by Sourdough is the oldest type of sourdough bread in recorded history, and people have been eating it for thousands of years. The components of creating a sourdough starter are very simple – flour and water. When they are mixed, a live culture is created in which yeast and bacteria ferment the sugars in flour, making byproducts that give dough its characteristic taste and smell. They are also what cause their rise in the absence of other agents for leavening.
My sourdough starter, affectionately known as the “Fosters” starter, was given to me by my grandparents, who got it from my grandmother’s college roommate. It followed me throughout my academic career across the country, from undergrad in New Mexico to graduate school in Pennsylvania to postdoctoral work in Washington.
Currently, he lives in the Midwest, where I work at The Ohio State University as a senior research associate, collaborating with researchers to characterize samples in a wide variety of fields from food science to materials science.
As part of one of the microscopy courses I teach at the university, I decided to take a closer look at the microbial community in my family’s sourdough starter with the microscope I use in my day-to-day research.
Scanning electron microscope
Scanning electron microscopy, or SEM, is a powerful tool that can image the surface of samples at the nanometer scale. For comparison, a human hair is between 10 and 150 micrometers, and SEM can observe features 10,000 times smaller.
Since SEM uses electrons instead of light for imaging, there are limits to what can be imaged in the microscope. Samples must be electrically conductive and able to withstand the very low pressures in a vacuum. Low pressure environments are usually unfavorable for microbes, because these conditions will cause the water in the cells to evaporate, deforming their structure.
To prepare samples for SEM analysis, researchers use a method called critical point drying that carefully dries the sample to reduce unwanted artifacts and preserve fine details. The sample is then coated with a thin layer of iridium metal to conduct it.
Explore sourdough starter
Since sourdough starter is created from wild yeast and bacteria in the flour, it creates a favorable environment for many types of microbes to thrive. A sourdough starter can contain more than 20 different species of yeast and 50 different species of bacteria. The strongest being the dominant species.
You can visualize the microbial complexity of the sourdough starter by imaging the various components that vary in size and morphology, including yeast and bacteria. However, a complete gene sequence would be required to fully understand all the variation present in the boot.
The main component that gives the starter texture are starch grains from the flour. These grains, which are colored green in the image, are recognizable as relatively large spherical structures around 8 micrometers in diameter.
The yeast, colored red is the cause of the boot. As the yeast grows, it ferments sugars from the starch grains and releases bubbles of carbon dioxide and alcohol as byproducts that cause the dough to rise. Yeast usually fall in the range of 2 to 10 micrometers in size and are round to elongated in shape. Two distinct types of yeast can be seen in this image, one that is almost round, at the bottom left, and another that is elongated, at the top right.
Bacteria, colored blue, metabolize sugars and release by-products such as lactic acid and acetic acid. These by-products act as a preservative and are what give the starter its characteristic pungent smell and taste. In this image, bacteria have pill-like shapes that are about 2 micrometers in size.
Now, the next time you eat sourdough bread or sourdough waffles – try them, they are very tasty! – you can imagine the rich array of microorganisms that give each piece its distinctive taste.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Daniel Veghte, The Ohio State University
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Daniel Veghte does not work for any company or organization that would benefit from this article, does not consult with, shares in or is funded by any company or organization that would benefit from this article, and has not disclosed any relevant affiliations beyond their academic appointment.
