Sourdough bread may look simple, but the tangy flavor and chewy texture that define it come from a complex community of microbes living in the starter. New research suggests that one of the most basic baking decisions, which flour you use, can help shape that microbial mix.
In a study published in Microbiology Spectrum, researchers examined how different types of flour influenced the bacteria and yeasts that develop in sourdough starters over time. While the same dominant yeast appeared regardless of flour choice, the bacterial communities varied depending on the flour used, pointing to a clear link between ingredients and fermentation dynamics.
Sourdough starters are made from just flour and water, yet they host dozens of microbial species. To explore how flour choice affects those communities, the researchers analyzed starters made with all-purpose flour, bread flour or whole wheat flour. Over weeks of regular feeding, they tracked which microbes became most abundant.
Across all starters, yeasts from the genus Kazachstania consistently dominated, regardless of flour type or feeding schedule. The bacteria told a different story. Starters made with whole wheat flour tended to have higher levels of Companilactobacillus, while those made with bread flour showed greater abundance of Levilactobacillus. All starters shared a similar overall mix of bacterial groups, but the balance among them shifted with the flour.
For bakers, that shift could matter.
“Because the microbial composition affects different traits, by altering the flour you could potentially alter how your bread tastes,” said Caiti Heil, PhD, an evolutionary biologist at North Carolina State University and senior author on the study.
Previous research has shown that sourdough microbes are shaped by many factors, including the surrounding environment, surfaces and even the hands of the baker. Flour adds another layer of influence by supplying different nutrients to microbes as they compete and grow. Whole grains, for example, contain more fiber and minerals than refined flours, creating different conditions inside the starter.
The project began as part of an educational program led by a postdoctoral researcher and sourdough enthusiast, who worked with middle school students to explore fermentation and evolution through hands-on experiments. Students tested different flour and feeding combinations to see how quickly starters developed, and the resulting samples became part of the scientific analysis.
Rather than pointing to a single “best” flour, the findings highlight how sensitive sourdough ecosystems are to small changes. Changing flour does not guarantee a specific flavor or texture, but it can nudge the microbial community in different directions, which may help explain why sourdoughs made with similar recipes can taste surprisingly different.
For home bakers and curious eaters alike, the study offers a reminder that fermentation is shaped as much by ingredients as by time and technique, and that even subtle choices can influence the foods we make and enjoy.
This study was funded in part by an Education and Outreach grant from the Society for the Study of Evolution. The authors also thanked students and teachers at Lakewood Montessori Middle School in Durham, North Carolina, for participating in activities generated from the data set.