How Does Sourdough Starter Work? [science of sourdough] (2024)

How does sourdough starter work? We know this clever mixture of flour and water is a living thing that can make can make bread rise, but what is really going on inside the jar of your own sourdough starter?

Now I was never really interested in science at school, but things have come full circle, since now my family's food revolves around the colony of wild yeast and bacteria living in a jar on my kitchen counter. Crazy right? And because the sourdough starter is such a big part of my life and my family's food routine, I wanted to make sure I knew exactly what was happening in that jar.

This fuelled my expedition into the depths of science to find out how does sourdough starter work?

It's also such valuable information to teach our kids ... teaching them how to bake sourdough, and including a sneaky science lesson on microbial ecology ... how good is that?

Quick Summary of Science of Sourdough Starter

If you're in a hurry, here's a quick summary of the science of sourdough starter (how does sourdough starter work?):

• A sourdough starter (also called starter culture) is asymbiotic culture of bacteria and yeast(SCOBY) that's used as a natural leavening agent.
  
• Mix equal parts of flour and water together, leave at room temperature and allow the fermentation process to occur. Bacteria and yeast in the flour and air around you enables the microbes to convert the sugars in the flour into ethanol, carbon dioxide and acids.
  
• Carbon dioxide makes your sourdough bread rise. Acids give the sourdough bread its unique flavor. Enzymes produced by the bacteria pre digest the flour, giving sourdough bread its wonderful health benefits.
  
• This complex ecosystem of yeast and bacteria give you more than just a loaf of bread!

You can read more about the history of sourdough here, as well as find a simple sourdough starter recipe here which will help you easily make your own homemade sourdough starter. You might also like these 20 interesting facts about sourdough!

So ... What Is A Sourdough Starter?

A sourdough starter is a simple mixture of flour and water that is left to ferment at room temperature. The natural yeast that is present in the flour and the air around you and your starter begin to ferment the flour and water mixture. You can find detailed instructions for making your own sourdough starter here.

During the fermentation process, yeast and bacteria convert the simple sugars in the flour into other organic substances like carbon dioxide (that rises your bread), ethanol and acids (lactic acid and acetic acid).It's the acids that give sourdough it's unique sour flavor.

Because a sourdough starter consumes the nutrients in the flour, it's important to discard half of the starter and feed it regularly (generally twice a day during the establishment phase). It's best to feed your sourdough starter with a kitchen scale to ensure accuracy.

You can use the sourdough starter discard in sourdough discard recipes like sourdough chocolate chip cookies and sourdough pie crust. It's the active sourdough starter that's used to make sourdough bread and sourdough pizza crust.

Because the wild yeast inside your sourdough starter are already present in the flour (read more about where wild yeast come from here), it makes sense to use good quality flour such as an unbleached, organic all purpose flour or rye flour that are already teaming with good microbes, ready to colonise. You might find that fresh flour milled from whole wheat berries establishes a sourdough starter more quickly than store bought flour.

Let's Explore The Science of Sourdough Starter

The Sourdough Microbiome

Did you know that it really is true that each sourdough starter is unique. While scientists (and home bakers) know that sourdough starters are populated with "wild yeasts" and many of these wild yeasts occur in each and every sourdough starter, there are still unique elements to each culture - dependent on the geographical area, temperature, type of flour etc. These microbial communities, being nurtured on kitchen counters throughout the world, really are fascinating, right!

Did you know that some of the wild yeasts in your sourdough starter actually come from your own bread bakers hands?

You can read more about the unique qualities of the Sourdough Microbiome inthis study from the American Society for Microbiology.

How Do Sourdough Starters Work?

Ok, so we know what is happening inside the jar, but how does sourdough starter work? How does this powerhouse of microbial activity actually make your bread rise and give you the best sourdough bread ever?

When you mix active starter into your dough, you’re introducing a whole bunch of tiny microorganisms into the mix. As the dough rests, those microorganisms get to work, breaking down the carbohydrates in the flour and producing carbon dioxide.

This carbon dioxide is what causes the dough to rise – it gets trapped in those gluten networks (the ones you create during the stretching and folding process), creating those lovely airy pockets in your bread (also referred to as an open crumb).

But it’s not just about the carbon dioxide. Remember those lactic acid bacteria we talked about earlier? Well, they’re doing more than just making the dough sour.

They’re also producing enzymes that break down the gluten proteins in the flour. This is why sourdough bread often has a lower gluten content than conventional bread made with commercial yeast (saccharomyces cerevisiae). It is easier to digest and can often be tolerated by people who are gluten sensitive (however it is still not suitable for celiacs or people allergic to gluten).

So, there you have it – the science of sourdough in a nutshell. It’s a fascinating world of yeast, bacteria, and gluten, all working together to create the perfect loaf of bread. And the best part? You get to eat the results.

How Do Yeast and Bacteria Coexist?

The wild yeast and bacteria in sourdough starter are able to coexist due to their different dietary preferences.

You see, making a sourdough starter is like building a bustling city, but with limited resources. Think of the resources as simple sugars (from the flour), like glucose, fructose, and maltose. Different strains of yeasts and bacteria have their own food preferences, like picky eaters at a potluck.

Yeasts like glucose and fructose, while bacteria like maltose (source). But the cool thing is that a healthy starter has a perfect balance of microbes that don't steal each other's food, so everyone gets what they want.

Now, both yeasts and bacteria know how to make their place cozy and comfy for themselves. Yeasts release ethanol, which might not sound so pleasant, but bacteria like L. sanfranciscensis are totally cool with it.

Meanwhile, bacteria release acids, which would make most microbes throw in the towel, but not wild yeasts! They're tough cookies and can handle the sour conditions.

The best part is that yeast cells keep producing amylase enzymes as they multiply, breaking down more starch into simple sugars to keep everyone fed and happy. All these microbes work together like a well-oiled machine with perfect symbiosis.

The magical world of sourdough starter is a such a fascinating place, where yeasts and bacteria live in peace, love, and harmony, creating delicious bread for all to enjoy.

Want More?

​If you want to read more about the science of sourdough starter, then there are definitely some more detailed resources out there that will help you deep dive into the complex ecosystem of yeast and bacteria. Here are some of the articles I enjoyed reading on this topic:

-McGill University - How Microbes Could Keep Your Bread Fresher for Longer

-Review of Sourdough Starters: ecology, practices, and sensory quality with applications for baking and recommendations for future research

-The Diversity and Function of Sourdough Starter Microbiomes