER: But Karl is thrilled to say things have been changing for sourdough.

DE SMEDT: And we see now, the latest 20—25 years there is a revival of sourdough and we think we are at the beginning of something very nice that will come in the coming years where sourdough will again take its place in the bakeries that it deserves.

TWILLEY: With that sourdough revival came a renewed appreciation for the diversity of microbes in sourdough starters—and they are diverse. As we discovered.

DE SMEDT: Come closer, come closer, because something very special is going to happen. You have to realize that what we have here is probably the most unique place in the bakery world.

GRABER: Karl led the group up the stairs and to a closed door.

DE SMEDT: Ready? Keep your eyes on the door, let’s go for some magic. Three, two, one…

BAKERS: Whoa! Ahhhh!

TWILLEY: And with that, we stepped inside the world’s one and only sourdough starter library.

GRABER: It’s a library, yes, but instead of bookshelves, there are 12 illuminated refrigerators with glass doors so you can see the jars inside. Karl’s collected 93 different sourdough starters from 17 different countries. And they look totally different from one another.

DE SMEDT: Some are liquid and some are stiff. And then some are very dark. Some are speckled. Some are almost looking like crumble, because they’re so dry. So there’s a lot of colors—dark to brownish to yellow, and then the normal white ones.

TWILLEY: Karl took some of the jars out and allowed us to smell the starters. Some smelled fruity, some were acidic, some were biscuity, some were creamy.

DE SMEDT: The Chinese, for example, one of them is very meaty. When I open the jar, it’s like almost a sausage, very savory. Some are really very pungent, when I open the jar and smell, you really feel the acids go into your nose, and it’s like if you were to have a spoon of very heavy mustard, the Dijon mustard—that reaction.

GRABER: Karl’s goal with this collection is to preserve the communities of microbes that make each sourdough unique. But for Karl, it’s also really fun.

TWILLEY: Karl is the keeper of the sourdough library. He can’t sell these starters or even give them away. Each unique microbial community still belongs to the baker who donated that starter in the first place. But Karl feeds them and takes care of them. And sometimes he plays with them, too.

DE SMEDT: I do take home some sourdoughs and I do some experiements and, yes, I do bake with them. And I discover some other things. Sometimes the fermentation power is totally different.

TWILLEY: When Karl is feeding the starters he puts them in small plastic buckets.

DE SMEDT: Some of them they just blow away the lid of these things. And other ones are just very, very slowly rising, fermenting. So there’s really differences in fermentation power, in flavor, in aroma, in the way the dough is feeling when you touch the dough, it’s different. So yeah.

GRABER: Karl’s point is that these starters are all different from one another. And the library itself is also unique. Nobody’s ever tried to conserve communities of useful food microbes for the future.

TWILLEY: Walking around the library, looking at these spotlit jars in their glass refrigerator vitrines, you really see each sourdough starter as a distinct, individual, precious thing. But how different are they microbially, really? Who’s living in those jars?

DUNN: Sourdough, in terms of the number of species we know how to grow, is toward the simple end. Often you’ll have two to four culturable bacteria species and one yeast species. It’s very likely, although we don’t know, that there are also things that are hard to culture in the lab that are in those sourdoughs, that make it a little bit more complex. But it’s toward the simpler end in terms of numbers of species. It’s not simple though in as much as different sourdoughs seem very different. And so if you were to look around the world, how many different species could you find in all of the sourdoughs? That’s actually a much longer list. And so an individual sourdough: simpler. This big picture of sourdough is far more complex.

GRABER: As Rob is explaining, a sourdough starter is an interesting creature, or, really, creatures. You can have a community of just a handful of different microbes that works perfectly together—as Rob says, maybe two to four species of bacteria, maybe one kind of yeast, and it’ll work. It’ll make sourdough.

TWILLEY: But what’s also probably true is that your sourdough starter could contain an entirely different community than mine, and they’d both still make sourdough. And it’s that diversity—that huge world of bacteria and fungi that can collaborate to raise bread—that’s what Karl is trying to collect.

GRABER: His library, as unique and impressive as it is, is probably just the tip of the iceberg. And maintaining this library is a lot of work—it’s not just collecting samples and putting them behind glass.

TWILLEY: Any baker can tell you what a commitment it is to keep a sourdough starter alive.

BARKER: I always describe it, if you have a sour culture, it’s like having a pet or a child, yeah?

GRABER: Paul Barker owns a bakery just outside London called Cinnamon Square. And he has many sourdough pets.

BARKER: You have to look after it. If you don’t feed it, keep it warm, or whatever. So unless you look after it, it will spoil, it will eventually die on you. So it’s a commitment to having a sour culture .

TWILLEY: In fact, there are even specialized sourdough hotels, where you can send your sourdough starter to be looked after if you’re going on a super long trip. A sourdough starter is really much higher maintenance than commercial yeast, so why do bakers use it? We asked Paul.

BARKER: Firstly, because the sourdough gives you a much different type of bread: different textures, more digestible bread, more nutritional breads. So I like the fact that you can get a totally different product. And you can be so creative with a sourdough, more so than a yeasted bread. So you can actually do a lot more with the shape in the baking, the decorations, I think—because you can get more from it whereas a yeasted bread, a commercially yeasted bread, you are just expanding your dough and baking it.

GRABER: Commercial yeast, as Paul explained—it makes the bread puff up, but that’s it. Paul knows that the microbes in his starter are giving him a different dough. It often has the right type of texture to allow him to play around more with the shape of his loaves. But what are those microbes actually doing to create these differences, and how are they doing it?

DUNN: So the microbes in the starter are starting to break down some of the hard-to-break-down things in the grain that you’ve given them to eat. And they are beginning to produce these gases that we think of as some of the really important flavors in the bread. But, as they metabolize the grains, they’re also also altering the structure of the carbohydrates that are present, which then is going to alter the nutrition of the carbohydrates, it’s going to alter the outside of the bread.

TWILLEY: As Paul has noticed and as Rob just explained, microbes improve the texture and the nutrition and even the look of the final loaf. They can even produce extra vitamins. But they also shape its final flavor—you can literally taste the difference between bread from different starter communities.

DUNN: And so butteriness—a lot of butteriness comes from which microbes are in your starter. The kind of sourness you have—how lactic it is versus how acetic it is—that comes from which microbes are in the starter.

TWILLEY: Rob told us that some sourdough bread has a particular gooey, melt-in-your-mouth feel that comes from a chemical called dextran, which is produced by a bacteria called Weissella. Weissella lives in some sourdough starters, but not in others.

GRABER: So: microbes are munching away on the flour, excreting things like buttery flavored lactic acid and yeasty farts that puff up bread. That much we know. But Rob and his fellow microbiologists don’t understand how all this microbial munching and excreting creates the differences between different finished loaves of sourdough.

DUNN: And the further you get down that chain of events, the less we understand about the mechanics of how all of that is happening. But what we do know is that all of the things that could influence those final flavors, final texture, final nutrition are things that we think of as predominantly microbial.

TWILLEY: So we don’t know. We really don’t know how the microbes are working their magic. We don’t even where they come from in the first place. But Rob wants to know. And so did we. And hence this giant 3-day experiment in Belgium. Which we have the exclusive first results from after the break.


TWILLEY: Back to Belgium. Where we are about to conduct an epic baking experiment in order to figure where the microbes that are in a sourdough starter actually come from in the first place.

DUNN: So, in order to make a starter, you take a simple set of ingredients and you expose them to open air and to your body and to your home, and it starts to grow. It’s like making a garden without ever planting the actual seeds. The mystery to me is: what determines which life forms are growing in that garden? And so that’s the fundamental mystery: why is your garden different from my garden when we use the same things to start with?

GRABER: Many bakers think they know the answer to this mystery.

VILCHIS: I think is flour. But the hands of the bakery is very important too to the results.

BAKER: I think it’s probably a combination of all of the variables.

MARCUS MARIATHAS: It’s mostly, in my opinion, the reaction within the flour and water. That’s where it starts.

BARKER: I would assume the environment is going to play a part in it as well. Because it’s going to be a lot of cross contamination in bakery from different flours anyway and you can end up with different types of sours.

MADDEN: I feel like every baker we talk to has a different assertion about where the microorganisms from that sourdough starter came from. Some people are very clear: it’s likely coming from the flour. If I use a different flour, I’ll have a different sourdough starter and a different sourdough starter must be different microorganisms. Some people have suggested that it’s the water. That’s why San Francisco sourdough is San Francisco sourdough and you can never make it in New York. There are claims about it being in the wood of buildings.

DUNN: What I like about this project is that as scientists we have not had to come up with our hypotheses because the community of sourdough makers has provided us with the longest possible list of what they might be.

TWILLEY: From that long list there are four main hypotheses: that the microbes that make each sourdough starter unique and individual come from (a) the wheat, (b) the water, (c) the environment, and (d) the baker themselves.

GRABER: Rob says we know that there are different microbes on different grains. Even within the same grain, there are different microbes on different strains of wheat—different heritage varieties, for example. Or wheat that is grown in different ways, like organic wheat. And then, even on the same plant, you can find different microbes in the germ of the grain versus the endosperm. The endosperm is what millers use to make white flour. So this means that whole wheat flour has different microbes than white flour does. Rob says these all these variables in the flour itself could certainly be influencing the sourdough starters.

TWILLEY: Then there’s the hypothesis (b), the water.

DUNN: Water can conceivably kill things in the starter. It’s unlikely to be adding things to the starter because we have a pretty good list of what lives in water. I think people are surprised often that all water they ever drink, even bottled water, has microbes in it, but they’re not the kinds of microbes we characteristically see in sourdough.

TWILLEY: In other words, Rob is saying that the water might prune particular microbes out of a sourdough starter garden, but it’s unlikely to be contributing any new microbes itself.

DUNN: The other thing though that that can then contribute to the starter is what falls from the air into the starter.

GRABER: This is hypothesis (c), the environment around you as you make the starter. Rob says that plants might have a particularly strong impact, because of the insects they attract and the microbes on those insects.

TWILLEY: And then there’s just the bacteria that are swirling around in the dust and air. Some of those come from pets, if you have pets. The majority of them, usually, come from your own skin and the skin of the people you live with.

GRABER: And finally, hypothesis (d), the baker.

TWILLEY: Specifically, the microbes living on the baker’s skin.

DUNN: We can think of many ways that microbes differ from one person to another person.

TWILLEY: For example, there’s that gene that determines whether you have sticky or dry earwax.

DUNN: And depending on which version of that gene you have, your skin microbes in your armpits, but also around your body more generally, are super different.

GRABER: There are also microbes on your skin that don’t live on your skin. They get there when you touch parts of your body that have other microbial communities. Like your gut microbes.

DUNN: And then we know that human women and human men differ greatly in microbes because of vaginal microbes. And so women have way more Lactobacillus in general, but especially in vaginal communities, and those sort of travel around through the day-to-day business of being a human.

TWILLEY: These vaginal microbes are particularly interesting because Lactobacillus is a key part of most sourdough starter communities.

DUNN: Yes. So, in some cultures, sourdough is mostly or exclusively something that women bake. And to me it’s really intriguing to think about does that have something to do about the unique sourdough community that emerges when women make sourdough versus when men make sourdough.

GRABER: This three-day Belgium adventure, the experiment we’re watching unfold—it’s designed to try to tease out where the microbes in the sourdough come from. A, B, C, or D.

TWILLEY: To be precise, it’s designed to isolate two variables from these four possible sources for the microbes in sourdough—the microbes on the different baker’s hands and in their environment. Those are the variables.

MADDEN: They were shipped the ingredients, they were given the same protocol, the same recipe.

GRABER: That is, these bakers were shipped exactly the same flour. Not the water, because based on the existing research, Anne and Rob don’t think the microbes in water plays a big role.

TWILLEY: Anne and Rob cultured the microbes out of that flour, so they already have a list of the microbes that are being contributed to the starter from the wheat.

GRABER: Like Anne said, the bakers were given very specific instructions about exactly how much flour and water to use and exactly how long to ferment their starters. The goal is to make this all as controlled as possible.

TWILLEY: So all these bakers, men and women, in different parts of the world, they all made their sourdough starter using the same flour according the same protocol. And then they put their starters in a baggie and they brought it with them to Belgium

MADDEN: And that was a really fun part, when we got to open them all up and they’re coming in and some of them smell like vinegar and some of them smell more like yogurt and some of them smell creamy.

GRABER: As soon as the bakers arrived, Anne and Rob opened packages of sterile swabs, like super long Q-tips, to get samples of those rich microbial communities in the starters.

MADDEN: Just one double swab per.

GRABER: Then we took a break from the science. We all introduced ourselves and met each other, and everyone talked bread.

TWILLEY: The final part of the experiment that day was refreshing the starters, according to the protocol.

BAKER: I’m going to put my starter in this bowl, first of all dilute it with the water, and then add the flour on top, mix it, put it back in here, and then we’ll wait until tomorrow.

GRABER: And that’s it?

BAKER: And that’s it.

TWILLEY: And then we all ate dinner together accompanied by lots of bread, and day 1 of the experiment was over.

GRABER: First up day 2? After breakfast featuring lots of bread, we got to everyone’s not-so-favorite part—getting swabbed to find out what microbes live on their skin.

TWILLEY: Paul from London was up first.

MADDEN: Now, I’m going to be swabbing your hands, and I’m going to ask that put your hands out just in a way that I can apply some pressure. And I’m going to spend a few seconds.


MADDEN: Just going over the front and then I’m going to ask you to flip and then I’ll do the back. And if we could not talk over the swab when it’s out so that we can not introduce some of our oral microbes.


MADDEN: Thank you.

TWILLEY: Anne was swabbing the baker’s hands because if any microbes are going from a baker’s body into their sourdough starter, they are probably getting in there via their hands.

DUNN: You know it will be wonderful in some future version to you know top-to-bottom swab all these bakers and really start to tease out, you know, which body part is really contributing. But we had to start somewhere and so we started with the hand connection.

GRABER: In case you’re getting a little grossed out, don’t worry. The bakers do wash their hands. And they should wash their hands. Anne made sure to emphasize that. Even after you wash your hands though, there are still microbes on them. They’re everywhere.

TWILLEY: So, next step: after their hands were swabbed, the bakers were allowed back into the test kitchen to be reunited with their starters. Which they could hardly wait. It was like parents at the kindergarten gate. But before they could be fully reunited, the starters all had to be tested with some cool science gear, to find out their pH and their organic acid content.

TWILLEY: Once again, the sourdough starters all looked—and smelled—completely different.

KASPER HANSEN: My sourdough is called Danish Dynamite.

GRABER: That’s right, Danish Dynamite.

CASPER: So a lot of activity inside. So, as you can see, up side of the glass here.

TWILLEY: It was like looking at baby photos, I’m not kidding. Everyone thought theirs was the prettiest of all.

GRABER: You’re smelling your sourdough?

TOMMASO RIZZO: Smell is buttermilk—smell, taste, aroma.

GRABER: Can I smell? Mmm, yeah, it’s got a little sweet. The bakers made their bread and left it to proof overnight. And, as that official science-experiment bread was rising, the bakers were set free in the test kitchen to let their pent-up creativity run wild.

TWILLEY: And they went to town. Hakan made this crazy Turkish bread that had lots of melted cheese and a cracked egg on it. Leticia, the Mexican baker, she was putting cocoa and raisins into a sourdough loaf. Someone made pita bread.

GRABER: I’m going to taste some of this. Mmm. Nicky, hot pita with garlic butter? It’s really good.

TWILLEY: It’s really good. That’s good. So look, let me do this.

GRABER: Mmm, the smell.

TWILLEY: I’m squeezing the bread like it’s a bellows on an accordion or something. Or trying to light a fire. This is what I’m doing.

GRABER: That smells amazing. It’s like as you squeeze the dough the scents in the air pockets just, like, get blown right at your face.

TWILLEY: So I stood here. Stavros, like, pumped the bread in my nose, and Vassilis was like “This is sourdough.” We sniffed bread and we ate bread, and then we ate more bread.

GRABER: And then we ate dinner. Which also had some bread.

TWILLEY: And then we rose bright and early on the third day, had some bread for breakfast, and went back into the kitchen to bake the science-experiment bread. But… there was some tension.

GRABER: Tommaso, for one—he’s from Italy—he didn’t want to put his bread in the oven when everyone was told it was oven time. He said the dough wasn’t ready for baking—it hadn’t risen enough. Rob whispered to us that he and Anne were having a hard time making sure that all the bakers kept to the scientific protocol.

DUNN: Yeah. So we’re thinking about it right now. There’s a tension between what people view as counting as a bread. And, uh, what we want.

TWILLEY: Tommaso was overruled. In the nicest possible way. And all the bakers’ dough went in the oven at the same time. And the same way that their starters had looked and smelled really different, despite having been made from the same flour using the same instructions, the dough looked really different as it went into the oven, too.

GRABER: You could see some really big air bubbles in some and none in others. Some rose a third of the way up to the tops of the baskets, some rose all the way to the top. Some were super bubbly on top, some were shiny and smooth. And then the bread came out of the oven.

GRABER: Oh. Those are pretty. (OVEN DOOR CLOSING)

TWILLEY: Some of the bakers were happy and some were not. So these are Tom’s, you like the look of them?

WALTER: I like them. Because when it’s cracking open, you see black line. And Karl calls it eyeliner—so we have to bake it so—eyeliner on the bread.

TWILLEY: And eyeliner is a good thing, right?

WALTER: Yeah, yeah.

GRABER: I learned something new—I never knew bread should have eyeliner on it. It’s basically the nice, dark, cracked edge you see at the top of the loaf. Tom’s loaf had really lovely eyeliner.

TWILLEY: This has a lot of nice fish eyes or blisters.

GRABER: Little bubbly blisters on the cooked crust are another sign of a great sourdough. But some loaves didn’t look as good. Like Paul’s. And this one doesn’t look like it did very much over here, it didn’t even crack.

TWILLEY: Which are yours?

BARKER: The ones that are looking very sad at the back. The two behind this one here.

GRABER: No, they’re not very…


TWILLEY: And then, as soon as it was cool enough, all of the loaves were sliced in giant bread-slicing machines. And the bakers were asked to evaluate a slice from each loaf. They had to assess its appearance, its smell, and, of course, the way it tasted.

TOM REES: So we’ve got kind of two different colors, I see already. One which is a bit grayer, and one which is a bit more yellowy, creamy color.

TWILLEY: And is that reflected in differences of smell too?

REES: Yeah, so the greyer ones—the greyer ones have less of an acidic aroma

BARKER: Some are creamy and some have gone kind of more reddy, kind of browny, sort of hints. So there was a distinct difference in the color, which is quite interesting. I wouldn’t have expected that considering we are all using the same flour, the same ratios of ingredients.

BAKER: Like the one of Guillermo is dense and stronger, and from Tom, it’s very fragile and very open. But the taste and smell is about the same.

VILCHIS: For example, Hakan is very very similar to Kasper. I think is the same bread. Incredible. Paul is the same than Guillermo.

HANSEN: It’s much more like wheat—not so fruity. Hakan and Tom, taste more—have more acid taste.

DUNN: And so in this case we know that all those differences from bread to bread are really microbial.

GRABER: But it might not actually be because the starter contained different microbes. The exact same microbes can create different smells and tastes just based on the temperature that they grow in, for instance. So these results, that the breads smell and taste different? Could just be because the temperature in London is different from Guadalajara.

TWILLEY: Sensory evaluation was not enough to answer this question. Instead, Rob and Anne had to take to their swab samples back to the lab and analye them.

GRABER: A few months later, we called Rob up to find out how it all went. (PHONE RINGING) Hey Rob! So Belgium ended. You packed up to go home. How did you feel?

DUNN: I felt super full.

GRABER: I felt really full, too, just so you know.

TWILLEY: I was never going to eat bread again. And then I did.

DUNN: No, I’m ready for more bread to be honest. There’s been time.

TWILLEY: Science takes time, but this science took a little bit longer than Rob wanted because his samples—the swabs from the bakers’ hands and the sourdoughs starters—they got held hostage in Belgium. Trying to get these kinds of biological materials across borders can be tricky. Rob is a patient man, but even he was getting a little frustrated.

DUNN: And then, amazingly, just last week, we got the first results from that decoding of DNA.

GRABER: Rob, Anne, and their whole team spent a day just marveling at the data and poking around. They were trying to figure out if they could make any sense of the data just by looking at it. Which, of course, they couldn’t.

DUNN: But then eventually we started to formally analyze what’s going on with the patterns of the data and that’s where it starts to get interesting. And so the first one of those analyses happened on Friday and the second one happened about two hours ago.

TWILLEY: So tell us! What did you find?

DUNN: Well, the first thing last week was a result we weren’t looking for, we didn’t anticipate. And I had no idea it was even possible.

GRABER: It’s about the bakers’ hands. Normal hands usually have Staphylococcus, and some armpit microbes, some bacteria that are the same as acne bacteria, maybe some random bacteria from things you’ve touched recently.

DUNN: When we looked at the bakers’ hands, their skin bacteria on their hands was about half sourdough bacteria. And so they, like, have sourdough paws.

TWILLEY: Sourdough paws!

DUNN: We’ve looked at zillions of hands. We’ve never seen anything like this. And so the first result is that the bakers themselves have changed in response to their occupation.

TWILLEY: Normal hands like mine and Cynthia’s and Rob’s—they are something like 2 to 4 percent Lactobacillus.

DUNN: On the hands of the bakers, it is like it’s the star of the show. It’s wild. I mean, if it’s right, you should be able to put flour and water on a baker’s hand and it should start to ferment immediately and become acidic.

GRABER: Working with sourdough has entirely changed the microbial environment on the bakers’ skin. They’ve been colonized by their pets! Rob wonders if the bakers spend so much time with their hands in acidic dough that the sourdough Lactobacillus microbes end up with a competitive advantage over normal skin microbes.

TWILLEY: So that is weird. But it’s not what Rob and Anne set out to find. What they were trying to understand from this 3-day Belgian breadfest is whether the microbes in the sourdough starter come from bakers’ hands—not whether bakers’ hands are somehow different from normal hands.

DUNN: So what we saw two hours’ ago was that there’s a group of bakers that has very different sourdoughs, and the unusual microbes in those sourdoughs are also on their hands.

GRABER: One question answered. The bakers who have weird bacteria on their hand have the same weird bacteria in their sourdough. There is a connection. Individual bakers do indeed seem to influence their starters. But so, does this difference influence the flavor of the resulting bread? Rob doesn’t know, he hasn’t done that research yet, but he has a hunch.

DUNN: I predict that second group has more unusual flavors. And we should be able to capture that. We’ll see.

TWILLEY: Stay tuned. Meanwhile, what Rob and Anne have done is sit down and compare the list of microbes that were in the flour and the list of microbes that were on the hands and the list of microbes that were in the starters.

DUNN: We get a total of about 193 kinds of bacteria in the sourdoughs. which is a lot more than the bakers tend to think is there, which is interesting in and of itself. Something like 80 of those are also found on hands. And roughly the same number seems to be found in the flour. And there’s overlap between the flour and the hands. We saw almost nothing in the water, so they’re probably not coming from the water.

TWILLEY: But they did see some microbes that weren’t accounted for, that were not from the hands or the flour. They were maybe microbes that were just floating around in an individual baker’s kitchen.

DUNN: Yeah, they could come from a leaf outside the bakery. It could come from a bowl or a spoon. But it’s not so surprising that we haven’t found where all those microbes are coming from—and, in some ways, that leaves the bakers some magic. Where does the stuff we’ve not measured yet coming from? Just magic. You guys can keep that.

GRABER: Rob also told us another new finding that totally contradicts what he told us back in Belgium, earlier this episode. Remember how he said that sourdough starters have three or four species of bacteria and maybe one species of yeast? Rob says based on these new samples he’s seeing ten species of bacteria in the average sourdough starter and maybe three species of yeast.

DUNN: We now have enough data to say that I was wrong when I was describing the simplicity of the starters. Which also means the whole literature is wrong.

TWILLEY: Folks, this is science in action. We think we know things, like about how many species of microbes live in a sourdough starter, and then we do some research and discover we don’t. But Rob pointed out that sourdough starters are still not particularly complex in microbe terms.

DUNN: And so part of the story that’s super fascinating to me is, you put out flour and water, all around the world, and somehow you can create a very similar ecosystem out of what for bacteria and fungi is a relatively small number of species. If you put out sterile soil in this many sites globally, you’d be looking at 20,000 species. And so, on the one hand, the individual starters are more diverse than we tend to think. On the other hand, that global picture is actually a lot simpler. So that was really interesting.

GRABER: Rob and Anne and their collaborators have really only just begun analyzing this data. Over the next six months, they’re going to be figuring out what types of compounds each species of bacteria can produce—not necessarily that they’re actually making those compounds in the starters, but that they can.

TWILLEY: And then they’re going to match those compounds to their possible effects in bread—different flavors, different textures, different nutritional values.

DUNN: The other part is we’ve barely touched the fungal data. And so that will mean we’ll be spending a fair amount of time on that even this coming week.

TWILLEY: So there’s much still to be done with just the data from our Great Belgian Bake Off. But there’s also just more sourdough research to be done in general. Our Belgian breadfest was only one of the sourdough experiments Rob and Anne have got going on the lab right now.

GRABER: They’ve already gotten about a thousand people from around the world to send in their sourdough starters. Rob and Anne want to get a big picture of sourdough diversity. They’re hoping to see patterns, like whether some species are more common in some areas of the world. And they’re already starting to see some results.

TWILLEY: Rob told us that, in terms of bacteria, there seems to be a shared sort of pool that colonizes grain and water mixtures all around the globe. In other words, the same bacteria are pretty much everywhere and then which end up in which starter seems to depend mostly on the flour and the baker, as we just learned.

GRABER: But they are seeing a little bit of geographic variation with bacteria. Some bacteria tend to live in more northerly Scandinvanian countries, for instance. That’s not the only anomaly.

DUNN: There’s a little bit of a hint so far that maybe France is kind of special.

GRABER: France is special.

DUNN: But the fungi we’re seeing globally have a lot of geography. And so there’s one one kind of yeast—a kind of fungus—that we’ve basically only seen in Australian starters. We know that the yeast can do a lot in terms of flavors and aromas. If that unusual yeast is playing a big role, then there could be a flavor that you could only actually savor when you’re in Australia. And we don’t know that yet. That’s a fun idea.

TWILLEY: Sourdough tourism is going to become a thing, just wait and see.

GRABER: One of the things Rob and Anne are going to do over the next year is bake some bread from these thousand starters that they received. That way they can start to assess flavor while controlling for the other ingredients. The ultimate goal is to arrive at microbial recipes for sourdough deliciousness.

DUNN: Once we do that, that will be the hope—that there is some mix that really gives you the perfect butteriness or the sourest souriness. Is souriness a word? I don’t know.

GRABER: Rob and Anne are also working with colleagues to tease out the evolutionary history of sourdough. They’re going to be working out how microbes in starters change over time. So, eventually, they’ll be able to tell you, if you’re using your great-grandma’s starter, are those your great-grandma’s microbes? Or, as listener Danae asked, if she gives her sourdough starter to a friend, will it change—and if does, how quickly?

TWILLEY: So there’s still tons to figure out about sourdough, but Rob is on it. And we’ll keep you posted as his results come in. It’s super exciting research. Not just because we love microbes.

GRABER: A round of applause if you haven’t keeled over yet from taking a shot every time we say microbes!

TWILLEY: We do love microbes, But we also love this research because it points the way to a future of even more delicious bread!

MADDEN: And so I think the question is the next step, which is: What microorganisms create what flavors and aromas and traits in bread that we want. And then we can start tracking down what microorganisms might be leading to those traits. And so you can imagine a future where you could think about the kind of bread you want. Maybe I want it to be crusty and kind of chewy with fruity notes. And by having that choice of bread, there’ll be a list of species that will work together to create that. So you’ll have a designer sourdough.


TWILLEY: Thanks this episode to the Burroughs Wellcome Fund for supporting our reporting on biomedical research.

GRABER: Thanks also to some of our Supreme Fan level Patreon supporters: Andy Allen, Lori Schultz, Justin So, Robert Wells, Alex Sol Watts, Eric Schmidt, Corinne Lewis, David Kohn, Matt Rooney. We cannot thank you enough for your generosity in helping keep Gastropod going.

TWILLEY: And a big thank you to Puratos, who hosted this experiment but also hosted Cynthia and me in Belgium. We have photos and links to Karl’s magical Sourdough Library on our website,

GRABER: Thanks so much to Rob Dunn and Anne Madden for letting us follow them around for three days and try not to get in the way of all their swabs.

TWILLEY: And thanks also to the lovely bakers, who couldn’t have been more of a fun group to hang out with while doing some cutting-edge science. And some competitive-level eating.

GRABER: We are going on a brief break over the holidays. But we’ll back in 2018. We have an amazing season lined up for you. If you’re on our sustaining supporters list, you’ll get a sneak peak at what’s coming up. Thanks to all of you who listen, who support the show, who write in, who take part in our Shareathon—we do this for you, and we couldn’t do it without you!
crumbs to try to identify their microbes. Could those microbes be the same as the ones in sourdough today?