Researchers investigate the role of acetic acid bacteria in determining the characteristics of sourdough, with potential implications for various complex microbial systems.
During the pandemic lockdown, many individuals sought out new hobbies, with sourdough bread-making becoming a popular choice. This ancient practice, dating back to the Egyptians, is not only sustainable, relying on natural ingredients and traditional techniques, but it also offers numerous nutritional advantages. Research indicates that sourdough is richer in vitamins, minerals, and antioxidants compared to many other breads. For those who have mild gluten sensitivities, sourdough may be easier to digest, as the fermentation process partially breaks down gluten. Furthermore, the lactic acid bacteria prevalent in sourdough are recognized as probiotics, which can enhance gastrointestinal health.
A Flavor Profile Years in the Making
The journey of sourdough bread begins with a sourdough starter, created when a mix of flour and water becomes colonized by various microbes, mainly bacteria and yeast. This microbial community, known as a microbiome, is what gives sourdough its unique rise, flavor, and texture. Unlike most breads, which use commercial baker’s yeast, sourdough rises naturally thanks to its wild microbial starter.
Many sourdough starters are passed down through generations, with some dating back thousands of years. Maintaining a starter involves taking a portion from an existing batch and combining it with fresh flour and water. With repeated transfers, the microbial community adapts to the sourdough environment, comprising yeast, lactic acid bacteria (LAB), and acetic acid bacteria (AAB). The unique flavors of different sourdough varieties stem from the distinct strains of yeast and bacteria present.
Testing Genetic Diversity
Recent advancements in sequencing technology have allowed scientists to quickly analyze microbial communities, including those in sourdough. At Syracuse University, a team led by biology professor Angela Oliverio has focused on acetic acid bacteria to explore how genetic diversity among these bacteria influences sourdough communities.
While past studies primarily examined lactic acid bacteria and yeast, the roles of AAB in sourdough ecology and their genetic diversity have been less explored. Beryl Rappaport, a Ph.D. candidate in Oliverio’s lab, spearheaded a study published in mSystems, a journal from the American Society for Microbiology, where she collaborated with fellow researchers, including Oliverio, Nimshika Senewiratne from the Oliverio lab, SU professor Sarah Lucas, and professor Ben Wolfe from Tufts University. They sequenced genomes of 29 AAB strains from a collection of over 500 sourdough starters and created synthetic starter communities in the lab to understand how AAB affect the sourdough properties. This research was backed by a National Science Foundation grant given to Oliverio this year.
“Although acetic acid bacteria are not as prevalent in sourdough as lactic acid bacteria, they are more commonly associated with other fermented products like vinegar and kombucha,” Rappaport explains. “Our study aimed to follow up on previous findings that suggested AAB significantly influence key characteristics such as aroma and metabolite production, thereby shaping overall flavor.”
To examine the effects of AAB on the sourdough microbiome, the team experimented with 10 different strains of AAB, some being closely related and others distant relatives. They conducted controlled experiments by adding each AAB strain to a community of yeast and LAB, while maintaining a separate control community containing only yeast and LAB.
“By manipulating which microbes and their concentrations we included in these synthetic sourdough communities, we were able to observe the direct impact of each AAB strain,” Rappaport shares. “As anticipated, every AAB strain resulted in a lower pH in the synthetic sourdough—indicating increased sourness—since they produce acetic acid and other acids during their metabolism. Surprisingly, however, closely related AAB did not produce similar compounds. There was significant variability in the metabolites, particularly those linked to flavor, even among strains of the same species.”
According to Rappaport, the diversity of strains within microbial communities is often underappreciated, partly because identifying and manipulating this diversity is challenging, given the immense variety of microorganisms present. For instance, the human gut microbiome alone comprises roughly 100 trillion bacteria! By focusing on closely related strains in the lab, scientists can begin to uncover essential interactions within microbiomes.
A New Starter Source
Rappaport believes their findings offer new insight for bakers seeking to enhance the flavor and texture of their sourdough.
“Our results indicate that AAB consistently acidified the starters we tested and produced a vast range of flavor compounds. Bakers who want their sourdough to be tangier or want to experiment with new flavors may want to source AAB-rich starters or attempt to cultivate AAB themselves,” Rappaport advises. “We hope this study highlights the diverse microbes present in sourdough and their critical roles.”
This research might also inform the health benefits associated with sourdough bread.
During fermentation, AAB produces acetic acid, which significantly assists in breaking down gluten and complex carbohydrates, thus improving the digestibility of sourdough. By understanding the genetic diversity of AAB and its effect on acetic acid formation, researchers can cultivate strains that enhance this process.
The Broader Impact
The team primarily uses sourdough as a model system due to its relatively simple microbiome, making it useful for repeated experiments in the lab. However, the implications extend far beyond just baking.
“Our discoveries are pertinent to those interested in more intricate microbial systems, such as the human gut or soil microbiomes,” Rappaport states. This is because studying sourdough can help address ecological and evolutionary questions that would be harder to explore in more complex ecosystems.
In the human gut, microbial communities contribute to resilience against infections and enhance the breakdown of complex carbohydrates, fibers, proteins, and fats. In soils, microbes play a vital role in decomposing organic matter and preserving ecosystem stability. Yet, numerous aspects of how genetic diversity influences these processes remain unclear.
By understanding the consequences of strain diversity on a microbiome, the team’s research could offer significant advantages for human health, wellness, and environmental sustainability.
