The microbial life of sourdough

New research has garnered insight into the relationship between sourdough and its starting ingredients' microbial ecosystem.

The large-scale study conducted by several US and Danish universities analysed 500 sourdough starters across four continents to discover how different microbes influence both a sourdough's aroma and how quickly the sourdough rises.

"We didn't just look at which microbes were growing in each starter," said North Carolina State University assistant professor and paper co-author Erin McKenney.

"We looked at what those microbes are doing and how those microbes coexist with each other."

Tufts University’s Benjamin Wolfe said while there have been quite a few small studies on microbial ecosystems in sourdough, the study is the first to be conducted at this scale.

The study

The researchers conducted DNA sequencing on 500 samples of sourdough starter, primarily from home bakers in the United States and Europe. However, there were also samples from Australia, New Zealand and Thailand.

Based on those findings, the researchers then selected 40 starters to represent the diversity they saw across the 500 submissions. Those 40 starters were then cultured and assessed in three ways.

First, the researchers worked with an expert panel of sensory professionals to assess each starter's aroma profile. Second, the researchers performed a chemical analysis of the volatile organic compounds being released by each starter. This analysis allowed the scientists to determine the structure of these aromatic compounds and the relative amount of each compound being released. Last, the researchers measured how quickly each of the 40 starter doughs rose.

The results

One of the findings that immediately struck the researchers was that geography did not matter.

"This is the first map of what the microbial diversity of sourdoughs looks like at this scale, spanning multiple continents," said Elizabeth Landis, co-lead author of the study and a PhD student at Tufts. "And we found that where the baker lives was not an important factor in the microbiology of sourdough starters."

The findings challenge several preconceived notions regarding sourdough.

"Lots of bakers felt sure that specific factors were responsible for variation between types of sourdough," McKenney said. "But what we found is that, while there could be tremendous variation between the microbial ecosystems of different sourdoughs, we could not find any single variable that was responsible for much of that variation."

"What we found instead was that lots of variables had small effects that, when added together, could make a big difference," said Angela Oliverio, co-lead author of the study and a former PhD student at the University of Colorado, Boulder. "We're talking about things like how old the sourdough starter is, how often it's fed, where people store it in their homes, and so on."

The researchers were also surprised to see that 29.4% of the samples contained acetic acid bacteria.

"The sourdough research literature has focused almost exclusively on yeast and lactic acid bacteria," Wolfe said. "Even the most recent research in the field hadn't mentioned acetic acid bacteria at all. We thought they might be there to some extent since bakers often talk about acetic acid, but we were not expecting anything like the numbers we found."

The acetic acid bacteria also played a powerful role in shaping both the aroma of the sourdough and how quickly it grew. Specifically, the presence of acetic acid bacteria slowed the rise of sourdough and gave it a vinegary smell, the researchers said.

Some of the findings were less surprising. For example, about 70% of the starters contained Saccharomyces cerevisiae, or baker's yeast. On the other hand, many people may be surprised that 30% of the sourdough starters didn't include the yeast most people associate with baking bread.

While the median starter contained only one type of yeast, the researchers found 70 different yeast types across all 500 sourdough samples.

"I think it's also important to stress that this study is observational — so it can allow us to identify relationships, but doesn't necessarily prove that specific microbes are responsible for creating specific characteristics," Wolfe said. "A lot of follow-up work needs to be done to figure out, experimentally, the role that each of these microbial species and environmental variables plays in shaping sourdough characteristics."

Landis said while bakers will find this interesting, the work is also of interest to microbiologists.

"Sourdough is an excellent model system for studying the interactions between microbes that shape the overall structure of the microbiome. By studying 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."

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