Sourdough starter is a simple blend of flour and water that bakers rely on to make bread rise. For scientists, it is also a powerful way to study how living organisms change over time. The familiar chewy texture and tangy flavor of sourdough come from a complex mix of microorganisms that ferment the dough. Research over the years has uncovered more than 60 types of bacteria and over 80 kinds of yeast in sourdoughs from different regions of the world. "We can use sourdough as an experimental evolution framework, to see what happens over time," said evolutionary biologist Caiti Heil, Ph.D.

In a recent study published in Microbiology Spectrum, Heil and researchers at North Carolina State University in Raleigh set out to explore how flour choice affects the microbes living in sourdough starters. Their analysis showed that yeasts from the genus Kazachstania were consistently the most common across all starters. In contrast, the bacterial communities varied depending on the type of flour used.

What This Means for Bakers and Flavor

The findings suggest that changing flour types could influence the microbial makeup of a starter. "And because the microbial composition affects different traits, by altering the flour you could potentially alter how your bread tastes," said Heil, the study’s senior author. More broadly, she explained that the results show just how responsive the sourdough microbiome is to environmental conditions.

Earlier research has shown that sourdough microbes are shaped by multiple influences, including the flour itself, the surrounding air and surfaces, and even the hands of the baker. Starters can be made with wheat, rye, barley, teff, millet, or other grains, each supplying a distinct set of nutrients that microbes depend on to grow.

A Classroom Experiment Sparks the Study

The research began with an educational project led by Enrique Schwarzkopf, Ph.D., a postdoctoral researcher in Heil’s lab and an avid sourdough baker. He created a program at a local middle school to teach students about fermentation and evolution. Schwarzkopf, who maintains a sourdough starter named Seth, encouraged students to test different flour combinations and feeding schedules to see which starter would grow the fastest.

To analyze the starters, the researchers used metabarcoding, a genetic method that quickly identifies which microbes are present in a sample. Each starter began with one of three substrates: all-purpose flour, bread flour or whole wheat flour. At the start of the experiment, the flours showed similar bacterial profiles and contained a variety of yeasts.

Unexpected Yeast Dominance

After several weeks of repeated feeding, the microbial communities shifted. The starters all ended up dominated by the same yeast, while bacteria showed greater diversity. Heil said she originally expected to find Saccharomyces cerevisiae, also known as brewer’s yeast, which is commonly used in baking and is central to much of her lab’s research.

Instead, Kazachstania emerged as the leading yeast in every starter, regardless of flour type or feeding schedule. Genetic analysis also revealed differences among bacteria. Starters made with whole wheat flour contained higher levels of Companilactobacillus, while those made with bread flour had more Levilactobacillus.

Flour as an Ecological Driver

Heil, whose work focuses on how organisms adapt to new environments and compete at the genetic level, explained that each flour type offers unique nutritional conditions. Linking those differences to the environments microbes experience, she said, can help scientists better understand how diverse microbial communities form, compete, and persist.