Sourdough, an incubator for microbial symbiosis

“Blues is to jazz what yeast is to bread. Without it, it’s flat.” – Carmen McRae, jazz musician

History of sourdough

Sourdough bread and other fermented foods have been around for centuries. The oldest leavened bread was excavated in Switzerland, dating from 3500 BCE. However, the oldest evidence of leavening was recorded by the Egyptians possibly when flatbread dough was left out and colonized by wild yeasts and bacteria. Throughout most of human history, sourdough was the source of leavening and the use of baker’s yeast (Saccharomyces cerevisiae) did not occur until the 19th century. For more detail about this history of sourdough, visit The Sourdough School.

The makings of sourdough

Sourdough bread is created from a dense concoction called a sourdough starter, a mixture of flour and water kept at room temperature. Flour typically contains a variety of yeasts and bacterial spores which become active with the addition of water. Additional bacteria and wild yeasts from the air colonize the starter and begins fermentation, a metabolic process that converts sugars into acids, gasses and alcohols. The starter is fed with additional flour and water daily to keep the microbes happy. As the sourdough starter ages, the unique flavor of sourdough bread is enhanced and a portion of the starter can be added to the bread making process to create a delicious loaf of sourdough bread or other goodies. Starters can be maintained for years and are even passed down along generations. Maintaining a sourdough starter may seem intimidating at first but this guide provides day-by-day directions on starting your own sourdough starter.

Photo credit: Maurizio Leo of
Photo credit: Maurizio Leo of The Perfect Loaf.

Yeast and lactic acid bacteria symbiosis

The dominant species in mature sourdough starters are yeasts and lactic acid bacteria (LAB) but it did not begin that way. The microbial community in the initial sourdough starter represents the microbial composition the flour, consisting of lactic acid bacteria, aerobic bacteria (ex: Bacillus sp. and Pseudomonas sp.), Enterobacteriaceae, yeasts and molds. To begin the fermentation process, amylases (enzymes found in the yeast cells) break down starch into sugars, some of which the yeast metabolizes. Amylase also breaks down starch into maltose which the yeast cannot metabolize. The bacteria ferments the maltose and the byproducts of maltose fermentation are subsequently metabolized by the yeast, producing carbon dioxide to leaven the dough. As the carbohydrates in the starter are metabolized and lactic and acetic acid build up, the pH of the starter decreases. This decrease in pH is well tolerated by yeasts but some bacteria species (for example, the Enterobacteriaceae) are killed. Additionally, LAB metabolism results in the production of antifungal hydroxy fatty acids, preventing mold growth. Therefore, as fermentation progresses, LAB and yeasts dominate the mature sourdough.

The dominant LAB in sourdough are in the genus Lactobacillus and the dominant yeast species are Saccharomyces cerevisiae (commercialized as baker’s yeast), Kazachstania exigua and Candida hulilis. However, the exact microbial community in a sourdough starter can be influenced by a variety of factors ranging from the flour used, microbes in the air, temperature, etc. De Vuyst and coworkers reviewed the sourdough microbiota from sourdough samples from many countries and noted the diversity in sourdough microbial communities amongst all samples.

Next time you munch on some tangy sourdough bread, you can thank the microbes hard at work creating that defining flavor that is sourdough. Has anyone kept a sourdough starter? Have you noticed any difference in your sourdough starter if you have lived in different places?

CC BY-NC-ND 4.0 Sourdough, an incubator for microbial symbiosis by Jennifer Tsang is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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