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A study of 500 starter sourdoughs from four continents offers new insights into the environmental factors that contribute to the microbial ecosystem of each sourdough.
The results, which also shed light on how different types of microbes influence both the aroma of a sourdough and the speed at which the sourdough rises, may surprise sourdough enthusiasts.
“We didn’t just look at what microbes were growing in each starter,” says study co-author Erin McKenney, assistant professor of applied ecology at North Carolina State University. “We looked at what these microbes do and how these microbes coexist with each other.”
“There have been quite a few small studies of microbial ecosystems in sourdough,” says co-author Benjamin Wolfe, associate professor of biology at Tufts University. “We believe this is the first large-scale study, which builds on all of this previous work.”
Diversity of sourdough starters
For this study, published in eLife, the researchers collected 500 samples of sourdough sourdough, mostly from amateur bakers in the United States and Europe, as well as Australia, New Zealand and Thailand.
The researchers performed DNA sequencing on the 500 samples. Based on these results, the researchers then selected 40 starters as representative of the diversity they found among the 500 submissions. These 40 starters were then cultivated and evaluated in three ways.
First, the researchers worked with an expert panel of sensory professionals (think of them as super-sniffers) to assess the flavor profile of each starter. Second, the researchers performed a chemical analysis of the volatile organic compounds released by each starter. This analysis allowed scientists to determine the structure of these aromatic compounds, as well as the relative amount of each of these compounds released by each starter. Finally, the researchers measured the rate at which each of the 40 starting pastes grew.
One of the discoveries that immediately struck researchers: Geography doesn’t really matter (sorry, San Francisco).
“This is the first map of what the microbial diversity of sourdoughs looks like at this scale, spanning multiple continents,” says Elizabeth Landis, co-lead author of the study and a PhD student at Tufts. “And we found that the baker’s place of residence was not an important factor in the microbiology of sourdough starters.”
In fact, the results challenge much of the conventional wisdom regarding sourdough.
“Many bakers were convinced that specific factors were responsible for the variation between types of sourdough,” explains McKenney. “But what we found was that while there could be huge variations between the microbial ecosystems of different sourdoughs, we couldn’t find a single variable that was responsible for much of this variation.
“What we found instead was that many variables had small effects that, when added together, could make a big difference,” says Angela Oliverio, co-lead author of the study and former student at the study. doctorate at the University of Colorado, Boulder. “We’re talking about things like the age of the leaven, how often it is fed, where people store it in their homes, etc.
Microbes working together
The fact that 29.4% of the samples contained acetic acid bacteria (so named because they produce acetic acid), also surprised the researchers.
“The sourdough research literature has focused almost exclusively on yeasts and lactic acid bacteria,” says Wolfe. “Even the most recent research in the field had not mentioned acetic acid bacteria at all. We thought they might be there to some extent, as bakers often talk about acetic acid, but we weren’t expecting anything like the numbers we found.
And, the researchers note, these acetic acid bacteria played a powerful role in the formation of sourdough aroma and how quickly it grew. Specifically, the presence of acetic acid bacteria slowed the rise of the leaven and gave it a vinegar smell.
Some of the findings were less surprising. For example, about 70% of starters contained Saccharomyces cerevisiae, or baker’s yeast. On the flip side, many people may be surprised that 30% of sourdough starters do not include the yeast that most people associate with baking bread.
In fact, while the middle starter contained only one type of yeast, the researchers found 70 different types of yeast in the 500 samples of the sourdough. The potential variety is therefore formidable.
“I think it’s also important to point out that this study is observational – so it may allow us to identify relationships, but doesn’t necessarily prove that specific microbes are responsible for creating specific characteristics,” says Wolfe. “A lot of follow-up work needs to be done to understand, experimentally, the role that each of these microbial species and environmental variables play in forming the characteristics of the starter.
“And while bakers will find it interesting, we think the work is of interest to microbiologists as well,” says Landis. “Sourdough is an excellent model system for studying the interactions between microbes that shape the overall structure of the microbiome.
“By studying the interactions between microbes in the sourdough microbiome that lead to cooperation and competition, we can better understand the interactions that occur between microbes more generally and in more complex ecosystems.
Other co-authors are from Tufts; the University of Colorado, Boulder; and North Carolina State University. The National Science Foundation funded the work.
Source: NC State
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