It's Alive! The Sourdough Experiment Part Two

Here’s a jar of flour and water, also known as my sourdough starter. It’s pale, bubbly, and sour-smelling. 

Photo by blog author.

My sourdough starter, like every sourdough starter, contains a unique combination of yeasts—which make sourdough bread rise—and lactic acid bacteria—which make sourdough bread sour. The external environment, the temperature, and the type of flour used influence which specific types of yeast and lactic acid bacteria are present in a sourdough starter. Each sourdough microbiome produces a distinctively flavored sourdough bread. The bread made with my starter tastes different than the bread made with any other starter, but the process for making all sourdough starters is basically the same.

Inside of a sourdough starter both yeasts and lactic acid bacteria break down carbohydrates in the flour for fuel. (To learn more about this process see my last post.)

Do yeasts and lactic acid bacteria compete with each other for resources in the starter or do they work together?

Well, they do a little bit of both.

Yeast and lactic acid bacteria do compete with each other for resources like carbohydrates, and nitrogen, used to make proteins.  But, despite occasional skirmishes, yeast and lactic acid bacteria work together to successfully survive in the sourdough starter.

We know that yeast and lactic acid bacteria both break down carbohydrates in flour. What I did not mention yet is that there are different types for carbohydrates in flour, like maltose, glucose, fructose, and sucrose. Yeast can digest some of these carbohydrates and lactic acid bacteria can digest others. Yeast and lactic acid bacteria collaborate to consume all these carbohydrates. Here’s an example of how the common sourdough yeast species Saccharomyces exigus and lactic acid bacteria cooperate.

 

Image by blog author

While the yeasts are digesting carbohydrates, they also release amino acids lactic acid bacteria need to survive.

But what do all these microbial interactions mean for us, the ones eating the sourdough?

While breaking down carbohydrates for their own fuel, yeasts and lactic acid bacteria make these carbohydrates easier for us to digest. Because of this, eating sourdough bread does not cause abrupt spikes and drops in blood sugar levels like eating bread made with packaged yeast does.

Sourdough yeasts also help us absorb minerals more easily. Here’s how:

Image by blog author.

In addition to being healthy, well-made sourdough tastes great! Yeasts and lactic acid bacteria in the sourdough starter produce lots of tasty compounds, like buttery diacetyls, and sour organic acids. Just think—most of the flavor and microbial complexities in sourdough bread originated in a jar of flour and water.

Sources.

It's Alive! The Sourdough Experiment Part One

 The first loaf of sourdough bread I made tasted good covered in melted cheese—in the way I imagine cardboard would taste good covered in melted cheese. My sourdough bread was dry, crumbly, flavorless, and barely edible.

The dry, crumbly bread. Photo by blog author.

What went wrong? I have a few ideas, but before I share my hypothesis for the Great Bread Failure, let me explain what exactly sourdough bread is.

Bread is leavened with yeast—a microscopic fungus which breaks down carbohydrates in flour and releases carbon dioxide and ethanol as byproducts. The carbon dioxide released by the yeast fills the bread dough with gas and makes the dough rise, giving the bread the fluffy texture we expect.

Dry packaged baker's yeast. Photo by blog author.

Most bread we make and eat today is leavened with dried baker’s yeast—Saccharomyces cerevisiae—a yeast species specifically bred for its leavening capability. S. cerevisiae isn’t the only yeast that can leaven bread. Wild yeasts growing on food and on practically every other surface can also be used to make bread—if we can catch them.

A sourdough starter, simply made of flour and water, is the way people harnessed the power of wild yeasts for centuries. This starter is used in place of dry yeast to make bread. Or, I should say dry yeast is used in place of a sourdough starter, as dry yeast is a fairly recent invention and originally all breads were sourdough.

My bubbling sourdough starter. The bubbles are carbon dioxide being released by the yeast.
Photo by blog author.

The water in the sourdough starter prompts flour enzymes to break down starches in the flour into carbohydrates that yeasts can digest. The yeast naturally found in the flour begin to ferment these carbohydrates and release carbon dioxide, ethanol, and a variety of other byproducts, including B vitamins and organic acids.

The yeasts in the sourdough starter are joined by lactic acid bacteria, a type of beneficial bacteria which lives on food and in soil. The lactic acid bacteria also ferment carbohydrates in the flour, but the main byproducts they produce are lactic and acetic acid. Lactic acid bacteria are responsible for the “sour” in sourdough. (To learn more about lactic acid bacteria check out my blog post on vegetable fermentation.) 

Once a sourdough ferments for about a week, it is ready to be used to make delicious sour bread.

Or in my case, not-so-delicious bread. My sourdough didn’t even taste sour!

The sourdough recipe I used called for baking soda, another leavening agent mainly used in quick breads. Baking soda reacts with acid to release the carbon dioxide which allows the bread to rise, but during the reaction the acid is neutralized. When I mixed baking soda with my sourdough starter, it fizzed like a baking-soda-and-vinegar volcano releasing copious amounts of carbon dioxide. While the baking soda helped my bread rise faster, it also neutralized all the tasty lactic and acetic acid in the dough, leaving my concoction decidedly un-sour and un-tasty.

Baking Soda. Photo by blog author.

Sourdough starters contain yeast, and baking soda is not needed to leaven the bread. However, sourdough breads take a very long time to rise, which is why some recipes call for baking soda to speed along the rising process. But baking soda destroys the sourdough’s flavor, and I don’t think the faster leavening time is worth it.

The baking soda explains why my bread wasn’t sour, but not why it was so dry and crumbly.

When I was making my sourdough, I got a little carried away with adding whole-wheat flour. Whole-wheat flour is very healthy, but the bran in it soaks up lots of water—more water than I accounted for. Since I didn’t add enough water to the dough I ended up with dehydrated and unappetizing bread.

Despite my struggle with my first sourdough attempt, I was determined to try again, and sourdough take two turned out wonderfully! I did not use baking soda and I used a mixture of both whole-wheat and white flour to avoid the dehydration problem.

The tasty sourdough bread! Photo by blog author.

Why was I so determined to repeat the sourdough experiment? One reason is because well-done sourdough is delicious and has many health benefits, but the reason I really am interested in sourdough is the dynamic and little-known relationship between the yeast and lactic acid bacteria in the sourdough starter. More on that relationship in my next post!

Sources.