Scientists explore microbial diversity in sourdoughs

When millions of people were confined during the pandemic, they went in search of new hobbies at home to help them cure their boredom. Among them, sourdough bread was made. As well as being sustainable through the use of natural ingredients and traditional methods dating back thousands of years to ancient Egypt, it is also valued for its nutritional benefits.

For example, studies have shown that sourdough contains more vitamins, minerals and antioxidants than many other types of bread. For people with mild gluten sensitivity, sourdough bread may be easier to digest because much of the gluten is broken down during the fermentation process. Additionally, many species of lactic acid bacteria, essential to sourdough, are considered probiotics, associated with improved gastrointestinal health.

A flavor profile years in the making

The process of making sourdough bread begins with a sourdough starter. These sourdough starters are created when microbes – communities of bacteria and yeast – stabilize in a mixture of flour and water. Known as the microbiome, this community of wild yeast and bacteria is what makes sourdough bread rise and contributes to its taste and texture. One way sourdough differs from most breads is that it relies on this sourdough from wild microbes to help it rise instead of packets of baker’s yeast.

Many sourdoughs are preserved over generations, with some samples dating back thousands of years. To preserve a sourdough starter, you extract a sample of a previous dough and mix it with new flour and water. With enough sourdough transfers, the microbial community will be composed of yeasts, lactic acid bacteria (LAB), and acetic acid bacteria (AAB) best adapted to the sourdough environment. What makes different sourdoughs unique are the different strains of yeast and bacteria that produce the distinctive sour flavor.

Testing genetic diversity

Advances in sequencing technology have allowed researchers to quickly profile microbial communities, such as the sourdough microbiome. At Syracuse University, members of biology professor Angela Oliverio’s lab studied acetic acid bacteria to determine how AAB genetic diversity impacts sourdough communities.

While previous research has focused more on lactic acid bacteria and yeast, the ecology, genomic diversity, and functional contributions of AAB in sourdough remain largely unknown. Beryl Rappaport, a Ph.D. student in Oliverio’s group, recently led a study published in mSystemsin which she and other sourdough scientists, including Oliverio, Nimshika Senewiratne of the Oliverio lab, SU biology professor Sarah Lucas, and Tufts University professor Ben Wolfe, sequenced 29 AAB genomes from a collection of more than 500 sourdoughs and builds communities of synthetic sourdoughs. in the laboratory to define how AAB shapes the emergent properties of sourdough.

“While not as common in sourdough as lactic acid bacteria, acetic acid bacteria are better known for their dominant role in other fermented foods like vinegar and kombucha,” says Rappaport. “For this study, we wanted to follow up on previous findings that, when present in sourdough, AAB appears to have a strong impact on key properties, including odor profile and metabolite production, which shape the overall formation of the aroma.”

To assess the consequences of AAB on the emerging function of sourdough microbiomes, their team tested 10 strains of AAB, some distantly related and others very closely related. They set up manipulative experiments with these 10 strains, adding each to a yeast and LAB community. They maintained a distinct community composed only of yeast and LAB to serve as a control.

“Since we can manipulate which microbes and what concentrations of microbes enter these synthetic sourdough communities, we could see the direct effects of adding each strain of AAB to the sourdough,” says Rappaport. “As we expected, each AAB strain lowered the pH of the synthetic sourdough (associated with increasing acidity) since it released acetic acid and other acids as byproducts of its metabolic processes. Unexpectedly, however, AABs that were more closely related did not release more similar compounds. In fact, there was high variation in metabolites, many of which were linked to flavor formation, even between strains. the same species.

According to Rappaport, strain diversity is often overlooked in microbial communities, in part because it is difficult to identify and manipulate diversity levels due to the vastness of microorganisms within a given community. . The human gut biome alone can host approximately 100 trillion bacteria. By zooming in on the diversity among closest relatives in the laboratory, researchers can begin to understand key interactions within microbiomes.

A new startup source

When it comes to baking, she says their findings offer bread makers a new direction for shaping the flavor and texture of sourdough.

“Since AAB reliably acidified the sourdoughs we worked with and released a wide variety of flavor compounds, bakers who want their sourdough to be more acidic or create new flavors can try getting a sourdough with AAB or attempt to capture the AAB themselves,” says Rappaport. . “We hope this study will help shed light on the diversity of microbes present in sourdough and their important functional roles.”

Their research could also have implications for the health benefits of sourdough bread.

During the fermentation process, AAB generates acetic acid, which significantly helps break down gluten and complex carbohydrates, thereby improving the digestibility of the sourdough. By examining the genetic diversity of AAB and its influence on acetic acid production, researchers can develop strains that optimize this process.

The wider impact

The team uses sourdough primarily as a model system, because the sourdough microbiome is relatively simple to cultivate and use for repeated laboratory experiments. But their results go well beyond baking.

“Our findings will be relevant to people interested in more complex microbial communities, such as the human gut or soil,” says Rappaport. Indeed, the sourdough system can be used to ask questions about ecology and evolution that would be more difficult to ask with more complex systems.

When it comes to the human gut, microbial communities can help build resilience to infections and improve the efficiency of breaking down complex carbohydrates, fiber, proteins, and fats. In the case of soil, microbes help break down organic matter and maintain the overall stability of the soil ecosystem. However, there are many unknowns about the impact of multiple levels of genetic diversity on these processes.

By recognizing how strain diversity can have community-wide consequences on the microbiome, the team’s knowledge could have numerous benefits for human health, well-being and environmental sustainability.