“Baking may be scientific or as romantic as you want it to be.”

Sourdough bread has cultivated people for thousands of years — maybe even more so throughout the bread-making hype of the COVID-19 pandemic — and bakers have perfected the craft of producing it over generations. Now, scientists are beginning to comprehend the identities and activities of the microbes in sourdough that are key to making a delicious loaf.

“Folks are creating sourdough all over the world — it is the most ancient form of bread-making and also an ancient form of fermentation,” said Elizabeth Landis, a microbiologist at Tufts University near Boston, Massachusetts. But she added,”We didn’t know a lot of the diversity of germs which are currently in people’s newcomers .”

Everything begins with flour, microbes, water and patience. After the water and flour are mixed, the microbes then ferment and feed off the mixture, producing carbon dioxide, then the exact same gas that makes the dough rise until it goes into the oven.

No two sourdough starters are the same, mostly due to the germs, especially yeasts, that shape the many characteristics of the sourdough’s flavor.

What makes starters distinct?
To examine those differences as well as the germs that cause them, an international group of researchers put a call out to get people from throughout the world to ship samples of their sourdough starters.

Erin McKenny, a microbial ecologist at North Carolina State University, and her team at the International Sourdough Project acquired over 500 samples from the U.S., Europe, Asia and Oceania. The samples included both commercial and homemade starters, a few with origins that can be traced back 200 decades. While previous research had studied the microbial composition of sourdough starters in little regional samples, the new study is the first to look at this a large sample worldwide.

To learn about the community of microbes living in each sample, the group cultured the microbes and sequenced their DNA.

Some areas — San Francisco, for instance — praise for having famous sourdoughs as a result of the regional germs, ingredients and environment. But contrary to popular belief, the researchers revealed that the geographic origin of the sourdough starter didn’t really help determine the composition of these microbes necessary for bread-making.

The diversity of microbes, the scientists discovered, depended more on other things, such as the kind of bread used to nourish the starter and if the starter was stored in the refrigerator or at room temperature.

“I am convinced the fermentation parameters are more impactful than the geography,” explained Guylaine Lacaze-Elleboudt, a food microbiologist at Puratos, a Belgian firm that supplies goods to bakers and runs on a sourdough library that stores starters from all around the world.

When researchers took a closer look at the fungi and bacteria present in the starters, they discovered that Saccharomyces cerevisiae, or baker’s yeast, was present in only 70% of them. There were 70 types of yeasts in most of the samples together, although most human starters had just one dominant yeast. “You only have one, two, possibly three different kinds of bacteria or yeast that dominate the entire thing,” said McKenny.

Lactic acid bacteria are also crucial in the practice of sourdough fermentation. These bacteria produce lactic acid, which creates an acidic microenvironment that allows only a few different bacteria and yeasts to develop. In addition they give sourdough its yogurt and cheesy scents.

What most astonished the researchers was the existence of bacteria which create lactic acid, which is present in vinegar, in roughly 30 percent of the starters. These bacteria slowed the rise of the dough, but they were also vital for its vinegary notes of sourdough’s odor, which, together with the lactic acid, give sourdough its tangy flavors.

From lab to bakery
The researchers then studied the selection of aromas in the sourdough starters. They picked 40 representative had practitioners sniff each newcomer — think of these as bread sommeliers — to set its own aroma profile.

The scientists identified that the volatile compounds from the starters using an extremely sensitive chemical analysis. This enabled them to demonstrate that the vinegary notes were indeed a product of the acetic acid bacteria present in certain starters.

Until now, scientists had concentrated on the parameters that would produce lactic acid bacteria create more or fewer lactic acid, but the discovery of the role of acetic acid bacteria”can change the way we approach the sourdough taste traits,” said Lacaze-Elleboudt, who wasn’t involved with the new study. “It opens new possibilities”

The perfect sourdough for Bellamy of Boulted Bread, who was also not involved with the analysis, is something which’s less sour and more fruity, floral and yogurty. The new results can help professionals like him — and home bakers too — figure out how to realize their objectives.

“Baking can be as scientific or as romantic as you want it to be,” Bellamy said. “And for me, personally, the comprehension of the science behind it’s educated the romantic parts of ittoo.”