The strange science inside your sourdough

The strange science inside your sourdough

We know sourdough’s flavour and texture comes from the activity of microscopic bacteria – but where exactly are they coming from?

In a room of refrigerators in Belgium live more than 110 jars of flour, water, and magic.

At least, that’s how you might sometimes think of sourdough starters, the cultures of bacteria and yeast that bakers mix into dough instead of commercial yeast to produce bread of delicious complexity, with a biting acidity or a firm creaminess, depending on the recipe.

Starters are made by leaving an inviting slurry of flour and water out on a counter and waiting until microbes colonise it, turning it bubbly and sour. But most bakers don’t have DNA sequencers on hand to see exactly what is living in their starters, which often look, smell, and taste different. To boot, just where the microbes come from? The air? The flour? The baker’s own microbiome?

Even whether a starter, refreshed with flour and water and passed from person to person over a hundred years, is really the same thing at the end that it was at the beginning, is an open question.

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Luckily, biologists and bakers are starting to delve into the mysteries of sourdough. To get an inside look at a very interesting experiment, Cynthia Graber and Nicola Twilley, the team behind Gastropod, a podcast about food culture and science, went with a pair of microbial ecologists to Belgium, to the sourdough library of Karl de Smedt. (You can listen to the resulting Gastropod episode here.)

Housed in a room at a company called Puratos, where de Smedt is head of the Centre for Bread Flavour, the library began to take its current shape when a Syrian baker of traditional chickpea cookies contacted de Smedt and asked if he would help document and preserve his starter, relates Twilley. The baker’s sons were interested in shifting from the traditional starter leavening to commercial yeast. He hoped that de Smedt would help keep the starter from disappearing.

The strange science inside your sourdough
View image of Baker kneading dough (Credit: Getty Images)

The library grew as de Smedt collected other notable starters, making sure to feed them with the exact same flour they would have had back home in an effort to preserve their unique character.  

When Twilley and Graber first entered the library, de Smedt counted down “Three… two… one…” and opened the door. “It was honestly like a treasure room in a museum,” said Twilley. Behind the glass fronts of the refrigerators were spot-lit jars, and a shimmering projection of leaves played across the ceiling.  

The bakers were there to bake bread with their starters, using exactly the same recipe, and see whether the breads tasted different

If the library reflects an effort to collect and preserve unusual starters, the experiment Puratos and de Smedt helped coordinate was an effort to understand how they get that way. In Belgium along with de Smedt, who is also the communications and training director at Puratos, and the podcasters were 12 bakers from around the world. The bakers were there because Anne Madden and Rob Dunn, microbial ecologists at North Carolina State University, had sent them all the same flour with the same instructions on how to make a sourdough starter.

They had brought their precisely concocted starters along so the scientists could take samples to see what microbes were there, whether they were different from starter to starter, and whether the microbes were also present on the hands of the bakers and in the original flour. The bakers were there to bake bread with their starters, using exactly the same recipe, and see whether the breads tasted different.

The results of the Belgian experiment are currently under review before publication. But Madden recently shared some details of what they found. First of all, the starters were not all the same, despite being made the same way with the same ingredients.

The strange science inside your sourdough
View image of Sourdough loaf being broken (Credit: Getty Images)

There were more than 350 strains of microorganisms across the starters, and most of the starters had yeasts from the Saccharomyces genus, the same group that common baker’s yeast belongs to. But one was dominated by completely different yeasts, from the genuses Naumovozyma and Kazachstania. Another recent project from the research group – called the Global Sourdough Project, which aims to look at the effects of geography on the make-up of different starters – found these yeasts to be a distinctive feature of Australian starters, says Madden.

Second, when the researchers swabbed the bakers’ clean, washed hands and cultured the resulting microbes, they found that the bakers’ hand microbiomes were a bit different to other people’s. They looked more like the sourdough starters’ microbial makeup, suggesting that perhaps the constant dunking of their hands in acidic bread dough had influenced the survival of the fittest on bakers’ hands and produced a different set of tiny inhabitants. (Make no mistake – your body and your food are playing grounds for natural selection, just like the larger-scale habitats we’re more used to thinking of.)

We are selecting for the microbes that do what we want and that give us a nice taste, handing an evolutionary advantage to the ones that please us

Perhaps most interestingly, however, almost all the microbes found in the starters themselves were either also found on the bakers’ hands or found in the flour. Only 31 out of more than 350 were not, suggesting that the common idea that starter microbes are wild and drift in on the air is less likely than that the microbes already in flour and on bakers’ hands are what’s making bread rise. Still, Madden notes, it’s hard to establish causality with the current data. Which way the microbes are flowing, from the hands to the starter or in the reverse direction, isn’t clear.

What is clear is that sourdough starters, and the bread made with them, represent human manipulations of the evolution of microbial communities. We channel the growth of particular members of these communities by raising or lowering the temperature of our dough, by feeding starters more or less frequently, and by giving them different kinds of flour. We are selecting for the microbes that do what we want and that give us a nice taste, handing an evolutionary advantage to the ones that please us.

In all those spot-lit jars, and in kitchens around the world, selection is at work, bringing us delicious bread.

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