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United States Department of Agriculture

Agricultural Research Service

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Image Number K7250-35

Bakers in the Bay Area insisted their sourdough bread couldn't be duplicated farther than 50 miles from the center of San Francisco. Didn't know why, either. It just couldn't.

Enter ARS researchers, who knew there was a scientific answer to the mystery of sourdough bread. They started looking for clues.

One puzzling thing about the brad was its high acetic acid content, which contributed to its tangy taste. Yeasts generally can't tolerate acetic acid.

Obtaining samples of starter doughs from five local bakeries, a scientist found in all five a bacterium never before discovered. Naming it Lactobacillus sanfrancisco, he spent several months and tried 30 different substances before finding a medium in which it would grow.

The other thing he found was an unusually acid-tolerant yeast, Saccharomyces exiquus. It worked with the bacterium in a symbiotic relationship to produce the bread's unusual flavor, crust, and texture. Comments the scientist: "It was a happy marriage between two noncompetitive bugs."

So now San Francisco-style sourdough bread can be baked anywhere in the world. Which turned out not to be bad news for the City by the Bay. Pure cultures of L. sanfranciscoare now grown commercially and are commonly used by San Francisco bakers to control the quality of their product.

Photo by Scott Bauer.



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Last Modified: 5/23/2006
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