At the height of the COVID-19 pandemic, devoted bakers suddenly found themselves spending more time at home. With that, the quest for the perfect sourdough bread was on.

But as the pandemic waned, some scientists still had a burning question: What actually makes sourdough bread so uniquely, deliciously sour?

Sourdough differs from its bread cousins in one major way. Rather than using baker’s yeast to rise, sourdough relies on “starter” — a living, fermenting colony of wild yeast and lactic acid bacteria.

While sourdough breads trace their roots to ancient Egypt, surprisingly little was known about the chemical compounds that deliver its special taste and smell. Some purists even say genuine sourdough bread only comes from San Francisco, born of the unique water and foggy climate.

That’s where a group of German scientists enter the picture. A team from the Technical University of Munich took a decidedly rigorous approach, using a technique known as “sensomics” to chemically analyze sourdough bread’s soft interior. It was more than an obscure culinary quest. Knowing the bread’s composition could help bakers produce consistent dough that isn’t too sour.

Using classic chemistry techniques, the team identified nearly two dozen scent and flavor compounds. Then, they ranked them in importance. The result was a sourdough “essence” of 10 key taste compounds and 11 crucial scent profiles.

After analyzing a variety of both yeast-based and sourdough breads from local bakeries and supermarkets, the researchers had their eureka moment:  It’s the balance of lactic and acetic [əˈsēdik] acids that are the key to sourdough’s special flavor — and its favor among humans over the centuries.