It gurgles. It bubbles. It's alive!

One of the side effects of lock down during the COVID-19 pandemic is people have more time on their hands and have been engaging in bread making at home. Specifically, sourdough bread and sourdough starters. Even Stephen Colbert did a segment in which he tried to grow a starter for sourdough. Indeed, if the Internet is to be believed, sourdough is on the rise!

A group of scientists, led by Rob Dunn of NC State and Anne Madden of The Microbe Institute, have been investigating sourdough as one of their projects to understand the microbes in our lives. In the case of sourdough starter, they asked questions about the hands of bakers and why do loaves have unique properties.

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In theory, growing a sourdough starter is not particularly complex. Place two tablespoons of flour (any type) and two tablespoons of water in a jar, cover with paper towel held in place with an elastic so insects can't get in, and leave it some place warm. After 24 hours, the starter needs to be fed, so remove a tablespoon of the mix and add a one and one-eighth of a tablespoon of new flour along with a tablespoon of water. (Discarding a tablespoon limits the volume of the starter.)

After a few feedings, sometimes a thin layer of liquid will develop on the surface. It is called "hooch" and indicates the starter needs to be fed more frequently. But generally speaking, a daily dose of flour is sufficient to keep the colony alive.

The starter is used to make bread in the traditional sense. Mixed in with flour, salt, and water, it will eventually result in the makings of a sourdough loaf. The microbes growing in the starter flourish in the dough and produce carbon dioxide that gets trapped within the structure. The gas provides the air pockets which make for a nice light and fluffy loaf. (The gluten in the flour provides the elastic network which catches the gases.)

While store-bought bread tends to be made with a single species of baker's yeast, traditional leavened bread is a result of a species of Lactobacillus, the same bacteria found in yogurt. But Dunn and colleagues have identified in total more than 60 different bacterial species and a half dozen strains of yeast in starters from various locations. It is the acids generated by the Lactobacillus which make sourdough taste a little sour but different starters tend to generate their own taste profile.

To examine why starters vary and what influence these variations had on the final product, Dunn and coworkers engaged in a series of experiments. In the first part, they asked 15 bakers from 15 countries to make the same starter, using the same flour and water. In theory, the only variable in this experiment was the bakers themselves and the air where they lived.

The bakers met at the Puratos Centre for Bread Flavour in Saint Vith, Belgium. Both the starters and the baker's hands were swabbed with the resulting organisms being cultured and identified. Analysis of the DNA found in the starter samples by Noah Fierer at the University of Colorado revealed several hundred species of yeast along with several hundred species of Lactobacillus and related bacteria. Different microbes, though, were found in the starters from different regions. One fungus, for example, was found to be exclusive to Australia. The diversity of organisms was an expected result.

What was an unexpected result was the cultured samples from the hands of the baker's. The surface of our bodies is covered in a sheath of microbes. We might think washing our skin with soapy water would remove them but it doesn't. For the most part, these microbes are either helpful or benign.

The most common microbes found on hands in the general public tend to be Staphyllococcus, Corynebacterium, and Propionibacterium, with only traces of Lactobacillus (two per cent for men; six per cent for women).

But the bakers' hands were totally different. On average, 25 per cent of the microbes on their skin was Lactobacillus, although for one individual it was 80 per cent. Similarly, nearly all of the fungi on their hands were yeasts such Saccharomyces found in the sourdough starters. It seems being a baker involves having your hands in dough so much, it changes the microbial colonization of the skin.

Further, for the most part, the bacteria were found in the flour used to make the starters. The result were loaves with distinct flavour and texture. Some were creamy while others were more sour even though each starter was generated from the same starting materials.

What is clear from the science is what folklore tells us - each sourdough starter is, to some extent, unique and dependent upon who is handling it. The other thing the scientists learned is sourdough bread tastes better with a little beer to wash it down.

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