A new paper examines how certain strains of bacteria, and specifically the genetic diversity of acetic acid bacteria, influence the smell and flavor of sourdough bread and even how it is processed by the body.
While previous research has focused more on lactic acid bacteria and yeast in sourdough bread, acetic acid bacteria (AAB) and its the ecology, genomic diversity, and functional contributions remain largely unknown.
Researchers from Syracuse University and Tufts University sequenced 29 acetic acid genomes from a collection of over 500 sourdough starters and constructed synthetic starter communities in the lab to define the ways in which AAB shape emergent properties of sourdough.
“While not as common in sourdough as lactic acid bacteria, acetic acid bacteria are better known for their dominant roles in other fermented foods like vinegar and kombucha,” says Beryl Rappaport, a PhD student at Syracuse University and lead author of the report along with Syracuse University Professor Angela Oliverio.
“For this study, we were interested in following up on previous findings which stated that when present in sourdough, AAB seems to have a strong impact on key properties including scent profile and metabolite production, which shape overall flavor formation.”
To assess the consequences of AAB on the emergent function of sourdough starter microbiomes, their team tested 10 strains of AAB, some distantly related and some very closely related. They set up manipulative experiments with these 10 strains, adding each one to a community of yeast and lactic acid bacteria.
“Since we can manipulate what microbes and what concentrations of microbes go into these synthetic sourdough communities, we could see the direct effects of adding each strain of AAB to sourdough,” says Rappaport.
“As we expected, every strain of AAB lowered the pH of the synthetic sourdough (associated with increasing sourness) since they release acetic acid and other acids as byproducts of their metabolic processes. Unexpectedly, however, AAB that were more closely related did not release more similar compounds. In fact, there was high variation in metabolites, many related to flavor formation, even between strains of the same species.”
According to Rappaport, strain diversity is often overlooked in microbial communities, in part because it is difficult to identify and manipulate levels of diversity due to the vastness of microorganisms within a given community.
By zooming into the diversity among closer relatives in the lab, researchers can start to understand key interactions in the microbiome.
The impact of this research is two-fold. When it comes to baking, she says their findings offer bread makers a new direction to shape sourdough flavor and texture.
“Since AAB reliably acidified the starters we worked with and released a large variety of flavor compounds, bakers who want their sourdough to be more sour or to create new flavors may try sourcing a starter with AAB or attempt to capture AAB themselves,” says Rappaport.
“We hope that this study helps to shine a light on the diversity of microbes found in sourdough and their important functional roles.”
Their research could also have implications on the health benefits of sourdough bread.
During the fermentation process, AAB generates acetic acid, which significantly aids in breaking down gluten and complex carbohydrates, enhancing the digestibility of sourdough. By examining the genetic diversity of AAB and its influence on acetic acid production, researchers can develop strains that optimize this process.
The team uses sourdough primarily for its use as a model system because the sourdough microbiome is relatively simple to culture and use for repeated experiments in the lab. But their results stretch far beyond baking.
“Our findings will be relevant to people interested in more complex microbial communities, like the human gut or soil,” says Rappaport. This is because the sourdough system can be used to ask questions about ecology and evolution which 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 efficiency in breaking down complex carbohydrates, fiber, proteins, and fats. In the case of soil, microbes help to break down organic matter and maintain overall soil ecosystem stability. There are many unknowns, however, about how multiple levels of genetic diversity impact these processes.
By recognizing how strain diversity can have community-wide consequences on a microbiome, the team’s insights could have wide-ranging benefits for human health, wellness, and environmental sustainability.
The team’s work was supported by a National Science Foundation grant awarded to Oliverio earlier this year.
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