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Different Bacteria Need Different Flour in Sourdough

Sourdough bread
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The type of bacterial ecosystem found in a sourdough starter depends on the type of flour used, according to a new paper.

“People bake sourdough all over the world, and our previous research shed light on the tremendous variation in the types of microbes found in sourdough starters, and how those microbes influence the aroma of sourdough and how quickly it rises,” said Erin McKenney, corresponding author of the study and an assistant professor of applied ecology at North Carolina State University.

“Our new work focuses on the role that different types of flour play in shaping those microbial ecosystems. As it turns out, the flour bakers use to ‘feed’ their starters plays a significant role in determining which types of bacteria thrive. And that, in turn, strongly influences the aroma that these sourdoughs produce.”

“In other words, our findings show that bakers can influence the aroma of their sourdough by using different flours, because those flours will foster different communities of bacteria.”

To get their findings, McKenney and her team first got baking. They created four sourdough starters using 10 different flours, for a total of 40 starters. The team used five flours that included gluten (unbleached all-purpose flour, red turkey wheat, emmer, rye and einkorn) and five gluten-free flours (teff, millet, sorghum, buckwheat and amaranth). The 40 starters were kept in the same growing environment and were fed once a day for 14 days.

The team then collected data from each starter. This included measuring pH and height, as well as recording the aromas produced by each starter. They also took samples for DNA sequencing to determine the diversity and abundance of bacteria in each sample.

“We found that the starters started out being fairly similar to each other, but that changed substantially over time,” McKenney said. “Over the course of the 14 days, we found that each type of flour formed increasingly distinct microbial communities. Essentially, it appears that different types of bacteria are able to make the most of the nutritional compounds found in different types of flour.”

In this way, the unique pairings of these bacteria and nutritional compounds create the distinctive smells of certain baked sourdough. 

“For example, the bacterial community in amaranth sourdough produces an aroma that smells almost exactly like ham,” McKenney said. “I’ve never smelled a sourdough that had such a meaty aroma. Rye produces a fruity aroma, buckwheat has an earthy smell, and so on. There’s a tremendous amount of variation.”

Rye flour’s fruity aroma can be attributed to the wide variety of bacteria it supports – a finding that came as a surprise to the McKenney and her team.

“One surprise was that rye flour fostered a much wider diversity of bacteria than any other type of flour,” McKenney says. “We found more than 30 types of bacteria in the rye starters at maturity. The next highest was buckwheat, which had 22 types of bacteria. All of the other flours had between three and 14.”

The researchers also found that 70% of flours produced starters with high levels of acetic acid-producing bacteria. Only starters made using teff, amaranth and buckwheat lacked the acetic acid bacteria.

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“These acetic acid-producing bacteria made up between 12.6% and 45.8% of the bacteria in the starters from those seven flours,” McKenney said. “So it’s playing a significant role in those microbial ecosystems. This is surprising because we didn’t even know this type of bacteria was found in sourdough until 2020. Our previous work found that it was not uncommon, but to see it at such high levels, across so many types of flour, was definitely interesting.”

All these findings, McKenney says, could offer some practical insights for sourdough bakers.

“This study offers insights into how bakers can modify the flour they’re using in their starters to get the aromas and flavors they’re looking for,” McKenney said. “We also found that the starters took 10 days to become ‘functionally mature,’ or ready for baking. And that’s useful for bakers to know, too.”

Reference: McKenney EA, Nichols LM, Alvarado S et al. Sourdough starters exhibit similar succession patterns but develop flour-specific climax communities. PeerJ. 2023;11:e16163. doi: 10.7717/peerj.16163

This article is a rework of a press release issued by North Carolina State University. Material has been edited for length and content.