A study of 500 sourdough starters from four continents provides new insights into the environmental factors that contribute to the microbial ecosystem of each sourdough.
The findings, which also shed light on how different types of microbes affect the aroma of a sourdough and how quickly the leaven rises, may surprise sourdough enthusiasts.
“We did not just look at which microbes grow in each appetizer,” says co-author Erin McKenney, an assistant professor of applied ecology at North Carolina State University. “We looked at what those microbes do, and how those microbes coexist.”
“There are quite a few small studies on microbial ecosystems in sourdough,” says co-author Benjamin Wolfe, associate professor of biology at Tufts University. “We think this is the first large-scale study that builds on all the previous work.”
Yeast-starter-diversity
For this study, published in eLife, the researchers collected 500 samples of sourdough starter, mainly from home bakers in the United States and Europe, and also from Australia, New Zealand and Thailand.
The researchers performed DNA sequencing on all 500 samples. Based on the findings, the researchers then selected 40 entrees as representative of the diversity they saw in the 500 submissions. The 40 starters were then grown and judged in three ways.
First, the researchers work with an expert panel of sensory professionals (consider them super-sniffers) to assess the aroma profile of each appetizer. Second, the researchers conducted a chemical analysis of the volatile organic compounds released by each appetizer. This analysis enabled the scientists to determine the structure of these aromatic compounds, as well as the relative amount of each of the compounds released by each starter. Finally, the researchers measured how quickly each of the 40 appetizers rose.
One of the findings that the researchers immediately noticed: Geography doesn’t really matter (sorry, San Francisco).
“This is the first map of what the microbial diversity of sourdough looks like on this scale, spanning several continents,” says Elizabeth Landis, co-author of the study and a PhD student at Tufts. “And we found that where the baker lives was not a major factor in the microbiology of sourdough entrees.”
In fact, the findings challenge much of the conventional wisdom regarding leaven.
“Many bakers felt that specific factors were responsible for the variation between types of sourdough,” says McKenney. “But what we found is that although there can be tremendous variation between the microbial ecosystems of different yeasts, we could not find a single variable that was responsible for much of the 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 a former PhD student. the University of Colorado, Boulder. “We talk about things like how old the sourdough starter is, how often it is fed, where people store it in their homes, and so on.”
Microbes work together
The fact that 29.4% of the samples contain acetic acid bacteria (so named because they produce acetic acid) also surprised the researchers.
“The research literature on sourdough has focused almost exclusively on yeast and lactic acid bacteria,” says Wolfe. ‘Even the most recent research in the field did not mention acetic acid bacteria at all. We thought they would be there to some extent, because bakers often talk about acetic acid, but we did not expect anything like the numbers we found. ‘
And according to the researchers, the acetic acid bacteria played a powerful role in shaping the aroma of the sourdough and how fast it grew. The presence of acetic acid bacteria slowed the emergence of sourdough and gave it a finger-like odor.
Some of the findings were less surprising. For example, about 70% of starters contain Saccharomyces cerevisiae, or bakgis. On the other hand, many people may be surprised that 30% of sourdough entrees do not include the yeast that most people bake with bread.
While the median appetizer contained only one type of yeast, the researchers found 70 different types of yeast in all 500 sourdough samples. The potential variety is therefore enormous.
“I think it’s also important to emphasize that this study is observational. It can therefore enable us to identify relationships, but does not necessarily prove that specific microbes are responsible for creating specific traits,” Wolfe says. “A lot of follow-up work needs to be done to experimentally determine the role that each of these microbial species and environmental variables play in the formation of leavening properties.”
“And while bakers will find it interesting, we think the work is also of interest to microbiologists,” Landis says. “Sourdough is an excellent model system for studying the interactions between microbes that form the overall structure of the microbiome.
“By studying interactions between microbes in the yeast microbiome that lead to collaboration and competition, we can better understand the interactions that occur between microbes more commonly, and in more complex ecosystems.”
Additional 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