Why no two sourdoughs are exactly the same

While sourdough starter is not the social media star it was in the early days of the COVID-19 pandemic, sourdough bread is still deliciously easy to make at home. It’s also a prime canvas for studying microbes. A study published October 4 in the open-access journal PeerJ found that using different types of flour creates different bacterial communities. These variations contribute to sourdough’s unique flavors and aromas. 

[Related: How to make a sourdough starter—and keep it alive.]

Professional and at-home bakers alike can influence the aroma of their loaves of sourdough bread simply by using different flours, according to study co-author and North Carolina State University microbial ecologist Erin McKenney.

“Our new work focuses on the role that different types of flour play in shaping those microbial ecosystems,” McKenney said in a statement. “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 this new study, McKenney and a team of researchers developed a protocol designed to reproduce what bakers actually do in their kitchens. They created four sourdough starters using 10 different flours, creating 40 starters in total. A sourdough starter is an active colony of wild yeast and good bacteria. It is cultivated by combining water and flour and then allowing the two to ferment. When it is continually fed, it forms a reliable “natural yeast” culture that can be used to raise or leaven breads and other pastries. 

The team used five flours that contain gluten: unbleached all-purpose flour, red turkey wheat, emmer, rye, and einkorn. They also used five gluten-free flours: teff, millet, sorghum, buckwheat, and amaranth. 

For 14 days, all 40 starters were kept in the same growing environment and were fed once a day. They collected data from each starter, measuring the pH and height all while recording the different scents each start was producing. The team also sampled the starters for DNA sequencing to determine the diversity and abundance of bacteria in each of the samples.

“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.”

[Related: A new kind of quinoa flour may be coming to a sugar cookie near you.]

A variety of bacterial communities thriving on different nutritional inputs can yield a wide variety of metabolic outputs, as different bacteria produce different smells.

“The bacterial community in amaranth sourdough produces an aroma that smells almost exactly like ham,” McKenney says. “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.”

The study came with a few surprises. The rye flour created a much wider diversity of bacteria than any other type of flour the team studied with over 30 types of bacteria at maturity. Buckwheat came in second place for most diverse bacteria with 22 types. All of the other flours had between three and 14.

They also found that seven out of the 10 flours produced starters that had high levels of bacteria which produce acetic acid, making up between 12.6 and 45.8 percent of the bacteria from the flours. This acid often acts as a leavening agent when it is combined with baking soda. Starters made using teff, amaranth, and buckwheat were all lacking the acetic acid bacteria. 

“So it’s [the acetic acid] 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,” said McKenney.

While McKenney and her team were working with bread under a microscope and in a lab, this kind of research also has more practical insights for sourdough enthusiasts. It shows how home bakers can modify the flour in starters to get the exact flavors and smells they are looking for. Starters take 10 days to become “functionally mature,” which is also helpful for bakers looking to time their next sourdough loaf.