Many claim that there is something special in the water or air that gives San Francisco sourdough bread its unique tang and local bakers often say that no one outside of the area can replicate the flavor and texture of San Francisco sourdough.
Sourdough bread is made from a starter – a slurry of water and flour brimming with microorganisms. Once fed, bacteria and yeast begin to eat up the nutrients in the mixture and produce an array of compounds that give sourdough its flavor. The microbes also produce carbon dioxide, which makes the starter rise. Once the nutrients in the flour have been digested by the microbes, growth slows down and the starter falls. The starter almost seems alive.
In 1971, a previously undescribed lactic acid bacterium was isolated from San Francisco sourdough. The scientists named it Lactobacillus sanfranciscensis. But it turns out that this bacterium is not unique to San Francisco. It has been discovered all over the world – in bakeries in France, Italy, and China, for example. Could it be possible to make San Francisco sourdough outside of San Francisco?
Now, many scientists are trying to answer questions like this one by teaming up with sourdough enthusiasts all over the world.
The Great Sourdough Extravaganza
One such scientist is Robb Dunn, a professor of applied ecology at North Carolina State University. “We know that when you make a sourdough, the species and strains of microbes in that starter [influence the flavor of bread],” Dunn said in an interview with Gastropod. “And yet it’s still pretty mysterious what determines which of those microbes are originally in your starter.”
Dunn and Anne Madden, a postdoctoral researcher in his lab, are determining what could affect the sourdough starter’s community. They recruited a group of 15 bakers to help. Each baker received the same flour and instructions for creating a starter from scratch. They cultivated these microbial pets for a month. The literature suggests that an active starter can develop in about 10 days, but the actual time can vary tremendously. “We wanted them to have a baking-worthy starter, so we wanted to make sure they had plenty of time [to grow],” says Madden. Then, Dunn, Madden, and the bakers all headed to Belgium for a three day sourdough extravaganza at the Puratos Sourdough Library.
When they arrived, they realized all the starters smelled and looked different… and so did the bread that the bakers made following the same instructions. Why were the starters so different when they were made with the same flour?
To answer this question, Madden and Dunn collected samples of the starters and the bakers’ hands to identify what microbes were present. They found that there were around 200 species of microbes in the starters. Of these, roughly 80 were also found on the baker’s hands and another 80 were found in the flour. Some of these overlap. Almost no microbes were found in the water, confirming Dunn and Madden’s hypothesis that microbes in the water do not play a big role in sourdough microbial communities. What about the microbes not account for by the hand or flour microbiome? These microbes likely come from the air.
Madden and Dunn also found that the baker’s hand microbiome was different from the hand microbiome of a non-baker. In non-bakers, their hand microbiome contains typically 2-4% lactobacillus, but in sourdough bakers, they found that nearly half of their hand microbiome consisted of sourdough bacteria.
Even More Sourdough Starter Data
Dunn’s team is also collaborating with Benjamin Wolfe’s lab at Tufts University in a different sourdough exploration. Here, they tasked themselves with an even bigger project. Instead of 15 sourdough starters, the researchers amassed 571 sourdough starter samples sent from all over the world. Their ultimate goals is to not only identify what microbes are present, but to also see how flour, geography, and age of the starter affect the microbial community.
So far they have noticed that starters from the East and West Coast of the US are not that different from one another and that starters bought from King Arthur Flour seem to be more similar to each other. Once they identify what microbes are living within the starters, they hope to identify connections between the microbes present, the compounds produced, and the aromas and flavors of the bread.
Making Herman Cake
Now let’s turn to Herman, the latest sourdough starter to jump on the citizen science bandwagon. The Herman Project gets its name from the German tradition of passing down sourdough starter to friends to make a “Herman cake” or “Friendship cake.”
Herman, is typically grown for ten days before it is divide for baking and for giving away to a friend. The recipient of Herman would do the same thing, passing its “descendants” on to friends. Gabriel Leventhal, a postdoctoral researcher in Otto Cordero’s lab at MIT, first came across Herman as a child. But in 2015, after years of scientific investigation, he began to think of the starter in a new way. “It suddenly occurred to me that what everyone is doing is actually a serial transfer evolution experiment with sourdough,” Leventhal recalled.
And thus, the Herman Project was born. The Herman Project catalogs the journey of Herman as its descendants are passed on from one participant to another and from one location to the next. Herman’s descendants and their travels across the world are kept track online and samples are sent back to the lab where the microbial community is determined. But this is more than a sample collection project for the Herman caretakers – its participants are actively steering the ecological and evolutionary changes in the sourdough.
“Everyone really is the scientist and participates in conducting at least four transfers of the experiment, much like we would partition such daily work among lab members,” Leventhal says. “The choice they make about what flour to use, what water to use, and where to store Herman all will influence the ecological and evolutionary outcome of the experiment.” The Herman Project is just at its infancy but Leventhal hopes to gather at least a few hundred data points.
This unique project combines the involvement of citizen scientists in the experimental process with the study of diverse microbial communities long term – a new territory for citizen science projects. “We have learned a lot about how single populations evolve from experimental evolution projects such as Richard Lenski’s long-term E. coli evolution,” Leventhal says. “Yet, in nature, microbes almost never occur as isolated clonal populations.”
Maybe one day, we will have an answer to what makes San Francisco sourdough so special or perhaps we can soon create specific starter formulation based on the desired sourdough bread aromas. We certainly have a lot to learn about these microbial communities bubbling away in our kitchens.
