Breaking down the microbiology world one bite at a time
Sourdough bread and its aromatic microbes.
Bread is an important part of many cultures, and who doesn’t like a warm baguette fresh from the bakery? Sourdough bread is a fermented food made from a microbial community of yeasts and bacteria. Its main members are yeasts, lactic acid bacteria and acetic acid bacteria. This community will create organic acids (such as lactic acid or acetic acid) which will give sourdough bread its distinguishable taste as well as CO2 that will make the bread rise.
To make sourdough bread, we need a starter, basically a stock of this microbial community that will be fed and maintained to make bread (think of it as a pet you need to feed every other day or less if it stays in the fridge). This starter is easy to make: you only need flour and water. It can also be acquired from commercial sources or someone you know who makes sourdough bread.
Most studies on sourdough bread have been done in Europe. In their study, E. Landis and colleagues collected 500 sourdough starters from across the world. Most samples came from the US, but also Europe, Canada, Australia, New Zealand and Thailand. To study their composition they sequenced the samples to identify their members. They got all the DNA present in the sample to read the DNA sequence and assign each DNA to each organisms. In all the samples they found a total of 7 different bacteria and 35 yeasts.
The authors found that most communities were dominated by a single yeast and/or bacteria. For example Saccharomyces cerevisiae (learn more about S. cerevisiae at the end of the article) was found in more than 77% of the starters and represented more than half the community abundance, while Fructilactobacillus sanfranciscensis (previously L. sanfranciscensis) was the dominant bacterium in most starters. The authors also found an underreported type of bacteria: acetic acid bacteria were found in almost 20% of the starters but at low abundance.
They then tried to understand where the differences in communities came from. They looked at their geographic differences, age, storage location (fridge vs room temperature), home characteristics etc. But out of the 33 parameters they looked at, only 10% explained the variation between community compositions.
However, they found that some taxa were enriched under specific parameters. For example, younger starters were often dominated by L. plantarum and L. brevis, while older starters often contained F. sanfranciscensis. The figure below shows the relation between the parameter and the organism: the further away from the line the more impact the parameter has on the presence of the yeast/bacteria.
Finally, the authors were interested in the aroma profiles and how the microbial composition could influence it. They selected 40 starters and measured their volatile organic compounds (VOC) – their chemical signatures. They found 14 dominant sensory notes such as yeasty, vinegar/acetic acid/acetic sour, green apple, fermented sour, and ethyl acetate/solventy. The main difference between the communities was change in abundance in the acetic acid bacteria. Although in very low abundance in the community, they seem to influence the variety of aromas found between samples.
Underrepresented groups in this study showed that they had a major impact in the aroma profiles of the different starters. Sometimes the little guys have the strongest powers.
Learn more about Saccharomyces cerevisiae :
From the authors on elife: Listen to Elizabeth Landis and Angela Oliverio reveal insights into the sourdough microbiome.
Podcast from Microbigals on S. cerevisiae: here
Link to the original post: Landis, E. A. et al. The diversity and function of sourdough starter microbiomes. Elife 10, 1–24 (2021).