The Sourdough Project

Rob Dunn Lab | Public Science Projects

sourdough-ballThere are millions of kinds of bacteria and fungi on Earth. We have found several thousand species in human belly buttons alone. Yet if you mix flour and water, the community of organisms that colonize the resulting concoction is almost always composed of a small handful of organisms that are able to leaven bread, yielding a sourdough starter. How this happens is one of civilizations great mysteries, a mystery at the heart of the bread making (and, for that matter, traditional beer brewing). Yet, while bakers understand how to make starters, the underlying biology of the species in these starters remains mysterious. Starters can produce similar effects on bread (and similar flavors), despite being composed of different species, a key different ingredient. Conversely, starters composed of the same species sometimes yield different flavors. Then there is the issue of what happens to starters over time. The organisms in starters are hypothesized, by some, to stay the same over time—an old growth forest of miniatures—even if their living conditions change. Few ecosystems are so (apparently) stable. Then again, starters can change through time, sometimes suddenly. Starters are, if anything, predictably mysterious. But not for long. We aim to understand the biology underlying the differences among starters and the changes (or lack of change) in starters through time.

Step 1: We Need Your Starters

If you have a sourdough starter that you love and cherish, we appreciate you taking the time to send it to us. We are honored to be able to study the organisms in your sourdough starter, especially if your sourdough starter is made using unusual ingredients, has unique flavors, is particularly old, or has a great story (this last part isn’t about the science, but we love great stories). [Thank you to all of you who have already submitted your sourdough starter, we are no longer accepting more starters!]

For the first thousand starters sent to us from around the world, we will use genetic techniques (metagenomics, in the lingo of my tribe) to determine which species are present in the starter. We will share this list of species with you so that you can get to know your starter’s micro-zoo by name.

Once we know which species are living in each starter, we will use statistical approaches to compare the different communities. By doing this comparison we can begin to understand the factors that influence which community lives in one starter relative to another, why your starter is different from that of someone else. Some bakers suggest (swear by, really) that the biggest factor influencing which species of microbes are found in a starter are the microbes that are on (and in) the bodies of the people who started it. Some say it is the grain that is used. Others, the climate. Others still, the water. Everyone might be right; it could be that each starter is influenced by many factors. But we don’t actually know. Once we know the identity of the organisms in each of a thousand starters from around the world (which may take us a while) we will be able to test the relative influence of each factor using statistical models — the same sort we would use to understand why there are, say, more bird species in the tropics. But there is more to the story, more than just ecology.

Step 2: Understanding the Evolutionary History of Sourdough Starters

Some sourdough starters are very old, heritage starters. Such starters have been passed along, person to person, through families. In some cases, they are passed from just one person in a generation to just one person in the next, the way the family’s prized silver might be passed. In other cases, they are shared with any child or grandchild who asks (and some who don’t). As these starters travel through time, something very interesting may be happening in them, something different from what happens in a young starter. The species in heritage starters may be, like Darwin’s finches in the Galapagos Islands, evolving unique characteristics. These characteristics may be a function of their circumstances and relative isolation. We don’t imagine that the yeasts and bacteria in starters grow wings or tails, but it is possible that they trade genes, or even lose some of those genes that prove less necessary. Each heritage starter is an evolutionary experiment that has already been conducted, but for which the data have not yet been taken. So we don’t really know the evolutionary stories of starters, not yet anyway. We will remedy this. Ben Wolfe, at Tufts University, and his amazing lab of wonder-scholars, are going to grow the microbes from each of one hundred of the most interesting starters (particularly some of the most ancient ones) and then use the microbes that they have grown to track the evolutionary histories of the starters. It is possible that old starters have a characteristic evolutionary signal. It is also possible that, through time, old starters diverge in unique ways such that the descendants of a particular starter might be identifiable based on the genes in their microbes. We might even be able to look at the evolutionary history of starters we know to be related, those passed among families. We don’t yet know. But thanks to Ben and his team (especially Liz Landis), and you, we will.

Steps 3 through 10: Where We are Headed

The work I have described above will tell us which factors tend to predict what is in your sourdough, perhaps even suggest where you might need to travel or what you might choose to add to your starter if you want to foster a particular species. It will also help to understand what happens to starters through time. But as an ecologist, I admit to a deep fondness not only for big geographic studies and comparative evolutionary ones, but also experiments.

In an experiment, one manipulates one or more variables of interest and controls for the other variables. In the context of sourdough starters, we could manipulate many variables, but one in particular fascinates me, the potential that the person who makes the sourdough starter imparts some of his or her flavor to the starter. The Lactobacillus bacteria, for instance, that play a key role in starters, include species known from human bodies. Lactobacillus bacteria inhabit vaginas, guts, and even, sometimes, armpits and belly buttons. It is entirely possible that when you make a starter that the particular Lactobacillus that you seed it with are your own symbionts. But how would we possibly know?

This is where the experiment comes in. This summer, in July, we will meet in Belgium with colleagues from the company Puratos. At the Puratos facility we will unite bakers from around the world. Each baker will, over a period of days, make each of several starters. We will sample the chef’s hands before they make the starters in order to know which microbes are present. We will then sample the resulting starters, after they are made. In addition, we will bake the breads using both the starters the chefs made and a subset of those starters that you have sent us. Once we bake those breads we will be able to say for a given set of ingredients how important the person who made the starter, and their microbes, were not only to which microbes were in the starter but also the chemistry and flavor of the bread. [We are now testing the protocols for this big experiment at Duke University, where Aspen Reese and Rytas Vigalys are working with students in their Microbial Ecology and Evolution class to do a pilot version of this experiment.]

All of this is just the beginning. We are also talking with Peter Kjærgaard, director of the Danish Natural History Museum, and Claus Meyer, of Nordic Food fame, about the potential of a project in Denmark. We want to work with children around Denmark to plant grains in school gardens, mill the grains in schools, as well as produce starters and breads in classrooms. In the process, we want to both do new science and reconnect children with their agricultural heritage and the wild diversity (of both grains and microbes) on which every bite depends. As for the United States, Anne Madden and Lea Shell are almost done with lesson plans that allow students in schools and universities to help us understand the ecology of sourdough by experimenting with sourdough starter ingredients and taking data on sourdough kinetics and ph.

Take Home

Meanwhile, if you do choose to participate in our project, know that we will care for your starter with love even as we give it the occasional exploratory poke or prod. Bread and the microbes on which bread depends are at the heart of our agricultural civilization, the ancient marriage between humans and grains. They are also at the heart of the ways in which we come together to break bread and savor the rich flavors of wild microbes and tamed grains. By this same token, we hope that by studying starters together, that you, Ben Wolfe, Anne Madden, Lea Shell, Liz Landis, Erin McKenney, Matthew Booker, Tate Paulette, and many others might understand and savor more than any of us could on our own.

What Can You Do Now?

We are no longer accepting sourdough starters, but we are beginning to process them in the lab, so updates are forthcoming.

Regardless of whether or not you have sent us a sample of your sourdough, if you would like to stay updated about our results or events related to the project you can sign up for email updates as well.

The Sourdough Team

In its broadest scope this projects involves professors who study the evolution of food microorganisms (Ben Wolfe, Tufts University), the ecology of microbes (Tad Fukami, Stanford/Natural History Museum of Denmark), human evolution (Peter C. Kjærgaard, Natural History Museum of Denmark), the ecology of life in homes (Rob Dunn, Natural History Museum of Denmark/NC State University), the history of food (Matthew Booker, NC State University), and the culinary arts (Michael Bøm Frost, Nordic Food Labs). But the truth is that the real leadership of this effort is being done by the six people you see in this picture, younger people, people in whom we would be wise to trust the future of food ecology, people in whom we would be wise to trust much else. These are, along with Lea Shell (not pictured), Anne Madden, Lori Shapiro, Angela Oliveira, Liz Landis, Lauren Nichols and Erin McKenney. Together this team (which recently convened the 2018 Sourdough Summit, pictured above) are the ones leading this project. Each of them has a kind of superpower, but collectively they have a combined set of skills for the work at hand like no other team on Earth, from public engagement to art, from culturing to sequencing. Everyone involved in this project, including all of the public participants, does their part, but this group, leads. Everyone else knows well enough to, most of the time, just get out of the way.