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.
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.
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.