The Biology of Sourdough

Patricia Gadsby and Eric Weeks wrote . . . . . . . .

About 34 years ago, Frank Sugihara recalls, he and Leo Kline, a fellow microbiologist, set out to “solve the mystery of San Francisco sourdough.” The two scientists were working with baker’s yeast in a Bay Area lab run by the U.S. Department of Agriculture, so perhaps it was inevitable they’d wind up studying San Francisco’s signature bread. This crusty loaf, with its chewy bite and sharp acidulated tang, was a long way from Wonder Bread, and few tourists left the airport without a loaf. Local lore attributed the bread to Basque migrants from the Pyrenees who arrived in San Francisco during the gold rush. Local bakers swore that no one could reproduce it outside a 50-mile radius of the city. When they gave dough to bakeries elsewhere, it inexplicably lost its “sour.” But was it—is it—truly unique?

Sugihara laughs. “It’s hard to say.”

The practice of making sourdough is as ancient as bread itself. For 5,000 years or more, humans have mixed flour and water, waited for the mixture to ferment, and when it was good and sour and full of gas, used it as leavening to make dough rise. They found that they could propagate their leavening by saving a bit of unused dough to sow the seeds of foment in the next batch. No one knew then that these “seeds” were actually living microorganisms—it was Louis Pasteur, in the mid-1800s, who showed that fermentation was caused by microbes. That knowledge led to the commercial production of baker’s yeast, strains of Saccharomyces cerevisiae bred for speedy growth. Today baker’s yeast rules. It makes short work of pumping carbon dioxide into dough, and it always delivers. Still, the old, slow bread-making ways didn’t disappear. Sourdough, for example, not only survived in San Francisco—it has gained new respect from artisanal bakers and sourdough hobbyists.

“Here’s a sourdough bâtard from Artisan Bakers in Sonoma,” says Danielle Forestier, a French-trained master baker in Oakland, just across the bay from San Francisco. “I’m checking the package,” she reports over the phone. “It’s made of unbleached flour, water, and salt. Three ingredients, lots of taste, great texture.” Yet a typical supermarket white bread has more than 25 ingredients and additives and still tastes vapid.

The difference is those fermenting bugs. The baker’s yeast in supermarket bread creates a virtual monoculture of S. cerevisiae. The sourdough bâtard, on the other hand, is a product of natural fermentation involving wild yeasts and bacteria. Almost all the bacteria are lactobacilli, cousins of the bacteria that curdle milk into yogurt and cheese. “These lactobacilli outnumber yeasts in sourdough by as many as 100 to one,” Sugihara says. It’s the acids they make that give sourdough its tartness. Not only that, say European researchers, the bacteria also contribute carbon dioxide as well as aromatic compounds that infuse bread with flavor and delicious smells.

Keeping a sourdough culture alive requires good time management and something like affection. An ecosystem begins to form as flour mixes with water to make a starter dough. Enzymes in the flour split starches into sugars. There are swarms of yeasts and bacteria everywhere—in the flour, in the environment, and on the baker. They converge on the sugars “like a rabble,” says Jürgen-Michael Brümmer, former head of baking at the Federal Institute for Grain, Potato, and Lipid Research, in Detmold, Germany. Not to worry, he says: The bugs will sort themselves out, and the “bread friendly” ones will come out on top.

As lactobacilli convert sugars to lactic and acetic acid, the dough noticeably sours, going down to the pH of mayonnaise, around 3.8. Most microorganisms drop out of competition at this point, but yeasts that tolerate acid come into their own and convert sugars into carbon dioxide and ethanol. Gas bubbles and fruity smells signal that fermentation is under way. Served regular meals of flour and water—”refreshments” in sourdoughspeak—selected organisms will multiply day by day. By day six or so, the culture should teem with bugs and be ready to raise dough. Not all the culture is used, and the remainder is fed flour daily so it can live on to make bread another day. A well-fed culture can last years. “I call it microfarming,” says Rick Kirkby, at the Acme Bread Company in Berkeley.

In their landmark San Francisco sourdough studies, Sugihara and Kline showed how nicely yeasts and lactobacilli live together. The principal yeast they found now goes under the name Candida milleri, and the principal bacterium, a species never found in nature before, is called Lactobacillus sanfranciscensis. Unlike baker’s yeast, C. milleri is exceedingly tolerant of the acid that the bacterium produces. What’s more, C. milleri doesn’t digest maltose, one of the sugars derived from flour starch. This is unusual for a yeast, and lucky for the bacterium. L. sanfranciscensis, it turns out, can’t live without maltose. That tight, mutually helpful relationship may have allowed some San Francisco bakeries to keep their sourdoughs alive for more than 100 years.

Once scientists knew what to look for, they started finding L. sanfranciscensis in starter doughs in other countries—in French levains and German Sauerteigs, for instance, and in the dough for Italian panettone. Wherever it shows up, says Michael Gänzle, a microbiologist at the Technical University of Munich in Germany, it probably comes from bakers’ hands.

What to make of the claim, then, that San Francisco sourdough can’t be authentically made elsewhere? Will a San Francisco starter stay true to form in, say, New York? Many bakers contend the culture will lose its zip. “Local bugs join the party, and before long you’ve got Lactobacillus newyorkensis,” says Jeffrey Hamelman, director of the baking education center for King Arthur Flour in Vermont.

Perhaps. Cultures are dynamic. Mess with their living conditions— room temperature, mealtime, brand of flour offered—and they will change. But how much? An ongoing project in France may offer a clue. “Generally, we find the yeasts stay the same,” says Bernard Onno, a French microbiologist looking into the biodiversity and dynamics of sourdough. But in stable, established cultures, he says, “the lactobacilli vary—not the species but the ratios between the species in the dough.” Perhaps, then, if you transplant San Francisco sourdough to New York, you should expect some reshuffling within the bacterial inhabitants. The culture, however, isn’t always to blame for taste changes. “Fifty percent of taste comes from culture,” Onno says, but the other 50 percent comes from savoir faire—the baker’s craft.

“I think you can make San Francisco sourdough pretty much anywhere,” Sugihara says. “It’s such a self-protective system.” Now 82, he still consults for bakeries around the world. “They do a good job of sourdough in Japan,” he says, “but they mainly sell it to foreigners.” He has also persuaded one large Japanese bakery to perk up its white bread with a shot of sourdough. “They call the bread My Heart,” he says with a chuckle. “As in ‘I left my heart in San Francisco.'”

Source: Discover Magazine


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Lactobacillus sanfranciscensis : The Key to the Perfect Sourdough

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Growing Conditions

Lactobacillus sanfranciscensis has been cultured for many years through a procedure called “backslopping”– that is, the repetitive re-inoculation of a starter by adding flour and water, and never using the entire starter at any given time. This ensures that the bacteria are given plenty of nutrients to grow and multiply, prolonging the life of the starter. Over time, backslopping produces a steady ratio of certain bacteria, and thus a baker develops his or her own special flavor. Commericially-sold starters often include the basics, along with L. delbrueckii, fermentum, and plantarum.

Perfect function of L. sanfranciscensis relies on a balance of certain environmental conditions. Though bakers can specialize in certain techniques, additions, and temperatures, Ganzel et al (1998) revealed that L. sanfranciscensis itself does have an ideal environment to perform at its best, albeit broad (Figure B) . The group studied the contribution of ionic strength, pH, temperature, and metabolic end products (lactate, acetate, and ethanol) on the stability and metabolism of L. sanfranciscensis and its yeast counterpart C. milleri. They found that L. sanfranciscensis grew best between 30 and 37°C, whereas C. milleri responded well to temperatures below 26°C — this is consistent with the “baker’s rule” that temperatures between 20 to 26°C are preferable for yeast fermentation.

L. sanfranciscensis thrives best in a pH range of 3.9 to 6.7, and it is able to grow in up to 4% NaCl. It can tolerate over 160 mmol acetic acid and temperatures between 30°C and 37°C. By contrast, its symbiotic partner C. milleri thrives between 20°C and 27°C. It can handle a larger range of acidity, pH 3.5 to 7, and up to 8% NaCl. However, this yeast’s growth is completely inhibited by 150 mmol acetic acid. Thus, a process that encourages lactic acid production at higher temperatures and fermentation at lower temperatures must be considered in the creation of a proper sourdough. If acetic acid must be added for sourness, it should be done after fermentation to prevent yeast inhibition. Acetic acid may also be produced naturally by the addition of fructose or by increased aeration.

Although sourdoughs are normally prepared without salt, some bakers still opt to use some in their mix. Ganzel’s group found that adding salt to the dough inhibited lactic acid bacteria, including L. sanfranciscensis. This allowed for a faster growth rate of yeast, which were less sensitive to the addition. Thus, salt may be added to encourage fermentation and slow the rate of souring. The bacteria also had a specific range of acidity that encouraged growth. L. sanfranciscensis grew best at pH greater than 4.5, and did not grow well at all under pH 3.8. During the process of sourdough growth and fermentation, the pH drops significantly. Thus, lactic acid production occurs rapidly and stabilizes once pH drops to 3.8 and L. sanfranciscensis is unable to grow. This must be considered in the addition of acetic acid to some commercially-produced sourdoughs– the pH should match that of a sourdough acidified by L. sanfranciscensis at a pH of 3.8. Furthermore, since S. cerevisiae can tolerate lactic acid but not acetic acid, it is important that the addition of acetic acid to commercially-produced sourdoughs occurs after fermentation so that the bread has a chance to rise.

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Lactobacillus sanfranciscensis : The Key to the Perfect Sourdough . . . . .


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Lactic Acid Fermentation in Sourdough . . . . .

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A Close Look at the Bacteria and Yeast in the Sourdough Starter

Vanessa Kimbell wrote . . . . . . . . .

I spent last week with Karl from The Sourdough Library, who has taken a sample of my sourdough starter back the library in Belgium, where there is collection of 91.. ( yes we are number 91) and he has sent the other sample to Marco Gobbetti, who I met in September, from the university of Bari, to be analysed.

It will be 45 days before the full yeast and lactic acid bacteria identification and analysis is complete, hopefully in time for the results to go in my book. In the meantime, this piece I wrote three years ago, will perhaps give you some indication of how fascinated I have been and perhaps give an indication of just how excited I am to have the lactic acid bacteria and yeast strains identified.

When I am teaching sourdough bread making on the course here, I often refer to my starter as a sourdough cultures. It’s a good name that reflects a little of what is going on, as sourdough is a complex ecosystem containing multiple species of yeast and lactic acid producing bacteria. Despite having had a close and somewhat intimate working relationship with my starter for more years than I can remember I had never actually see my starter close up I was recently lucky enough to be invited to Lalemand, the UK’s largest yeast manufacturing factory in the UK and I was thrilled when they offered to take a look at my starter and analyze it in the laboratory. Seeing my starter under the microscope was almost as exciting as opening my stocking on Christmas day. The technician dropped some of my starter into a solution and placed it under the microscope – and there they were – my yeasts and my bacteria. Wild sourdough yeasts don’t live alone in a monoculture. Unlike baker’s yeast, dormant cells of bacteria float through the air all around us and hang about in wheat waiting for a place to call home. When they find wheat they begin to reproduce and their digestive process is “fermentation.” To see these microorganisms close up with my own eyes and seeing both the yeast and bacteria side by side was magical.

The yeast and the bacteria have a mutually beneficial symbiotic partnership sharing all the available nutrients form the flour. Rather than compete for food they cooperatively protect their ecosystem from other uninvited bacteria. The yeasts are tiny oval shaped one-celled fungi and when they have access to oxygen aerobic fermentation produces carbon dioxide gas (CO2.) These are the bubbles that you see in the bread dough which is what makes your bread rise. I’d just fed the starter the night before on the basis that I wanted it to be as lively as possible. So I was even more fascinated to see them move. The yeast’s made their way around the petri dish like footballers running around a pitch in a random manner.

When they are aerobic then they are at their most active. Of course, when the yeast ferments in the absence of oxygen (anaerobic fermentation) it produces alcohol and slows down, which is why when you see a sourdough culture that has been left to ferment for a while without being aerated or fed develops a thin layer of alcohol (called hooch) on the surface. It is this alcohol combined with lactic acid that provides an additional flavor dimension. I’ve been baking with sourdough since I was a little girl and I never tire of it. The idea that there is an ecosystem in a pot that behaves differently according to the conditions that it lives in fascinates me. The Sourdough bacteria are much tinier than the yeast and are lactic acid-producing bacteria (lactobacilli) (these can also be found in numerous other fermented foods and it is these bacteria that produce the unique flavors and textures. They are also responsible for the increase the nutritional value of sourdough bread, through digesting the indigestible bits of flour.

The bacteria eat carbohydrates, fats and proteins and then produce acids, most notably lactic acid and to a lesser extent acetic acid (vinegar). The pH of sourdough changes according to the stage of fermentation it is at but in general it has a pH 3.5 – 5. It is this acidity that keeps out of pathogenic microorganisms such as botulism bacteria, E. coli bacteria and spoilage fungi as it is unable reproduce in an environment with a pH below 4.6. Nevertheless I was shocked as the technician took a swab and labeled up the peti dish as E-coli. I had never considered that my sourdough might have bad bacteria in it. “You’ll have to wait three or four days until we know if it has any E-coli or other nasty’s in it.” he said with a grin.

I have to admit I really wasn’t expecting this aspect to be tested, but was greatly relived to have the results back which gave my starter the all clear. I was confident that the Lactic acid bacteria had protected the ecosystem and prevented pathogenic microbes from invading the sourdough ecosystem and upsetting the balance. It is these same organic acids that keep sourdough bread mold-free far longer than bread made with baker’s yeast.. All in all it was fascinating to see my sourdough ecosystems is healthy and resilient, especially as this one is an heirloom sourdough starter that has been passed down for many generations.

Source: The Sourdough School

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