Sourdough starter is a simple mixture of flour and water that bakers rely on to leaven bread. It is also a powerful way for scientists to study how living organisms change over time. The familiar chewy texture and savory flavor of sourdough comes from the complex mix of microorganisms that ferment the dough. Research over the years has revealed more than 60 species of bacteria and over 80 species of yeast in sours from different regions of the world. “We can use yeast as an experimental evolutionary framework to see what happens over time,” said evolutionary biologist Caiti Heil, Ph.D.
In a recent study published in Microbiological spectrumHeil and researchers at North Carolina State University in Raleigh decided to investigate how the choice of flour affects the microbes living in sourdough starters. Their analysis showed that yeast from the genus Kazakhstan were consistently the most common among all starters. In contrast, bacterial communities differed depending on the type of flour used.
What this means for bakers and taste
The findings suggest that changing flour types could affect the microbial composition of the starter. “And because the microbial composition affects different properties, by changing the flour you could potentially change the taste of your bread,” said Heil, lead author of the study. More broadly, she explained that the results show how sensitive the yeast microbiome is to environmental conditions.
Previous research has shown that sourdough microbes are shaped by many influences, including the flour itself, the surrounding air and surfaces, and even the hands of the baker. Starters can be made from wheat, rye, barley, teff, millet, or other grains, each of which provides a different set of nutrients that microbial growth depends on.
A classroom experiment will stimulate learning
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 high school to teach students about fermentation and evolution. Schwarzkopf, who runs a sourdough starter named Seth, encouraged students to test different flour combinations and feeding schedules to see which starter grew 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 started 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 different yeasts.
Unexpected yeast dominance
After several weeks of repeated feeding, the microbial communities shifted. All the starters ended up being dominated by the same yeast, while the bacteria showed more diversity. Heil said she originally expected to find Saccharomyces cerevisiae, also known as brewer’s yeast, which is commonly used in baking and is the focus of much of her lab’s research.
Instead, Kazakhstan proved to be the leading yeast in every starter, regardless of flour type or feeding schedule. Genetic analysis also revealed differences between the bacteria. Appetizers made with wholemeal flour contained higher amounts Companilactobacilluswhile those made from 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 type of flour offers unique nutritional conditions. Linking these differences to the environment microbes experience can help scientists better understand how diverse microbial communities form, compete and persist.
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