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The science of sourdough bread: inspired by fermentation.

Updated: Feb 25, 2021

The science of sourdough is quite fascinating.

It all starts with life, bacteria and fermentation.

The fermentation process started a long time ago.

Let's turn the clock back 100 million years. The first bacteria are born.

Bacteria are living organism and just like the rest of us they need to eat.

So what is fermentation? Fermentation is the chemical reaction resulting in bacteria feeding on carbohydrates (complex sugar) and transforming these into CO2 and ethanol, in the case of ethanol fermentation, or lactic acid, in the case of lactic acid fermentation. It is an "anaerobic" process, that takes place without the need for oxygen.

Since this is a blog about science, I felt compelled to illustrate the fermentati0n process in a formula, that will look familiar if you have studied chemistry (even just in high school):

C6 H 12 O 6 , C 12 H 22 O 11 -> CO 2 + C 2 H 5 OH (ethanol) + acetic acid +

lactic acid. Without going into the details, C6H12O6 and C12H22O11 represent complex sugars that will be transformed by bacteria into carbon dioxyde (CO2), ethanol and acid.


Fermented food are commonly used around the world


Common food we are all familiar with such as beer , wine, yogurt , vinegar, cured olives , cheese, miso, kefir, kimchi are all the result of fermentation. Fermentation also happens as part of our digestion process, in the gut. These "good" bacteria that are in our digestive system (intestin) are the gut microbiome. Less well known, but also important for all mammals is the fermentation process happening in our muscles when they run out of oxygen. They use a compound named ATP (adenosine triphosphate) to release sugar that translates into energy and muscle contraction (movement).

The science of fermentation, also known as zymology, was only discovered in the 1850s by Louis Pasteur and has seen a resurgence of interest in the last decades for its key contribution to our overall health.


What are the two types of bacteria found in a sourdough starter?


One of these types of bacteria, yeast, originally comes from the fungus family: it has been around for over 100 million years on planet earth, and its first recorded use by humans on a large scale (to bake bread) dates back about 5000 years in Egypt.

The most common type of yeast is the species Saccharomyces cerevisiae that is the primary contributor in beer fermentation but also commercial dry yeast. Over 1500 species of yeast have been recorded and a typical home made sourdough starter will contain at least a handful of different types.

The other common type of bacteria present in sourdough is the LAB (Lactic Acid Bacteria). As its name indicates, it produces Lactic Acid. LABs are a strong contributor to the complex taste of sourdough bread. The most common one is (L. acidophilus). It is found in yogurts and has a creamy taste. Other types produce more of a sour taste (heterofermentative).





Fun fact: L. Sanfranciscensis was discovered in the bay area in 1971. It was thought originally to be unique to San Francisco, but as it turns out, it can be found worldwide.

Commercial yeast is produced from molasses (sugar), ammonia, phosphoric acid in a fermenter at 86F. Sugar is added to speed up the process.


Symbiose between yeast and LABs: the secret sauce behind a healthy culture


The magic behind sourdough starter is the story of symbiose between living organism that complement each others and eliminate intruders.

On one end, LABs produce acid that eliminates a lot of unwanted bacteria, except yeast that has a high tolerance for acidity. On the other end, yeast produces ethanol which kills a lot of bacteria except LABs. LABs are much smaller molecules than yeast, and the ratio in a starter is about 100:1. LABs and yeast are the most dominant active bacteria found in the sourdough microbiome, however new research provides insights on the presence of AAB, Acetic Acid Bacteria producing (as you guessed) acetic acid.

There are some other important actors taking part of the sourdough fermentation.

One of them is enzymes. Enzyme is a protein found in flour. It acts as a catalyst that assists in breaking down starch (complex sugar) into glucose. It is also present in saliva, so when we chew bread we release sugar that provides a relative sweet taste.


Gluten gives the crumb its texture and elasticity


Gluten is a protein that is naturally present in wheat grain. It gives the crumb its texture and elasticity. Once water is added to flour, gluten is activated and develops a network of strands. Heat and Kneading (or stretch and fold) also help in this process.

Once the fermentation begins, CO2 released by the process is trapped by the gluten network and the dough starts to develop.

Just in case you've wondered, the strength and elasticity of gluten in flour is measured in the baking industry using a farinograph.

Many people in recent years have developed severe gluten intolerance, also called celiac disease. Some studies have shown that sourdough bread is easier to digest than commercial bread and can help alleviate mild gluten intolerance.


What happens during the baking process?


Unlike some other fermented food, bread does not contain any live culture: the yeast dies at about 130F. Air pockets set, pockets "set", giving the baked product a soft and spongy texture. Ethanol and water evaporates, and gas cells expand with the heat. This process is called the oven spring, and occurs in the first 10 min of baking. Excess gas escapes following the scoring pattern. Gluten coagulates to give the bread its final shape. Sugar caramelizes at the surface giving the crust its beautiful roasted color.

Now the next time you bake bread, think about all these chain reactions that occur. Us bakers are just one small cogwheel in the complex machinery called fermentation.



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