The Science Behind Sourdough

Last November, I joined Lucy Ordaz, a food science teacher, in her classroom to open the world of bread making to about 100 eighth-graders. We taught them the science behind sourdough—at least how to make sourdough bread (or try to make it). There were several successful attempts in making it, but not all her classroom teams succeeded.

I have been in her classroom a few times before and each was a STEM experience, once before cooking and twice for in-depth outdoor education. This round I had not thought much about the science part of the sourdough process.

Here is what I learned. First, you cannot pack years of sourdough experience into two 40-minutes sessions of classroom time and expect much success. Two, there really is science behind sourdough bread making, what with using units of measure, mixing ingredients, and adding heat. It is all applied science. But then I realized I needed to learn more myself so I started looking into the science behind sourdough.

To begin Emily Buehler seems to have a good overall description of the science:

“A sourdough starter is created by mixing flour and water, and then allowing the microorganisms—wild yeasts and bacteria—that live in the flour and air to thrive and multiply. Over time, a stable population of these microorganisms develops. When used in bread, the microorganisms perform fermentation reactions, producing the gas that makes the dough rise and the molecules that give it flavor.”¹—Emily Buehler

Then the folks at the Compound Interest chemistry website, explained the scientific process of bread making in four very simple steps: mixing ingredients, kneading dough, fermentation, and baking.

“Sounds pretty straightforward, right? Perhaps,” they say, “but at a molecular level, there’s a lot more happening.”² Since both yeast leavening and fermentation use microbes to produce carbon dioxide that bubble up in the dough making it raise, clearly organic chemistry is at work. But sourdough takes it a step further into applied microbiology.

Commercial loaves of bread are most often made with baker’s yeast, which is a more recent tradition associated with beer production. Barm, the foam that forms on beer during fermentation, is used to make this baker’s yeast. This became popular in the 1930s, quickly replacing unattended family starts all over the country. It is quick-acting and makes bread rise in less than a quarter of the time it takes to make sourdough bread.

But something is lost with speed. Traditional sourdough baking relies on an age-old blend that marries both bacteria and wild yeast to make the chemistry work, which increases the depth of flavor in the bread well beyond commercial loaves.

In sourdough, there is complex biology between the wild yeast and lactobacilli. Water triggers both yeast and bacteria to begin predigesting the carbohydrates in the flour. Then in a unique symbiotic relationship, the yeast enzymes breakdown flour starch turning it into sugars;  first using the enzyme amylase to break down the starch to maltose all the while deepening the flavor of the bread.

“The wild yeasts found in sourdough are unable to process maltose. Luckily for them, the bacteria in the sour dough mixture can, and since maltose is simply two glucose molecules joined together, it produces food for both the bacteria and …the bacteria are able to feed on any dead yeast cells. The ultimate result is that it produces carbon dioxide and ethanol.”

The bacteria use ethanol to produce lactic acid. This “acidity created by the lactobacilli is good for the yeast but inhospitable to other organisms. A sourdough starter is able to be kept at room temperature (if fed properly) and the acidity of the bread acts as a preservative even after baking.”³

With all this chemistry going on, bubbles of carbon dioxide develop becoming trapped in the flour proteins, glutenin and gliadin. These proteins, “collectively referred to as gluten …are inert, but as soon as water is added into the mixture the fun begins. The proteins are then able to line up with each other and interact. They can form hydrogen bonds and disulfide cross-links between their chains, eventually forming a giant gluten network throughout the dough. Kneading the dough helps these proteins uncoil and interact with each other more strongly, strengthening the network.“⁴ 

Adding our third “perfect ingredient,” salt for flavor, also strengthens the network of gluten making the dough more elastic and able to capture carbon dioxide bubbles, which makes the bread rise. Kneading the dough makes these gas bubbles more uniform throughout the dough, improving the final texture.

And finally, applying heat causes a whole series of reactions between the sugars and amino acids that add flavor and helps the crust brown better. 

Where Does the Wild Yeast and Bacteria Come From

There are traces of yeast and bacteria all around us. These beneficial microorganisms are found naturally on the surface of all grains, fruits, vegetables, and they are also found in the air and soil. This makes each sourdough strain have variations of bacteria and wild yeast unique to their locale.

That is probably why San Francisco likes to claim they have the best sourdough when it actually is just the organic chemistry from their area bringing unique yeast and bacteria together to produce their tangy signature flavor.

In fact, in 1971, two researchers,  Leo Kline and  Frank Sugihara, set out to discover what was making the San Francisco sourdough so unique. They found a previously uncatalogued bacteria in the San Francisco starters that had not been seen before. In its honor, it was finally  named Lactobacillus sanfranciscensis.⁵

However, since those studies, these same bacteria have been found in both France and German levains, as the starts are often called there. Interestingly, the start at Abigail’s Oven, hails from San Francisco.

A funny thing, I have had my start for 18 months and I would have thought by now it would have morphed into a Utah start. Not so, according to Martha Levie, one of the owners at Abigail’s. In a recent interview, she told me that the “mother start’s” chemistry destroys invading bacteria.

What’s Going On In Your Sourdough Starter

Emily Buehler explains, “A sourdough starter is created by mixing flour and water, and then allowing the microorganisms — wild yeasts and bacteria — that live in the flour and air to thrive and multiply. Over time, a stable population of these microorganisms develops. When used in bread, the microorganisms perform fermentation reactions, producing the gas that makes the dough rise and the molecules that give it flavor.”⁶

Interestingly both bacteria and yeast use carbohydrates in flour as fuel in a wet start. As they are fueled they both produce carbon dioxide gas bubbles that are trapped in the dough. The process continues to expand from starter to dough making the entire loaf rise.

In comparison, commercial baking yeast is very specialized and is very fast-acting. It is easy to produce, but it doesn’t adapt well and is intolerant of acidic environments. Because of its rapid chemical interactions bread made this way does not develop the depth of flavor found in artisan sourdough bread.

Sourdough Breaks Down Gluten and Helps with Mineral Absorption

The longer process used to raise sourdough bread allows the proteins (gluten) to breakdown into amino acids that are easier to digest. “This gluten breakdown is why some people who have a gluten sensitivity can tolerate sourdough wheat bread.”⁷

Bacteria in sourdough helps to “activate phytase, an enzyme that breaks down phytic acid, an anti-nutrient found in all grains and seeds.⁸” This allows you to absorb and retain minerals that phytic acid would otherwise bind and take out of your body.

The Culture of Bacteria and Yeast

In a healthy sourdough start, the percentage of lactic acid bacteria far “outnumber yeast cells in a mature sourdough starter by roughly 100 to one. In fact, a levain isn’t stable without the lactic acid bacteria that symbiotically live with the wild yeast.”⁹

In their research, Kline and Sugihara wrote that an “aspect of this [San Francisco] sourdough system containing these bacteria and certain yeasts is its self-protective nature, i.e., its incredible resistance to contamination by other microorganisms which has been maintained for decades …these bacteria may produce related antibiotics,”¹⁰ thus preserving the original microbiology of the starter. So much for combining my Alaskan, Utah and San Francisco Sourdough starts.

Conclusion

The folks at Modernist Cusine sum up this whole chemistry experiment well:

“A starter’s composition will stay the same only in a perfectly maintained sterile environment, more like a laboratory setting than a bakery. The community of microorganisms will fluctuate and adjust to whatever foods they are given and whatever living conditions they experience. If one strain finds the environment more welcoming than the others, it will quickly grow and crowd its neighbors.

“But locking in a specific population of bacteria is not important. What matters is creating a hearty colony of yeasts and lactic acid bacteria that behaves predictably; in other words, as long as the levain is fed on the same schedule and kept at about the same temperature and hydration, it will ripen and mature as expected.”

So feed your start every day with at least a third cup of flour and a quarter cup of water, or store it in the fridge for a week. If you are feeling just too busy, tuck a quarter cup away in your freezer and use it when you can again.

In the comment section below, tell us about your sourdough start.

Sources

¹ Emily Buehler, “What’s the Difference Between Levain and Starter?”, Kitchn, OCT 4, 2017
² Compound Interest, “Baking Bread: The Chemistry of Bread-Making”
³ Cultures for Health, “Introduction to Sourdough”
⁴ ibid.
⁵ Leo Kline and  Frank Sugihara,  “Microorganisms of the San Francisco Sour Dough Bread Process,” Applied Microbiology, Mar. 1971, p. 459-465
⁶ Buehler
⁷ “Sourdough Bread Made from Wheat and Nontoxic Flours and Started with Selected Lactobacilli Is Tolerated in Celiac Sprue Patients,” Appl Environ Microbiol. 2004 Feb; 70(2)
⁸ “Prolonged Fermentation of Whole Wheat Sourdough Reduces Phytate Level and Increases Soluble Magnesium,” J. Agric. Food Chem. 2001, May 4, 2001
⁹ “The History of Bread Yeast”. BBC. Retrieved December 24, 2006.Modernist Cuisine Team, “Sourdough Science”
¹⁰ Kline and Sugihara
¹¹ BBC