Sourdough science: how a sourdough starter is made

When I first started baking sourdough, I was frustrated by all the conflicting information I was reading. Feed it 1:1:1! Feed it 1:10:10! Feed every 12 hours! Feed every 24 hours! I decided to put my undergrad to good use and start reading more about sourdough starters and fermentation.  We will be getting a little  into the weeds here, while staying beginner-friendly!

Part of the reason I started Yeast Coast Sourdough is to remove this barrier of entry, so people an start baking faster rather than wait weeks for their starter to hopefully establish. You can learn more about our starter kits here if you'd like to skip all this!


Without further adieu, let's get into it. 

First things first: what exactly is a sourdough starter?


For thousands of years, leavened breads were made via fermentation. Thought to originate in Egypt, bakers learned to keep a small piece of their dough to save for their next batch. This was the first sourdough starer: a living culture that creates rise in doughs. 

What's the difference between commercial yeast and sourdough starter?


The majority of modern breads are made with commercial yeast: the packet you can buy at the grocery store. This is a monoculture of only a strain of yeast called Saccharomyces cerevisiae, bred for speed and consistency. When mixed into dough, the yeast microbes consume the simple sugars, producing carbon dioxide and ethanol primarily through respiration. These gas bubbles become trapped in the gluten network as the dough proofs, creating the final bread structure and rise. This happens fairly rapidly, often in 1-2 hours.

Sourdough starter is a symbiotic culture of wild yeast, lactic acid bacteria, and sometimes acetic acid bacteria. The microbes utilize both respiration and fermentation (without oxygen present), producing carbon dioxide, ethanol, as well as organic acids, primarily lactic acid and acetic acid. The gases produced by the yeast produce the rise, just as they do in commercially yeasted bread. The acids created by the bacteria present in the culture create the signature tangy flavor present in sourdough breads. Sourdough rises much more slowly than commercial yeast due to the complex taxonomy of the culture, fermentation, and pH drop. The longer the fermentation, the stronger the flavor as those organic acids build. 

How is a sourdough starter made?


If you've landed here you probably already know this: water and flour are mixed together and over time, a sourdough starter develops. Of course it's not magic; there's microbiology going on behind the scenes! How does flour and water turn into a microbiome?

The majority of the microbes inside a sourdough starter will be naturally present in the flour used. Additional species are introduced via the environment, water, and fascinatingly, the baker's hands! A 2020 study found that bakers' hands shared a more similar Lactobacillales community with their own starter compared to a random starter. The microbes immediate begin vying for survival, consuming sugars, excreting waste, and multiplying.

In the first few days of creating a sourdough starter, there will be a large bacterial bloom known as the "false rise". The starter will explode in volume rapidly, often tripling in size overnight. Many beginners get excited at this stage, thinking their starter is ready to bake already! The first microbes to take over are generally members of the Leuconostoc genus, naturally present on grains. They quickly outcompete the rest of the microbes present, exploding in population. These bacteria produce large amounts of carbon dioxide and produce a foul odor. You'll know your starter isn't ready yet when you smell it.

In addition to the CO2, the Leuconostoc create acid waste. As the waste accumulates in the jar, the pH begins to drop, becoming more acidic. The wild yeast and lactic acid bacteria/acetic acid bacteria we are attempting to cultivate thrive in these conditions, while pathogens die off at a pH of 4.2 The Leuconostoc are pushed out of the environment they themselves created, and quickly outcompeted by our acid-tolerant sourdough starer microbes. A sourdough starter typically stabilizes around 3.5 - 3.7 (remember - the closer to 0, the more acidic!). Once the culture stabilizes, congratulations! You have a ready to use sourdough starter! It will be immature, weak, and more delicate in the initial weeks but with proper care will be with you forever.

This is another benefit to beginning with an established sourdough starter rather than creating your own. Each batch of Yeast Coast Sourdough Superstarter originates with our mature, robust master culture. Once you activate it, it retains its original strength and resilience, already in balance! Learn more about our starter kits.


How does a sourdough starter work?

A sourdough starter contains multiple species of microbes. There are always yeast present, as well as lactic acid bacteria and often, acetic acid bacteria. Although they both consume sugars found in flour, they each find their own niche in the microbiome rather than competition. (warning more science ahead, but we'll keep it simple).

Flour naturally contains very small amounts of glucose, a simple sugar, and large amounts of starch: long chains of glucose. These chains are too large for the microbes to break apart to utilize for energy. 
Flour also contains enzymes called amylase, which break these chains into smaller pieces, often maltose, which is formed by two linked glucose molecules. 

The bacteria in the starter prefer to consume this maltose, while the yeast prefer glucose. The bacteria split each maltose into two glucose molecules, utilizing one for energy while excreting the other as waste. The yeast cells are then free to use them for energy. 

Once you mix your dough, fermentation immediately begins as the microbes consume the carbohydrates in your flour. Yeast produces carbon dioxide and ethanol gases, which become trapped by the gluten network as gas bubbles. Bacteria create organic acids, lowering the pH, speeding up the amylase enzymes, and creating the signature sourdough tangy flavor.

It's important to remember that fermentation does not stop once you shape your dough. Especially if you do a cold proof in the refrigerator the microbes' metabolic activity will slow dramatically, but they will continue to produce gases and acids until the dough is baked. 

No Two Sourdough Starters are Exactly Alike!

Studies of sourdough starters have found over 25 species of yeast and 75 species of Lactic Acid Bacteria. While some starters are dominated by a single species of each, many sourdough starters contain many types of each. It is not yet known the specific properties each species or combination contribute, but the taxonomy of a sourdough starter directly influences the bread's flavor profile! Your starter will also change over time to create your own unique culture that thrives best in your environment. 

Why use a dehydrated sourdough starter instead of making one from scratch?

As we discussed, it takes 2-4 weeks for a sourdough starter to stabilize, and then around 3 months to mature enough for reliability. A dehydrated sourdough starter like our Yeast Coast Sourdough Superstarter, is an active, mature starter in a dormant state. It does not need to stabilize or go through this maturation process, it just needs to wake back up! That means you'll be ready to bake in a few days rather than a few months.

Interested? Shop our sourdough starer kits!

Any questions?



References/recommended reading

Reese AT, Madden AA, Joossens MLacaze G, Dunn RR2020.Influences of Ingredients and Bakers on the Bacteria and Fungi in Sourdough Starters and Bread. mSphere5:10.1128/msphere.00950-19. https://doi.org/10.1128/msphere.00950-19

Elizabeth A Landis, Angela M Oliverio, Erin A McKenney, Lauren M Nichols, Nicole Kfoury, Megan Biango-Daniels, Leonora K Shell, Anne A Madden, Lori Shapiro, Shravya Sakunala, Kinsey Drake, Albert Robbat, Matthew Booker, Robert R Dunn, Noah Fierer, Benjamin E Wolfe (2021) The diversity and function of sourdough starter microbiomes eLife 10:e61644. https://doi.org/10.7554/eLife.61644

Lee, Jessica Audrey. (2010). Yeast Are People Too: Sourdough fermentation from the microbe’s point of view. https://www.researchgate.net/publication/302180762_Yeast_Are_People_Too_Sourdough_fermentation_from_the_microbe's_point_of_view

Gerez, C.L., Rollán, G.C. and de Valdez, G.F. (2006), Gluten breakdown by lactobacilli and pediococci strains isolated from sourdough. Letters in Applied Microbiology, 42: 459-464. https://doi.org/10.1111/j.1472-765X.2006.01889.x

Luc De Vuyst, Patricia Neysens,
The sourdough microflora: biodiversity and metabolic interactions,
Trends in Food Science & Technology,
Volume 16, Issues 1–3,
2005,
Pages 43-56,
ISSN 0924-2244,
https://doi.org/10.1016/j.tifs.2004.02.012.



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