Unveiling the Microbial World of Sourdough: A Look Under the Microscope

A researcher at Ohio State University studied the microbial community in the sourdough starter his family was cooking with. To do this, he used a scanning electron microscope, which allows him to photograph the surface of samples on a nanometer scale. About writes ScienceAlert.

In contrast, human hair is between 10 and 150 micrometers thick, and a scanning electron microscope (SEM) can see features 10,000 times smaller than that.

Because SEM uses electrons instead of light to produce images, there are limits to what can be imaged with a microscope. Samples must be electrically conductive. They must also withstand very low pressure in a vacuum. A low-pressure environment is usually unfavorable for microbes, because under these conditions the water in the cells evaporates, deforming the structure.

To prepare samples for SEM analysis, a technique known as critical point drying is used. In this process, the sample is dried to reduce unwanted artifacts and preserve details. It is then coated with a thin layer of iridium metal.

Sourdough is made from wild yeast and bacteria in flour. These ingredients create a favorable environment for many types of microbes to thrive. Sourdough can contain more than 20 different types of yeast and 50 different bacteria. The most stable become the strongest species.

You can visually observe the microbial complexity of a starter by visualizing the various components that differ in size and morphology, including yeast and bacteria. But for a more detailed analysis of the diversity present in sourdough, it is impossible to do without following the whole genome.

The main component that gives the sour texture is the starch grains of the flour. These particles, colored green in the image below, are identified as relatively large spherical structures with a diameter of about 8 micrometers.

Red yeast causes sourdough. As they grow, they ferment sugar from starch grains and produce carbon dioxide bubbles and alcohol as byproducts. They make the dough rise. Yeasts usually range in size from 2 to 10 micrometers. They have different shapes – from round to long. The photo shows two types of yeast: one that is almost round in the lower left corner and one that is elongated in the upper right corner.

The blue colored bacteria metabolize sugars and produce by-products such as lactic acid and acetic acid. These by-products act as preservatives and give the sourdough starter a sour smell and taste. In the image, the bacteria are like tablets in shape, about 2 micrometers in size.

Now when you eat sourdough bread, you will be able to glimpse the rich array of microorganisms that give each bite a unique taste.

Previous Electron Microscope caught “dance” of proteins and lipids.