Every sourdough baker has a starter with a story. Some have been fed daily for years, passed between friends, carried across cities. The bread they produce can taste completely different from one kitchen to the next — tangier, chewier, more complex — even when the basic recipe looks identical on paper.

Scientists have long suspected that the invisible microbial communities living inside each starter hold the answer. New research confirms that one of the most powerful influences on those communities is something bakers handle every time they feed their starter: the flour.

A study published in Microbiology Spectrum, led by researchers at North Carolina State University, set out to examine how flour type shapes the microbes living in sourdough starters. What they found challenges some assumptions about which organisms drive fermentation — and opens practical questions for anyone who bakes regularly.

What Lives Inside a Sourdough Starter

A sourdough starter is, at its most basic, flour and water. What makes it alive is the ecosystem that develops within it — a community of bacteria and yeast that ferment the dough, produce the gases that make bread rise, and generate the acids responsible for that characteristic tangy flavor and chewy texture.

Researchers have identified more than 60 types of bacteria and over 80 kinds of yeast in sourdough starters sampled from different regions of the world. That diversity reflects how sensitive these microbial communities are to their environment. Earlier research has established that sourdough microbes are shaped by multiple factors — the flour itself, the surrounding air and surfaces, and even the hands of the baker handling the starter regularly.

"We can use sourdough as an experimental evolution framework, to see what happens over time," said evolutionary biologist Caiti Heil, the study's senior author.

How the Study Began

The research originated in an educational project rather than a conventional laboratory setting. Enrique Schwarzkopf, a postdoctoral researcher in Heil's lab and an committed sourdough baker who maintains a starter named Seth, created a fermentation and evolution program at a local middle school. Students were given different flour combinations and feeding schedules and asked to track which starter grew the fastest.

That classroom experiment became the foundation for a more rigorous scientific investigation. Each starter in the study began with one of three substrates: all-purpose flour, bread flour, or whole wheat flour. Researchers used metabarcoding — a genetic identification method that rapidly determines which microbes are present in a sample — to track how the communities in each starter changed over time.

At the outset, the different flour types showed similar bacterial profiles and a variety of yeasts. After several weeks of repeated feeding, the picture shifted considerably.

The Yeast Result Nobody Expected

Heil had expected the dominant yeast to be Saccharomyces cerevisiae — the organism commonly known as brewer's yeast, widely used in commercial baking and central to much of her laboratory's previous research. Instead, a different genus emerged as the consistent leader across every single starter tested.

Kazachstania dominated all of the starters regardless of flour type or feeding schedule. This result held across all three flour conditions, suggesting that once a sourdough starter is established and fed regularly, this particular yeast is well-positioned to outcompete others in the environment the starter creates.

The bacterial results told a more varied story. Unlike the yeasts, bacterial communities responded differently depending on the flour being used.

1. Starters made with whole wheat flour developed higher concentrations of Companilactobacillus, a bacterial group associated with specific fermentation characteristics.

2. Starters made with bread flour showed elevated levels of Levilactobacillus, a different bacterial profile entirely.

3. All-purpose flour starters fell between these two outcomes, reflecting the intermediate nutritional profile of the flour itself.

What This Means for Flavor and Baking

The connection between microbial composition and bread character is well established. Different bacteria produce different acids and compounds during fermentation, which influence how the finished bread tastes, how the dough behaves during proofing, and what kind of crust and crumb structure develops in the oven.

The practical implication of the new findings is direct. "Because the microbial composition affects different traits, by altering the flour you could potentially alter how your bread tastes," Heil said.

Each flour type supplies a distinct nutritional environment for the microbes living within it. Whole wheat flour contains more complex compounds from the outer layers of the grain. Bread flour has a higher protein content and a different carbohydrate profile. All-purpose flour sits in the middle of the spectrum. These differences create distinct ecological conditions — and the microbial communities that thrive in each one reflect those conditions.

For Heil, whose broader research focuses on how organisms adapt to new environments and compete at the genetic level, the sourdough starter offers an unusually accessible window into those processes. "The results show just how responsive the sourdough microbiome is to environmental conditions," she said.

There is something quietly compelling about the idea that a decision as simple as which bag of flour to open can set in motion a cascade of microbial competition that ultimately shapes the flavor of a loaf of bread. Bakers have known intuitively for generations that flour matters. Science is now beginning to explain precisely why — one invisible organism at a time.