Written on behalf of Leaven Foods, blog found on Substack, along with Cameron Martino and James Gaffney, Co-Founders of Leaven.
In this first installment, of what will be an ongoing “Science of Sourdough” series, we introduce sourdough microbiome basics. Microbes! What are they? Where do sourdough microbes originate? How does the sourdough microbial community form?
What is a Microbe?
They are everywhere, all over us, inside us, on everything we touch, consume, and interact with! Microorganisms, or microbes, are a wide-ranging collection of living creatures that exist at a scale too small for our eyes to see. They were only discovered by us once we could harness the optical power of microscopes, though their influence on the world was far from microscopic.
The tree below represents life on Earth, broken down by phylum. Bacteria represent a large portion of the known phyla, while all animals, including humans, are represented by the single, tan branch labeled Animalia (Hug et al. 2016). We are predominantly interested in two of these phyla in the context of sourdough: Firmicutes, which encompasses Lactic Acid Bacteria (LAB), and Fungi, which contain yeast species.
Microbial Ecology of Sourdough Starters
Increasingly, sourdough is being used by scientists as a model ecosystem to study microbial communities and their interactions. The ability to control and easily sample the sourdough environment, coupled with the relatively low diversity of microbial species present, makes sourdough an ideal place to study the microbiome. While we won’t do a deep dive into what has been discovered about microbial interaction and biochemistry until the next article, we can touch on some of the forces that influence sourdough microbial community structure.
When creating a starter, the growth of bacteria and fungi is initiated by the hydration of the flour. The initial microbial community is composed of a variety of both wanted and unwanted microbes, including molds, yeasts, and both fermentative and non-fermentative bacteria. A subset of fermentative bacteria, primarily LAB, convert carbohydrates from the flour into acids, and by doing so, slowly lower the pH of the starter over successive feedings.
The acidification of the environment acts to select acid-tolerant microbes while excluding undesirable bacteria and molds, such as Aspergillus, a genus responsible for bread spoilage. Acid-tolerant yeasts increase in number as the sourdough matures, thereby increasing the leavening capacity of the community. In this way, the interplay between the LAB and yeast generates a flavorful and functional leaven for bakers (Calvert et al. 2021).
Species Breakdown
DNA sequencing and microbial isolation have provided a global view of the microbes that inhabit a typical starter. The below bar chart illustrates the microbial diversity of 500 starters from the US and Europe at the species level using DNA sequencing. The top panel displays all detected species of bacteria and the bottom panel displays the yeast.
In studying the 500 starters, Landis et al. concluded that most starters exhibit one dominant species of yeast and 1-3 species of bacteria. Generally, yeast were found to be less diverse than their bacterial counterparts. The yeast Saccharomyces cerevisiae was dominant in 77% of samples studied, with species from the genus Kazachstania as the second most common. The most common bacteria are Lactobacillus sanfranciscensis, followed by L.brevis and L. plantarum (Landis et al. 2021).
While species-level surveys are informative, they struggle to capture the total breadth of functional diversity in each sourdough. There are many differences between strains of the same species. An analogy is the species Canis familiaris, which includes all dog breeds from the Chihuahua to the Great Dane. Akin to dogs, different strains of the same species of microbe can have profound effects on fermentation and the properties of the final loaf. Similarly to how we breed our pets, microbes can also be propagated over time, and certain traits can be enhanced through domestication.
The most common species found in sourdough provide great examples that demonstrate the impact of strain-level variation. The yeast species Saccharomyces cerevisiae, when domesticated for baking, produces much more gas (CO2) than those domesticated for brewing (Bigey et al. 2021). Moreover, sourdough Saccharomyces cerevisiae strains are often better adapted to consume the different sugars found in flour and grow at a different rate compared to domesticated baker’s strains of Saccharomyces cerevisiae (Bigey et al. 2021). Bacteria also have impactful strain differences, for example, some Lactobacillus sanfranciscensis strains can make exopolysaccharides from sucrose which results in more flexible dough (Rogalski, Ehrmann, and Vogel 2021).
Where do these Microbes come from?
Not only are microbes everywhere, they are inside everything, too. Starting at the farm, the soil, insects, and wheat kernels harbor an array of microbes including species of yeast and LAB found in mature starters. These microbes are transferred with the wheat, pass through the mill due to their microscopic size, and are retained in the resulting flour.
The next stop is the bakery or home kitchen, where the milled flour and the associated microbes are used to make sourdough. As a starter is mixed, additional microbes are seeded from both the person making the starter and from the local environment, also known as the “house” microbiota. Work by Reese et al. suggests that the origin of microbes breakdown as follows: 60% from the flour, 26% from the hands or skin of the baker, and the remaining 14% is from unknown sources, likely the air, water, or equipment (Reese et al. 2020).
Following the series put on by Noel Brohner of Slow Rise Pizza Co., the next few of Leaven’s Newsletters will cover Founder Cameron Martino’s lectures on the Science of Sourdough!
In our next piece, we will cover microbial biochemistry and discuss how microbes interact with each other, their environment, and even the baker, so stay tuned!
References:
Landis, Elizabeth A., Angela M. Oliverio, Erin A. McKenney, Lauren M. Nichols, Nicole Kfoury, Megan Biango-Daniels, Leonora K. Shell, et al. 2021. “The Diversity and Function of Sourdough Starter Microbiomes.” eLife 10 (January). https://doi.org/10.7554/eLife.61644.
Reese, Aspen T., Anne A. Madden, Marie Joossens, Guylaine Lacaze, and Robert R. Dunn. 2020. “Influences of Ingredients and Bakers on the Bacteria and Fungi in Sourdough Starters and Bread.” mSphere 5 (1). https://doi.org/10.1128/mSphere.00950-19.