Recording of the October 15 live online event.


Watch the replay of this event held on October 15, 2020 (transcript below)

As the Covid-19 pandemic forced people in lockdown, many retreated to their kitchens. Photos of frothy sourdough starters and freshly baked bread — the fruits of at-home experiments with just water, flour and a spoon — flooded social media. These nourishing endeavors were fueled by living microbes in the sourdough starter, which originated in the flour, the air and even the baker’s hands. Researchers, sometimes with help from home bakers, are still uncovering how various species of yeast and bacteria — and the conditions in which they thrive — contribute to each unique loaf.

Panel participants

Erin McKenney studies how microbial communities form over time and how they adapt to their environments. For more than a decade she has collected fecal samples from hundreds of animals in zoos and the wild, to learn how gut microbes affect health — and how humans can leverage or upset that balance. More recently, McKenney has expanded her research to sourdough and other fermented foods that can be studied without laboratory equipment. She is using this citizen science research to empower students of all ages and all over the world, including via the Wild Sourdough Project.

Guylaine Lacaze has a master’s degree in food microbiology and has been immersed in sourdough for nearly 20 years. She characterized sourdough fermentations for Biofournil (a French bakery in Le Puiset Doré) in collaboration with ENITIAA (the French graduate school of agricultural and food industry science) and worked at Lallemand (a producer of yeast and bacteria starters). In 2003, Lacaze joined Puratos to work on ready-to-use sourdoughs. She’s currently R&D manager for sourdough and grains at Puratos; among her many jobs is providing scientific input to the Puratos Sourdough Library.

Karl De Smedt always wanted to be a baker-confectioner. After graduating from bakery-confectionery school in Brussels in 1988, he worked for six years as a patissier in a Brussels bakery. In 1994, De Smedt joined Puratos as a test baker, and then worked as a demonstrator, product manager, corporate trainer and product training manager. As of 2002, a flour allergy limited De Smedt’s activities in the bakery, but he remains passionate about sourdough breads. He currently is the curator of the Puratos sourdough library and has travelled the world over to collect starter specimens.

Bob Holmes is a science writer who has covered ecology, evolution, microbiology and other life sciences for nearly three decades. He is also author of Flavor: The Science of Our Most Neglected Sense (W.W. Norton). He has a PhD in ecology and evolutionary biology, and is also an enthusiastic home cook.





Bob Holmes: “Hello and welcome to ‘The Science of Sourdough’ and thank you for joining us today. I’m Bob Holmes with Knowable Magazine from Annual Reviews, and this is the first event in our ’Reset: The Science of Crisis and Recovery’ project. This is an ongoing series of webinars, opinion pieces and journalism that explores how the world is navigating the coronavirus pandemic, and its consequences and the way forward.

“So, one way that many of us have been coping with the coronavirus pandemic is by baking, and sourdough bread has been a big part of that. You’ve probably noticed it on social media — all over the world, people are posting photos of their sourdough bread. The starters that leaven those loaves turn out to be interesting in their own right because they’re actually very complicated, complex microbial ecosystems that scientists are really just beginning to understand. With us today are some of the scientists who are spearheading that effort.

From Belgium we’re joined by Guylaine Lacaze, the head of sourdough research at Puratos, a baking consulting company, and Karl De Smedt, curator of the sourdough library at Puratos, which is what they’re sitting in right now. And from North Carolina, we’re joined by Erin McKenney, microbial ecologist at North Carolina State University. And Erin has brought another guest. Or several guests, I guess. Would you introduce them please, Erin?

Erin McKenney: “Yeah, this is Bavid Doughie. This is my sourdough starter. I’m down to one these days. At some point I had, I think, 24 starters, but it was a bit much so I’ve scaled back; but Bavid Doughie is the one.”

Bob Holmes: “Erin, can you tell us a little bit more about what’s actually in there?”

Erin McKenney: “Yeah, so let’s see. I am currently feeding Bavid Doughie a mixture of King Arthur all-purpose flour, and sprouted whole-wheat flour from Lindley Mills, a local mill. So it’s really put some kind of pep in his step. But as far as the microbes, compared to commercially leavened loaves, it’s not just yeast, right? We have these relatively complex communities of many different types of yeast, as well as lactic-acid-producing bacteria and acetic acid bacteria.”

Bob Holmes: “OK, and to get a sense of where those microbes came from, I know Guylaine has been part of a study, and Karl as well, involving bakers from around the world, where they sent identical flour out to bakers all over the world and had them prepare sourdough starters. Guylaine, could you tell us a bit more about that and what you found? Where did those microbes come from?”

Guylaine Lacaze: “In fact, in this study, we would like to know if once a microflora is established in a mother dough, is it possible to change it by using another flour than the one used to refresh it? So that’s why we sent the same flour to 17 bakers. And they had to refresh their own sourdough with this flour.

“We analyzed the sourdough before the experiment, regarding the bacteria and the yeast present inside. And we analyzed it regularly during one month. We’ve seen — so it was only after one month — that the flora was the same than the one present in the beginning. So it means that if your mother dough is stable, if you change the flour, and it will not have a lot of impact on your flora. But again, it was only one month of refreshment. Maybe if we go further, we should have more impact.”

Bob Holmes: “OK. And one of the things I think that you’re finding in general is that – well 

actually, let me just ask. Karl, maybe you’re the one to ask this to. Since you’re sitting in front of refrigerated or incubator cases full of sourdoughs, presumably not all sourdoughs are the same? As I recall, you did another experiment where you baked identical bread with five different starters and found quite different results. Can you tell us a bit about that?”

Karl De Smedt: “Yes, well, one thing that we know for sure now is that no two sourdoughs are the same. Well, we have ... Today, I added No. 130 here in the library. From all these starters, we see that they’re all unique. Some may look the same, but they all have a different combination or a different consortium of microorganisms in there. Some have the same flavor profiles, but still, they all are different.

“So I would say that, and I think that Erin and Guylaine can confirm that, that every sourdough is unique. And so what we did is we took five of the samples here that, according to us, were the most different ones, and we used them in five different breads with the same baking process. And we saw that there were quite some nice and interesting differences in the flavor profile, and sometimes in the texture, and also in the fermentation power that they have. Some have more fermentation power than others. So, yes, we could say that every starter is unique.”

Bob Holmes: “OK. Let me just pause it at this point and remind our audience that we will be taking questions after a half-hour or 40 minutes or so. If you want to submit questions, there’s a Q&A tab right at the bottom of the screen. Just click on that and you can type in the questions, and they’ll be passed along to our panelists. So, that’s just a reminder.”

“So let’s go back in the process for a moment. A sourdough starter begins with just flour and water. And then the environment contributes. And a lot happens in the next 10 to 14 days. Erin, I know you’ve studied this, can you talk to us a little bit about the evolution of a starter — how it goes from flour and water to a living ecosystem?”

Erin McKenney: “Absolutely. So that’s actually a process, that colonization process or the stages of succession from a glorified papier-mâché paste and just your flour and water to this bubbling, doubling, leavening community of organisms. It’s a process in ecology that I’m very interested in. I’ve studied that process of succession in guts of lemurs and other animals. And I have, more recently, studied it in jars of sourdough.

“Last summer, I partnered with four incredible middle-school teachers here in Raleigh, North Carolina, and an undergraduate researcher from Florida. And together, we grew 40 starters. So four replicates each grown from 10 different types of flour to map out what are the steps of succession. How does a sourdough starter grow up, right? And does that process change depending on the type of flour that you’re actually growing that starter from?

“And I should say, also, we were tremendously lucky to get our flours freshly ground from Boulted Bread bakery, which is also a local business here in Raleigh. So it was some of the freshest flour that we could have obtained. So really fantastic activity. What we found is, certainly in that first day, so we sequenced the DNA from just the flour and just the water, and then from the starters after one, two, three, six, 10 and 14 days, right? And that was based also off of measurements of the pH.

“Seeing how the pH drops precipitously in the first three days, and measuring the height over time to see how it increases. And that’s kind of a proxy for measuring the activity of those lactic-acid- and acetic-acid-producing bacteria for the pH, right? The more acid that you have, the lower your pH is going to go. And then the higher your sourdough starter is rising in the jar, that’s indicative of healthy, prospering yeast communities that are producing a lot of carbon dioxide.

“Using those kinds of behavioral or functional proxies, we were able to identify Days 1, 2, 3, 6, 10 and 14 as really interesting days. So we want to figure out what microbes are in there. The bacterial communities are changing very dynamically, so at Day 1, the starter looks essentially the same as the flour inputs — so flour plays a very heavy role, or those environmental inputs possibly from the mill where the flour was ground.

“Whatever microbes are in the flour tend to be present at very high relative abundance on Day 1, but even as soon as Day 2, we start to see a shift toward acid-producing bacteria that at first are more affiliated with plants and with bodies, right? They tend to be opportunistic, but they are producing a lot of acid, which is dropping the pH very rapidly below the threshold of any yeast that happened to be in the flour or in the environment to start with.

“So that’s when we see the pH crashes, and that’s — if you’ve ever grown your own starter at home — this is the point where I said, when I was growing mine, ‘What has gone wrong? What have I done?’ I’ve been studying microbes in labs for over a decade and now I’m completely humbled. And that’s when I learned to trust the microbes, right? Because from Day 3 to about Day 6, you see this shift in the community toward bacteria that are completely acid-resistant because they are also acid producers. You also see a shift toward yeast that are also acid-tolerant.

“So then you see a shift from more generalists that filter into that sourdough from this flour, from the environment, from our bodies, so echoing the findings from the bakers’ study. But then we see the shift toward specialists that really thrive not only in the generally acidic environment of a sourdough starter, but we saw specific differences according to which flours we were feeding, which makes me think that there’s also a nutritional component.”

Bob Holmes: “OK. And one of the take-homes there, I mean, a lot of people I think give up on their starter at Day 3, because it’s this black, slimy, putrid-smelling thing. And I think part of your message there is don’t worry, it’ll get better. It’ll solve itself.”

Erin McKenney: “Microbes have been on this planet way longer than we have. They know what they’re doing generally. Trust the microbes, stick with it. Yep, have faith.”

Bob Holmes: “OK. Because the things that make it black and putrid are decomposers and putrefaction bacteria, but they get replaced. Is that the point? OK.”

Erin McKenney: “Right. Yep, they are not favored in an acidic environment. That’s one of the beauties of sourdough, is that very low pH is actually selecting against those things that smell like something we really don’t want to eat. That’s when we ... If you stick it out to Days 10 to 14, we also smelled all of those starters. So we were able to also couple aromas and kind of those different functional differences unique to starters of different ages and unique to starters grown from different flours.”

Bob Holmes: “OK.”

Erin McKenney: “Yeah, so we can say now that the smells that we detect, those differences are real. They are significantly different, and we can all smell them, right? We all enjoy eating bread, we all enjoy it and respond positively to that smell of bread, and we respond negatively to the smell of rotting, right?

“But there is a definitive shift. As your microbial community shifts, so too does the functionality of that community.

Bob Holmes: “Right. OK. So that’s the first 14 days to when you get to a point where the sourdough becomes essentially ready to use. Let’s ask the folks in Belgium next, I think, what do we know about how much starters change after that? Is it over a scale of like months and years? Is the sourdough starter stable or do the microbes stay the same?”

Guylaine Lacaze: “From our experience, once your starter is stable or once your flora is defined, it will be very difficult to change it, very difficult. If you always take the same temperature, just use the same water content and so on, it will be really difficult to change the flora. Nevertheless, we know that, for instance, if you have different species of lactic-acid bacteria in your mother dough, this will remain over time, but it is possible that the ratio between the species move a little bit.”

“And then that’s why sometimes it is possible that the characteristics of a sourdough may change even if the dominant species remain the same.”

Karl De Smedt: “That meaning, to say that in a nonscientific explanation, you can look at it, at the starter, as a city. Paris was Paris 1,000 years ago, and there were families in Paris that still to date thrive in Paris, and within 1,000 years, they might be there or other families might come and go, it all depends how the walls will remain, and the name will remain, but it will depend on the conditions in the city. Is everybody happy with these conditions? Or will they just say, ‘No, this is not my place, I move to another place,’ and they will go. That’s a bit how this ecosystem in a sourdough is actually behaving. Some families will become bigger, and some families might become smaller, but they might remain, or they might disappear depending on how you treat your sourdough.”

Bob Holmes: “OK. So you’re sitting in front of cases of starters there. If I were to bake a bread with one of those starters and not tell you which one, could you recognize it? Are there some of them that you could recognize?”

Karl De Smedt: “I think I’ve never been to put to the test. I never had people baking with sourdoughs from the library and then say, ‘Come on Karl, which one is now, which one?’ I think it will be very difficult to recognize. But we might see, I’m sure that if we bake with the one that Guylaine has here, a bread, and we do it exactly the same with what I have here, we might see the difference. We will see the difference.”

Bob Holmes: “OK, yes, if you compared them they would taste somehow different.”

Karl De Smedt: Yes, but fact that your sourdough out of a lot of 10 breads and say, ‘Find yours’ — ooh, very tricky. We might get trained on that. Maybe if we really train on that we might be able to, but it’s going to be hard.

Erin McKenney: “Sourdough sommeliers.”

Bob Holmes: “Yes. So different sourdough starters have different microbes in them and produce different flavors. How much do we know about that process, about what accounts for the different flavors? And can you assign those flavors to particular microbes? Can you taste or smell a starter and say, ‘Ah, that one’s got a lot of blank bacterium?’”

Guylaine Lacaze: “I think that first, there is the fact that we have the two microorganisms, so the lactic acid bacteria and the yeast. So it’s the first … because each of these microorganisms will produce a different kind of molecules. So the lactic acid bacteria produce the acids, so the lactic and the acetic acid, OK?

“Within this lactic acid bacteria, some lactic acid bacteria species produce more acetic and other produce only lactic, OK? So it’s already a big difference within the bacteria and between the bacteria and the yeast. And the yeast from their side, they will produce mainly alcohol, sugar alcohol, aldehydes also, during the fermentation.

“But now, to see between one species to another one, if they produce specific molecules, it’s quite difficult because it depends on, also, on the flour and on the precursors present in the sourdough.

“The flavor production during the fermentation of the sourdough, something which is also interesting in that there is a co-work between the lactic acid bacteria and the yeast. And for instance, I said that the yeast produce superior alcohol and this is thanks to the action of the lactic-acid bacteria, which release in the sourdough precursors which are used by the yeast to produce some specific compounds.”

Bob Holmes: “So that the yeast and the bacteria are working together to produce the compounds that make the flavors. OK. What all the home bakers listening right now will want to know, I think, is how much control they have over the final flavor of the sourdough, and I gathered that there’s quite a bit of control in terms of how the baker can influence the flavor of the bread. 

“Erin, you’ve already mentioned starting with different flours. And I know you’ve seen quite different flavors depending on whether you start with corn flour or buckwheat flour and things like that. Can you talk a little bit about that?”

Erin McKenney: “So yeah, and I would say first, just to echo what Guylaine was saying, there’s an added complexity to trying to identify a specific bacterium because there’s so much variation, not only metabolically based on what foods are available, like the flour type, but also, there’s so much strain-level diversity within a species, right?

“To avoid competing with each other, they actually might utilize different metabolic pathways. So you might have the same species of microbe producing different compounds under different conditions. I always think of it as very fractal in its complexity. You don’t have a single player performing a single role at any point; you have, not only the cooperative efforts that Guylaine was talking about, but you also have a lot of variation, even within a single species of organism. Speaking to differences based on the flour types, one of the most interesting and consistent aromas that we detected in our succession project was, the amaranth starter smelled like ham.”

Bob Holmes: “This is a starter that’s made with amaranth flour instead of wheat flour.”

Erin McKenney: “With amaranth flour. Yeah, so one key take-home from that project was five of our 10 flours were gluten-containing flours, and five of them were gluten-free grains. We were able to grow mature successful starters that actually rose. I think buckwheat tended to have the least rise over a 24-hour period, but all 10 flour types produced starters that rose. So you can create and grow a successful starter even from a gluten-free flour, which I know is a question that resonates among bakers. That’s something we come up with all the time. So maybe that’ll knock one off the Q&A.”

Bob Holmes: “Yeah.”

Erin McKenney: “But yeah, so we not only have those differences in performance, but we also see differences, or we smell differences, in those aromas. We developed with Michael Bom Frost’s lab in Denmark, they helped to develop a sourdough sensory wheel. So, similar to a wine wheel, with these different categories of, ‘Do I smell the broader super category of grain cereal? Oh, well, is that more like porridge or more like toasted grain?’

“And then thinking about the more sour fermented vinegar smells that I would think of as broadly a category of more acetic acid, right? And again, no matter who’s producing it, right? Let’s see, when I did a taste test with middle school students — we had 275 middle school students grow starters last, last spring. So I think it would have been 2018, in January and February. They grew up their starters, they voted for the best ones within each school and then Boulted Bread baked bread from all of their best starters using the same base recipe, and then we did taste tests to see what are those flavor differences. Because it’s one thing to smell a starter in a jar and another thing to taste the final product, right?

“So I actually took — you can’t pass up bread — so I took bits of the bread from all three different schools and had my own little taste test that night. And the millet loaf was so bright, it was almost lemony, very effervescent, it was incredible.

“Whereas the red turkey wheat, and you can tell that this had lasting impact because I can remember this two years later, but the turkey wheat had a lot more toasted caramelization flavors, almost some coffee flavors. So definite Maillard reactions on the crust as well as a different crumb structure and flavor. And differences, more yogurty tartness or that dairy sour that I would identify with those lactic acids.”

Bob Holmes: “OK. Yep. OK, and there’s a lot of other ways that bakers can control the nature of their sourdough, I suppose. One of them being temperature. I’ve had some questions from the audience as well about temperature. Guylaine or Karl, would one of you like to talk about the effect of temperature, how much it matters what temperature your kitchen is at as you’re growing the starter?”

Guylaine Lacaze: “Yeah, so about the temperature, so if you speak about mother dough which is already stable, if you refresh it with low temperature, you will promote the yeast growth and activity. So if you want to have a sourdough which leavens quite well your bread dough, it is sometimes better to have a lower temperature fermentation … to avoid too much acidity production by your bacteria.

Bob Holmes: “OK, so lower temperature means more yeast and that means more of the fruity sorts of flavors and less of the sharp sour flavors that come from the bacteria which prefer heat? OK, so if you want a really sour sourdough, then you ferment it at a higher temperature.”

Guylaine Lacaze: “That’s correct. … because to give an idea, when we say warm temperature, it’s in degrees Celsius, so it’s more than 30 Celsius degrees.” 

Karl De Smedt: “Which is 86 in Fahrenheit.”

Guylaine Lacaze: “And we speak about cold temperature when you are below 25 Celsius degrees.”

Karl De Smedt: “So 25 is 77 Fahrenheit.So below 77 Fahrenheit, you favorize the yeasts.”

Guylaine Lacaze: “And also, when you start your mother dough, if you start it at lower temperature too, it seems that you promote the development of a type of lactic-acid bacteria, the one who produce the acetic. Maybe Erin has seen this type of development according to the temperature.”

Erin McKenney: “So just to make sure I’m on track, you’re talking about the lower temperatures also favoring the production of the acetic acid?”

Guylaine Lacaze: “Yeah, and also the growth, when you start from scratch your mother dough. If you use low temperature, so 20-25 Celsius degrees of temperature, you will start to favorize the growth of the lactic-acid bacteria, which are heterofermentative, so the one who produce the acetic acid.”

Erin McKenney: “So my best guess there, without having done any quantitative experimentation in the lab, right? So much is speculation still, right? It’s really exciting that we have so much opportunity for future research. But my thought there is, you are favoring the growth of those yeasts at low temperatures.

“But as the yeasts run out of resources and have to switch over, they start to get stressed. And if they produce alcohol, then that alcohol can be then converted also to acetic acid. So that’s my thought, is that you might actually be favoring a different metabolic pathway at a lower temperature.”

Bob Holmes: “Hmm. OK.”

Erin McKenney: “Does that make sense?”

Bob Holmes: “Yeah, it does.”

Karl De Smedt: “The fermentation in cooler temperatures, you keep it shorter, no? To avoid this run out of … metabolism.”

Erin McKenney: “Right. So it’s kind of like your triangulation point, right? You want enough yeast activity but not too much acetic acid, but you still want a decent rise. Yeah.”

Bob Holmes: “One of the things that really surprised me was that it seems to matter, to the flavor of the starter, whether you have a stiff starter or a runny starter. Would someone explain what’s going on there, please?”

Guylaine Lacaze: “Here again, I think here it’s due to the metabolism changes. With a liquid sourdough, you have less oxygen than in a firm sourdough. In a firm sourdough, a lot of oxygen is available for the lactic-acid bacteria, which produce the acetic. And when these bacteria have this oxygen available, they produce even more acetic than the lactic.

“And so, well, in general, the firm sourdough are more acetic and with the stronger…” 

Bob Holmes: “You get a sharper sourdough if it’s stiffer?”

Karl De Smedt: “You get more tang in them, yeah.”

Guylaine Lacaze: “Yes, whereas if we want a more creamy, smooth and less acidic … You feel less the acidity in this liquid sourdough.”

Bob Holmes: “OK. One of the other practical things that home bakers are going to want to know is, if they’re like me, you bake every week for a few weeks, and then something happens in your rest of your life and the sourdough gets neglected for a while, what do you do to keep a sourdough going, if you’re going to bake sporadically? How do you keep it alive and what’s your best process? What advice for someone who’s going to leave their sourdough starter for three weeks, say?”

Karl De Smedt: “I can take that one. What we do here in the library, that’s exactly what we do here in the library. Here in the library, we kind of neglect our sourdoughs.

We don’t bake with them every week, we maintain them in the fridge and we only feed them every two months. Every two months, we have the original flours of all these starters, and yesterday and the day before yesterday, we fed them all. And what we do in order to preserve the microorganisms, two months is the absolute maximum you can go. If you go longer, if you neglect your starter for more than two months, you might start losing some of the microorganisms that are inside because they just are too hungry.

“So when you neglect your starter in the fridge, well, try not to neglect it for more than two months, because some of the microorganisms you have might disappear and your starter might change. And then, you need to bring it back to condition. And that’s not just with one feed, we feed here three times with six hours in between. And that’s what I do at home when I bake.

“I do not bake every week. Well, now with lockdown a bit more than before, but I refresh my starter three times before I bake with it. So I feed it at midnight or at 6 o’clock, when I come home, I feed it once. I feed it again at midnight, and then at 7 or 8 in the morning, I feed it the last time, and then at lunch or after lunch, I start baking with it, because you expect from a sourdough, when you want to bake with it, you ask it to run a marathon because it has to deliver and you do not start or you make a bike race of 150 kilometers, you don’t do that on an empty stomach, you need to be well-fed, you need to have the fuel to do that.

“And that’s what you do by refreshing your starter a couple of times, you bring it back to speed in order to deliver at the end and give you a nice loaf of bread with a nice volume and in good flavor and not something that is low in volume where the gluten network is destroyed because of the acidity and you have a lot of tang in the bread. That’s not what you look for, right, when you bake it.

Bob Holmes: “OK, cool. We’ve had a couple of questions from the audience about water. Does choice of water make a difference? One of the questions actually came from someone who’s a professional coffee taster. And coffee tasters spend a lot of time optimizing the mineral content of the water for producing the flavor that they want. Does the same factors matter to sourdough?”

Karl De Smedt: “Yes. Maybe Erin has something from her side and then we can explain from our side. I don’t know if Erin ...

Erin McKenney: “Let’s see, from my experience in the Sourdough Succession Project, we went with distilled water just to make sure that everything was controlled from the same source. Because it stands to reason that your mineral content is also going to favor different metabolic processes or different activities in these different microbes, whether it’s different species or favoring different metabolic pathways or even favoring different strains, right?

“So in order to control for that, not having studied it, right, to me this is another exciting frontier for sourdough research. But to the home bakers who say, ‘You know what? I don’t necessarily want to go to the store and buy distilled water,’ I often will just have a pitcher or a jar and just leave it out overnight to make sure that I’m not dumping a lot of fresh chlorine from pipes into the starter, but I have also in a pinch used water straight from the tap. And it has been fine.

“I think that’s been more possible with a more resilient, mature sourdough starter. Once you have high-population densities and a more complex, more mature community, that you might be safer or have more resilience to the introduction of chlorinated water.But it is possible.”

Bob Holmes: “OK, so before we go to more audience questions, there’s one other thing I want to make sure that I cover with you folks, because both groups have ways for home bakers to participate. And maybe we’ll start with Belgium, and the Quest for Sourdough. Karl, can you talk a little bit about the Quest for Sourdough — what it is and how people can participate?”

Karl De Smedt: “Yes, well, absolutely. Thank you. So the Quest for Sourdough is an initiative we started back in 2016. When we had the library inauguration done in 2013, we opened the library and we had 43 sourdoughs. And so we were looking to have an idea of what is going on in the world of sourdough, and where are all these sourdoughs.

“And we have, of course, a network of customers all around the world from whom, we know, who have sourdoughs. But then there’s so much more outside, there is small pizzerias, there is schools, there is universities, there is research centers, there is, of course, this amazing crowd of home bakers, who they all have sourdoughs, but we don’t know these people.

“So we developed this website,, and there we ask, well, everybody who owns a sourdough to register it online, or you can even if – Erin, you have 14 or 24, I don’t know, you can even make your own inventory online of all these different sourdoughs just by registering it in a couple of steps where you need to give a name to your sourdough and then you can tell us how you made it, if there is a story behind how it smells, how it tastes, you can upload pictures. And as such, today, we have more than 2,100 starters registered.

“And some of these starters that we had found on the Quest for Sourdough are now already part of the library, where we think that they should be preserved for the future. So for us, it’s a tool to get a view on the world of sourdough and to find the sourdoughs that deserve a place in the sourdough library.

“And my dream is, of course, that maybe in two years or five years, we can have all these starters analyzed, and maybe placed here in the library. We will see how it evolves. Technology gets faster and cheaper, and maybe one day we will be able to do that and ask everyone who has registered his or her starter to send us some of it to keep it here in the library ...

Bob Holmes: “OK, great. And then Erin, let me just digress for a moment and say Erin’s been posting some links on the chat panel. We will also have those links on our own web page. Go to and click on the event, down at the bottom there will be links as well. So Erin, can you talk a little bit about the Wild Sourdough Project, which is something you’re hoping people will participate in?”

Erin McKenney: “Absolutely, and yeah, that’s just my inner teacher is coming out, give the information as you’re talking about it. So now you have the links to the two main projects that I’m going to plug for you. The Wild Sourdough Project, I worked with Lauren Nichols, Anne Madden and Rob Dunn to launch this project in mid-April, partially in response to Covid-19, to quarantine, and to everyone tapped into the zeitgeist of home sourdough baking and starting starters, and we thought this is a tremendous opportunity, one, to engage even more the global general public in thinking scientifically about sourdough, right?

“And two, it’s an opportunity for us to follow up on questions that sprung up during our initial Global Sourdough Project. So, in the Global Sourdough Project, we did cast a wide net. We were able to start to explore and investigate and characterize the major bacterial and yeast components of sourdough starters worldwide, but we weren’t able to get at questions of how does the age of a starter affect its activity and membership. And we weren’t able to get at the question of how do different flour types affect your starter’s behavior.

“So this was the perfect opportunity since folks are getting interested in sourdough anyway. Grow your starter, monitor its increase in height over time, take a picture of it, so we can see a side view and a top view to see how bubbly is it. Does it look frothy? Or does it have more of a placid, calm surface that hides the bubbles beneath? And just thinking about how do these functional behaviors, and of course the smells, all these aromas that are produced at different times across your starter’s life, and, at the point of its maturity, do those different functional behaviors — that undoubtedly impact your experience when you’re eating the bread that you bake with your starter — do they vary consistently, depending on where you are, or what type of flour you fed that starter?

“So that’s the first big plug – is for the Wild Sourdough Project. And the second one, this is also kind of a follow-up and a simplification of the Sourdough for Science initiative that I developed three or four years ago at this point, and that is particularly aimed for any educators. I imagine this includes many parents at this point, with so many students at home. I worked, again, with incredible teachers, right here in Raleigh, North Carolina, to create, design and align 10 or a dozen lesson plans that are centered around sourdough, but address curriculum standards that are required at the national level.

“So not North Carolina-specific, but actually US curriculum standards for sixth, seventh and eighth grades, not only in science and STEM, right? We included mathematics, but we also included a literature component for reading comprehension. So to really support student engagement in the scientific process, and to support our incredible teachers in the classroom. So that’s the second link – Students Discover. If you’re interested in incorporating maybe a more structured investigation, there’s an entire teacher’s packet that we developed on that website.”

Bob Holmes: “Cool. Thanks. OK, I’m going to go to some listener questions now. A reminder to the people that are in the audience, you can submit your own questions by clicking on the Q&A button on the bottom of the screen, and send them in, we’ll get through as many as we can over the next 20 minutes or so.

“Several of the questions that have come in so far have had to do with geography, with place. Here’s one who said, ‘A family member just mailed me her dehydrated 50-plus-year-old starter from the Yukon in Canada. When I revive the starter where I live in Nevada, will it change much just through natural exposure to our local microbiota?’ And several other people have asked more or less the same sort of question, someone noticing that, she’s shared her starter with people all over North America and they seem to get very different results. What do we know about, about how much a starter changes in a new place?”

Erin McKenney: “We actually tried to answer that question as much as we could with our Global Sourdough Starter Project data set, specifically looking at some different hub starters, right? Starters like — King Arthur distributes their starter, and the Carl Griffith starter that’s sent to anyone who wants a sample of it, right? Considering these hubs that then have many descendants.

“And what we have found — and this echoes what Guylaine was saying a few minutes ago — is that it depends on the conditions, right? It depends very much if the person you mail your starter to uses a completely different fermentation temperature or hydration, or flour type, right? Then you’re going to experience some changes.

“Ecologically, we might classify that as a disturbance event, right? Almost the equivalent of mowing your lawn or even chopping down trees and then letting that plot of land regrow a forest. You may not get the same forest at the end. So if we have, I think it depends on the ‘disturbance level’ compared to the original starter conditions. Does that echo with what you all would say at Puratos?”

Guylaine Lacaze: “Yeah, yeah, I totally agree with that, yeah.”

Karl De Smedt: “It’s what we try to do here in the library, is actually we try to keep the same. So to do so, we only feed, we have six moments of refreshments per year. So we do not open the jars that much. It’s not like feeding every day. So we limit it to six times per year. We use the original flour, or the original flours, because some of the sourdoughs contain up to six types of flour. So the owners send us every year, they send us the quantity of flour that we need in order to feed the starters. And so, we try to keep them as such.

“Now this year in summer, we wanted to do an experiment where we wanted to invite the owners of previous years who gave us a starter in 2014, ‘15, ‘16, ‘17, ‘18. And we wanted to ask them to bring back their starter that they have today in their bakery that we could compare it with what we have today in the library, and the initial analysis to see how did they change. Is there a change and what has changed? But OK, Covid decided differently but it’s definitely something we did not cancel, we delayed. So we will definitely do that experiment. And that will give us a bit more insight on what’s going on.”

Bob Holmes: “OK. This is sort of a related question. Several listeners have asked about how to maintain consistency from loaf to loaf. That how to keep your starter being dependable so that it’s at least the same, even if it’s — regardless of whether it’s the same or different than it was three years ago, at least to make sure that this loaf is similar to the last loaf. What’s the secret there?”

Erin McKenney: “I think you’ll notice that Karl and I both laughed.”

Karl De Smedt: “Measuring. It’s measuring. It’s measuring. That’s what we do as a commercial company. We produce sourdough to deliver it to customers in a consistent way, because we use the same microorganisms, the same flour and we follow the exact parameters of temperatures and speed of mixing and flour and everything is measured until we obtain the product. Like the winemaker or a brewer or a cheesemaker, they are following their process and they try to control what they do to get the consistency.

“At home, well, you do not measure that. So you go with the flow, you go with the temperatures, you do not play with it. So making a sourdough is easy. Making a loaf with that, that’s already complicated for many. Many people have to struggle and find the right way to make a loaf and make every day 1,000 loaves or 10,000 loaves like big bakeries. That’s even more challenging. And then to say that these loaves have to be the same on the first of January and the 31st of December year in, year out, forget it, Bob.”

Erin McKenney: “Yeah, I would agree with that completely, I think to guarantee consistency demands exactness and control. And you saw I have a paper towel over my starters, though I’m not hermetically sealing. So depending on what other ferments I have going in my kitchen, there may be different environmental inputs over Bavid Doughie’s life depending on if it’s a load of kimchi, or sauerkraut, or if I’m going for apples or tomatoes on a given week, right?

“Whatever’s growing in my kitchen or back when we would have company over inside, you’d have a sudden influx of, and someone brought their dog. So I think there are environmental factors that we may or may not be able to control. My home temperature also, despite a thermostat, as many days as we can we keep all the windows open so then, you get an environmental influx of temperature as well as microbes. So, yeah, I think it depends on how much you as a home baker require that consistency in your life. Because it would take a tremendous amount of time and energy to guarantee that consistency.”

Bob Holmes: “OK.”

Karl De Smedt: “But it depends more with who you live and for who you bake because they are the most critical. At least at home that’s, I don’t care, but my wife goes, ‘Whoa, it’s too acid.’ And my son, ‘Papa, there was something wrong this time.’ And so, yeah, if you have judges around you will determine how good or how bad it is.”

Bob Holmes: “OK, yep. A couple questions have come in about the library. One of them actually, is, what’s the oldest sourdough you’ve got in there?”

Karl De Smedt: “Well, it’s difficult to say. There is no carbon dating system for a sourdough. It doesn’t exist, so we need to trust or rely on what people tell us. We know we have sourdough from Switzerland, which is our No. 74, and that’s a sourdough that’s a sixth generation. And it is from a bakery in Simplon Dorf and goes back to 1868. We have a starter from Tokyo, Japan, which goes back to 1874, and then we have two Chinese sourdoughs.

“And when I went to China to pick up the sourdoughs, I asked to the lady — it was an old lady — and I said, ‘How old is it?’ She said, ‘Well, I had it from my grandmother, and her grandmother had it from her grandmother.’ And we don’t know. actually, how many generations these starters go back.

“What we do know is that the entry in the library, we know exactly that No. 43, we have had it since 1989. And it’s a starter from San Francisco. We don’t know when it was created, but we have it for that time in our possession. Within 20 or 50 years when the library will still be there and people will look into it, we will know exactly the age, and maybe that is the key to find out or to find a way to determine the age of a starter, but for the moment, we just have to trust on what people tell us.”

Bob Holmes: “OK. And sticking with the library, is there a strangest and most unique sourdough there?”

Karl De Smedt: “Strangest. The most strange sourdough.”

Guylaine Lacaze: “Like the one from Japan …”

Karl De Smedt: “Yeah, go for it.”

Guylaine Lacaze: “Yes. So we have a sourdough from Japan, which is based on rice, but not on flour, really on rice.”

Karl De Smedt: “It’s with cooked rice.”

Guylaine Lacaze: “Yeah, on cooked rice, yes. And this one is really different. Because it’s now we have the second one from rice but from flour that’s within the library. So now we have 122 sourdough. And most of them are from wheat and rye, and only two from rice.”

Bob Holmes: “OK.”

Karl De Smedt: “This one is from Tokyo and this one we got this morning from Germany, a small town near Cologne, Remscheid. This is white flour and this is cooked rice.”

Bob Holmes: “Interesting. OK, well, while we’re talking about flour, several questions from the audience as well about if you’re going to make a starter out of wheat flour, does it matter whether you use red flour or softer flour? Does it matter whether you use whole wheat or white flour, things like that, what do we know about that?”

Erin McKenney: “Another amazing question. And it’s like future research is needed, we need to keep track of all these Q&A because I think it’s a list of future projects that could be on the priority list for us and for other researchers. 

“I think it certainly matters, we don’t know how specifically or to what extent necessarily it matters. But off the top of my head, a white flour is going to contain much less of proportion of the bran, or of the germ of a wheat kernel, right? And we do know from kind of the ecology of a grain and of grasses that yeasts from the environment may be associated with the bran on the outside of the grain, even from the field that the wheat was grown in, but also the germ, some grasses – and these grains are typically considered grasses – some grasses actually have a close symbiotic relationship with some yeasts or some fungi.

“So by including more of the bran and more of the germ, in that milling process, in those different flours, whether it’s a whole grain compared to just a typical white flour, then you are potentially including more of those components of the grain that would have microbes associated with them. 

“I worked with high school students in 2017 and I told them, ‘Bring a cup of flour that you use most often, or the most interesting flour you have; and we’ll plate it, we’ll grow it on bacterial agar plates and selective nutrient agar to grow the yeast and the bacteria living in your flour.’ And this blew their minds, because I think No. 1, we don’t often think of our food as containing microbes all on its own, we have this strange idea that our foods are sterile. And they’re not, absolutely not.

“But then to see the differences, overwhelmingly, we tended to see that whole grain flours tended to have more yeast grow out of them whereas the white flour, the all-purpose, we would call it in the States, I think, strong flour, right? Might be the non-US term, right? For those more processed flours that have more of the endosperm, maybe a higher starch content, those tended to have a lot more Lactobacillus; not because they’re Lactobacillus living in that portion of the grain, but I think because those Lactobacillus are favored heavily by that starch component.”

Guylaine Lacaze: “And if I can add something about the difference between the wholemeal and the white flour. In the whole meal, there is more nutrient too because it’s important to feed your bacteria and your yeast when you refresh your sourdough. And with wholemeal flour, you bring them more vitamins, minerals, than with standard white flour. So it helps the bacteria and the yeast to grow all the way.”

Erin McKenney: “I made a time-lapse video in a couple graduated cylinders with the same sourdough starter fed white flour, a rye flour and a whole-wheat flour a couple years ago, and we did a time-lapse film. We even thought ahead to put rubber gloves over the top of the cylinders, and you could see over a period of 24 hours, the gloves inflated and then it was like the tubes were waving at you.

“But monitoring the growth to see the peak height reached by the different flours versus the rate, the speed at which it reached that peak and then fell back down. What I saw was that the microbial metabolism really mirrors our own metabolism of those same flours. So as Guylaine was saying, if you’re feeding more nutrients and more complex nutrients in a whole grain flour, we saw a higher and a more sustained rise, whereas the white flour, it was like a sugar crash, right? It got really high really fast and then ran out. And I think that’s the way at least I feel sometimes, if I eat something that’s made with a highly processed, a lot of white flour.”

Bob Holmes: “OK, maybe, we’re just about out of time. I think we’ve got time for maybe one more question. And this is something that I suspect echoes what a lot of homemakers wonder, which is, at least from time to time we struggle with getting the sourdough to lift enough, to have enough power, fermentation power to leaven the bread properly. Are there strategies — if you’re struggling with that — are there strategies to get sourdough to leaven better, to rise better?”

Karl De Smedt: “Yeah. Yeah, I think, well, it’s bringing it in shape to run the marathon?So there is different ... you can feed it a couple of times, with six hours in between to really bring it up to activity.But what happens most of the time is that people ... you accumulate sourness. All these organic acids, they accumulate in your starter and that has an impact on the yeasts, actually, that can thrive in your sourdough.

“So a good trick to do is to give your sourdough a bath. Now, maybe we don’t have time to explain everything, but I have on the Quest for Sourdough, I have a video where I show how to give your sourdough a bath. If it’s a liquid or a stiff one, you just have to add flour into it that it gets really stiff and then you can put it into small pieces; you tear it apart, you put it in lukewarm water like you would give your baby a bath and actually then the organic acid is released. 

“And that’s coming from the method of Piedmontese. Piedmont is a region in Italy where bakers tend to have stiff starters and they maintain them in water. And so during the ... their mother is always bathing in water and as such the acids release in the water and you can really see that when you measure the pH of that water, it’s really acid. And as such the sourdough remains less acid and has more fermentation power and potential, and a good environment for the yeast to thrive. When you see your sourdough is lacking fermentation power, give it a bath.”

Bob Holmes: “Interesting. OK, I think we’re out of time. So let’s stop there. Thanks to all our panelists for participating and the audience as well for joining us. There’s more information, as I said, including links to Citizen Science websites, on the Knowable website,, click on the event and scroll to the bottom. And thank you, all. This event was funded by a grant from the Alfred P. Sloan Foundation, with major heavy lifting from the good people at our parent company, Annual Reviews. Thank you very much for joining us. Good night.”