Sourdough Starter Science: How Fermentation Works

Sourdough Starter Science: How Fermentation Works

A mosaic of wild yeasts and bacteria, sourdough is a playground of nature’s tiny wonders, working in harmony to rise, ferment, and flavor. Journey with us as we delve into the captivating world of fermentation, exploring the intricate relationships between yeast, bacteria, and dough.

The Science of Fermentation: Interactions between Yeast, Bacteria, and Dough

Sourdough is unique because it’s a symbiotic culture of bacteria and wild yeast. Saccharomyces cerevisiae might be the most famous yeast due to its role in baking and brewing, but it’s not the primary player in sourdough. Here are the most common ones:

Wild Yeast: Unlike commercial yeast, wild yeast is less aggressive and ferments at a slower pace. The most common wild yeast found in sourdough is Saccharomyces exiguus.

Lactic Acid Bacteria: This bacteria helps give sourdough its acidic taste. Lactobacillus is the most prevalent genus, and within it, species like Lactobacillus brevis and Lactobacillus sanfranciscensis are frequently found.

How Does the Yeast and Bacteria in a Sourdough Starter Leaven the Bread?

Leavening in bread refers to the process where dough rises. In sourdough, this is primarily achieved by the fermentation activity of yeast and bacteria:

Sourdough fermentation begins when enzymes in flour break down starches into sugars. Yeast and bacteria consume these sugars, producing CO2, acids, and alcohol. CO2 gets trapped in the dough, causing it to rise and create the airy crumb structure.

Sourdough fermentation starts when the naturally present enzymes in the flour begin to break down its complex starches into simple sugars. These sugars are consumed by the yeast and bacteria to produce carbon dioxide gasses, acids and alcohol as by products.

Yeast Activity: Yeast cells consume sugars present in the dough, producing carbon dioxide (CO2) as a byproduct. This CO2 gets trapped within the dough, causing it to expand.

Carbon dioxide is the primary leavening agent in many breads, including sourdough:

As CO2 is produced within the dough, it causes the dough to expand. This results in the airy and porous structure characteristic of bread.

Gluten network of the dough traps CO2 gasses which creates pockets in the dough. When baked, these pockets become the holes or “crumb” of the bread. The activity and quantity of yeast determine the size and distribution of these holes.

Bacterial Activity: Bacteria in sourdough, particularly lactic acid bacteria, produce organic acids which imparts the characteristic tangy flavor of sourdough bread. While these acids don’t cause rise like CO2, they contribute to the dough’s elasticity and extensibility, assisting in trapping the CO2 produced by yeast.

How Do Yeasts Contribute to the Flavor of Bread?

While yeasts are pivotal in bread rising, they also play an essential role in flavor:

Alcohol Production: As yeast metabolizes sugars, it produces not just CO2 but also alcohol. These alcohols evaporate during baking, leaving behind aromatic compounds that contribute to the bread’s distinct flavor.

Interaction with Bacteria: Yeast and bacteria collaborate in sourdough. As bacteria produce acids, they create an environment where certain flavorful compounds from yeast fermentation are more pronounced.

What is the Optimal Temperature for Yeast Activity?

Temperature plays a pivotal role in yeast activity:

Optimal Range: Generally, yeast thrives and reproduces most rapidly between 78°F (25°C) and 82°F (28°C). However, the exact optimal temperature can vary depending on the specific yeast strain.

Limiting Extremes: Temperatures below 50°F (10°C) slow down yeast activity, often used in processes like cold fermentation. Conversely, temperatures above 95°F (35°C) can inhibit or even kill many yeast strains.

What is the Role of Lactic Acid Bacteria in a Sourdough Bread?

Lactic Acid Bacterias and the acids they produce are responsible for the unique character and benefits of sourdough:

Lactic Acid Bacteria (LAB) are crucial in sourdough. They produce acids that give the bread its tangy flavor, improve dough structure and texture, act as natural preservatives, and enhance the bread’s nutritional value by breaking down phytic acid and producing prebiotic compounds.

  • Flavor Development:

Sourdough Starter contains Lactic Acid Bacterias which produce both lactic and acetic acid which provides more complex flavors. A balance of lactic and acetic acid is desirable in sourdough as lactic acid provides smoothness (akin to yogurt) while acetic acid provides sour bite (vinegar). 

The development of lactic acid favors warm and loose dough, while acetic acid favors cool and stiff conditions; you can control temperature and hydration of sourdough culture to determine what flavor you want your bread to have.

  • Dough Structure and Texture:

The acids lower the dough’s pH, which can strengthen the gluten network in weak doughs. The acidic environment created by LAB is inhospitable for many unwanted microbes, acting as a natural preservative and extending the bread’s shelf life. LAB competes with potential pathogens and spoilage organisms, further safeguarding the dough.

  • Nutritional Enhancement:

LAB helps in the breakdown of phytic acid, which enhances mineral absorption from the bread. LAB fermentation produces prebiotic compounds, promoting gut health. Some argue that live LAB in freshly baked sourdough can act as probiotics, though the high baking temperatures likely kill most of these beneficial bacteria.

How Does the Microbial Composition of Sourdough Starter Differ from Conventional Breads?

The principal difference between sourdough starters and the yeast used for conventional breads lies in their microbial composition:

  • Diversity: A sourdough starter contains a mix of wild yeasts and bacteria, while conventional breads typically rely on a single strain of commercially cultivated yeast, most commonly Saccharomyces cerevisiae.
  • Flavor: The combination of wild yeasts and bacteria in sourdough creates complex flavors through fermentation. The lactic and acetic acids produced by the bacteria give sourdough its tangy taste. Conventional bread, using just commercial yeast, has a milder, less complex flavor.
  • Rising Time: Sourdough fermentation is typically slower than conventional bread fermentation. This longer fermentation allows for flavor development and a denser crumb structure. Conventional breads, with their cultivated yeasts, rise faster and tend to have a softer, more uniform crumb.
  • Natural Fermentation: Sourdough relies on naturally occurring microbes present in the environment, while conventional breads use commercially produced yeasts which are introduced to the dough in a controlled manner.

Microbial Symbiotic Relationships and Balance in Sourdough

How do Yeasts and LAB Coexist in a Sourdough Starter Without Competing for Resources?

Yeasts and Lactic Acid Bacteria (LAB) can break down different carbohydrates present in the flour. For instance, while yeasts primarily metabolize simple sugars like glucose and maltose, certain LAB strains target more complex carbs, ensuring they aren’t directly competing for the same resources.

Yeasts, while consuming sugars, produce CO2, helping the bread rise. LAB, on the other hand, generates acids which create an environment that enhances the action of certain yeasts, and this relationship becomes mutually beneficial. The acids produced by LAB can also help break down flour components into simpler sugars, providing additional resources for yeast.

The microbial diversity in a sourdough starter comes from the environment, including air, surfaces, and flour. Factors like flour type, temperature, humidity, and feeding frequency influence the microbial composition. A well-maintained heirloom starter can retain its unique microbial character over generations, despite environmental changes.

How Do Yeasts and LAB Make Their Environment Inhospitable to Other Microbes?

One of the primary products of LAB fermentation is acid (like lactic and acetic acid). These acids reduce the pH of the sourdough, making the environment acidic and unsuitable for many unwanted bacteria and molds.

By rapidly consuming available resources, yeasts and LAB essentially starve off many potential invaders. Moreover, certain LAB strains produce bacteriocins, proteins that inhibit the growth of other bacteria.

What Factors Affect the Microbial Composition of Sourdough Starter?

  • Flour Type: Different flours come with their own microbial communities. For instance, whole wheat might harbor a different microbial profile than refined white flour.
  • Environmental Factors: Temperature, humidity, and even altitude can influence which microbes thrive in the starter.
  • Feeding Frequency: Regular feeding of the starter can promote the growth of fast-acting microbes while sidelining slower ones.
  • Water: The water used can introduce new microbes or chemicals (like chlorine) that might affect existing microbial communities.

How Does the Type of Flour Affect the Microbial Composition of a Sourdough Starter?

Whole grains, rye, or other non-wheat flours often contain different nutrients, which can support a diverse microbial community. Whole grain flours that retain bran and germ have more microbes naturally present than their refined counterparts.

The type and amount of fiber and starch can determine which microbes flourish. Some might prefer the complex carbohydrates in rye, while others might thrive on the simpler ones in white flour.

Does the Microbial Diversity and Flavor Differences in Starters Vary?

A starter with diverse microbes will often produce a bread with a more complex flavor profile due to the range of fermentation byproducts. Starters can vary regionally, with each having its own unique blend of microbes, leading to distinct flavors. This is why a San Francisco sourdough might taste different from one in Paris.

Is There a Possibility that Heirloom Sourdoughs Have Retained the Same Microbes Over Generations?

While the environment continuously introduces new microbes, established colonies in a well-maintained heirloom starter can remain dominant. This means that, to some extent, they might retain a microbial “fingerprint” over generations.

Factors like changes in flour type, water source, or even a lapse in regular feeding can introduce shifts in microbial communities. However, dedicated bakers who maintain consistent practices can preserve a starter’s unique microbial character over time.

Origins of Microbial Composition in Sourdough

Where Does the Yeast in Sourdough Starter Come From?

Unlike commercial breads where yeast is introduced, sourdough harnesses the power of the environment. Yeast is naturally present in the environment, including the air we breathe, the surfaces around us, and even the flour used in the starter. 

When creating a sourdough starter, the mixture of flour and water offers a medium for these wild yeasts (and beneficial bacteria) to grow and multiply.

While there are many yeasts in the environment, some are more common in sourdough:

  • Saccharomyces exiguus: Often found in sourdough, it can metabolize maltose (a sugar present in flour) and coexists well with lactic acid bacteria.
  • Candida milleri (or Candida humilis): Another frequent sourdough inhabitant, this yeast contributes to the bread’s rise and flavor.

Where does Lactic Acid Bacteria in Sourdough Found?

Lactic Acid Bacterias are found all around us. LAB plays a role in fermenting a variety of foods. This includes dairy products like yogurt and cheese, as well as vegetables in pickles and sauerkraut.

Our physical bodies have LAB as a part of their natural flora. They can also be found on plant surfaces and in some soil and water samples.

What is the Difference Between Homofermentative and Heterofermentative LAB Strains?

The distinction between homofermentative and heterofermentative LABs boils down to the byproducts they produce during carbohydrate fermentation:

  • Homofermentative LAB:

These bacteria predominantly produce lactic acid from the fermentation of sugars. Examples include Lactococcus lactis, often used in cheese production.Nearly all of the consumed sugar (about 85% or more) is converted to lactic acid.

  • Heterofermentative LAB:

These bacteria produce a mix of compounds from sugar fermentation, including lactic acid, carbon dioxide, and ethanol (or other compounds). Leuconostoc mesenteroides, for instance, is used in sauerkraut and pickles. A more diverse set of byproducts results, often leading to a complex flavor profile in fermented foods.


The world of sourdough bread is a harmonious blend of science, tradition, and gastronomy. By understanding the microscopic marvels that contribute to its making, we gain a deeper appreciation for each bite we take. It’s a journey that connects us to our ancestral bakers, reminding us of the beauty of nature and the enduring legacies embedded in food. Whether you’re a seasoned baker or a curious eater, the story of sourdough is a testament to the magical confluence of microbes, environment, and human touch, creating a bread that’s as rich in flavor as it is in history.


Hey there! I'm Sam, your go-to pal for all things sourdough. I've been baking and kneading for 10 fun-filled years, and I can't wait to share the joy of turning simple ingredients into heavenly sourdough bread with you. Grab your apron and let's dive into this amazing world of sourdough bread together on this blog.

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