Zeitgeist: Rising in the yeast

For most of human history, sourdough was the dominant form of leavened bread. It wasn’t until about 150 years ago that commercial baker’s yeast, Saccharomyces cerevisiae, was adopted as the most common yeast used to bake bread (as well as to brew beer and make wine). Prior to that, for many thousands of years, bread was ­rudimentary. It mostly existed as flatbread-like forms. The discovery of 36,000-year-old fragments of grinding stones, at the Cuddie Springs archaeological site in western New South Wales, suggests that Aboriginal people were the first bakers. In north­eastern Jordan, charred food remains made from grain 14,400 years ago suggest the early origins of cooked bread-like products. 

However, it wasn’t until the Neolithic Age, approximately 12,000 years ago, that the development of agriculture and grain harvesting allowed humans to find solace in the warm, nourishing embrace of carbohydrates. Neolithic sites with bread remains have been found throughout Europe and southwest Asia. 

Evidence of bread production is written on the walls of Ancient Egyptian tombs and temples, showing the bread-baking process in detail. Ancient baking tools such as ovens, grinding stones and pottery have been found containing traces of yeast. The Ancient Egyptians are credited with the development of the first sourdoughs, which is likely to have been purely accidental; a flour-water mixture may have been left out for too long and, just by happenstance, wild yeast found its way into the dough. 

It’s the motherlode

This accidental process has not really changed apart from now becoming accidentally-on-purpose. Sourdough starters today are created by leaving a combination of flour and water to interact with its environment. This rich microbial community varies in every single sourdough starter; the geography, the microclimates, types of grains, and even the individual baker’s hands all play a part, making every starter unique.

The importance of sourdough is so esteemed that an entire library dedicated to preserving starters exists in Belgium. Karl De Smedt, the world’s first and only sourdough librarian, has a diploma in bakery. He joined Belgian bakery supplier Puratos back in 1994 as a test baker, then became a technical advisor and product trainer, and today is in charge of the Center for Bread Flavor. 

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Sourdough librarian Karl De Smedt displays some of the library’s physical collection of 125 starters. Every new arrival is analysed; so far more than 1400 strains of wild yeast and lactic bacteria have been recorded. Credit: Puratos Center for Bread Flavor

The library was opened in 2013 by Puratos, who have long been doing research and development into bread – part of a network including the Italian universities of Bari and Bolzano and the Microbe Institute. “Once in the library we have the chance to investigate them. They are part of scientific studies or projects to better understand how they work, what they do, how to maintain them and so much more,” says De Smedt. 

De Smedt has 125 starters from around the world housed in the library’s refrigerator. These samples are numbered (not named, as some sourdough enthusiasts do), and refreshed every two months with the original flour from which it was made, to replicate the conditions of the original bakery. A sample from each starter is sent to a lab to analyse and document its microorganisms. According to the Puratos website, more than 1,400 strains of wild yeast and lactic bacteria have been isolated and recorded so far from the project. These microorganisms are stored at -80°C to preserve the biodiversity of the starters for the future. 

In addition to the physical library, the Sourdough Library has an online database, which has 2,279 starters registered to date. Many of these have been given names and had their qualities described. “For a starter to become part of the Sourdough Library it must be made from a spontaneous fermentation and not prepared with a commercial starter culture,” explains De Smedt. “We look at its origins, the type of cereal or grain that is used, plus the history behind it.”

Some of the more unconventional starters have included one from Mexico which is fed beer, eggs and lime. There’s a Japanese variety made with cooked sake rice. But the most bizarre starter, says De Smedt, is number 108 from Verona, Italy. It contains cow dung. With such a unique preservation project, he has applied to UNESCO for the library to be recognised as a World Heritage listing under criterion 6 (an association with living traditions) and criterion 10 (containing the most important and significant natural habitats for in-situ conservation of biological diversity).

“Although some starters look the same, they smell and taste different,” he says. “Each starter is unique; no two have the same composition. They all have their unique consortium of yeasts and lactic acid bacteria that influences the flavour.”

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Karl De Smedt. Credit: Puratos Center for Bread Flavor

The knead to know more

The beauty of sourdough is that, once established, healthy and thriving, a starter has the potential to evolve. As long as you keep feeding it, starters can keep on living. Some have a long history in kitchens, with a tradition of up to 200 years old, passing through many environments, and many hands. But, there’s no way to date a sourdough starter. 

“From a microbiological point of view, they are only as old as the last time they were refreshed with new flour and water,” explains Kate Howell, Associate Professor of Food Chemistry and Director of Research Training at the University of Melbourne. 

There are many reasons people are interested in studying sourdough starters. Compared to the baker’s yeast S. cerevisiae, the types of yeast found in sourdough, generally from the Kazachstania genera, are not well described. According to Howell, by isolating the DNA from an older yeast in a dried sample of an old starter, you could compare the genome to yeast that exists nowadays. Genes are likely to have been enriched or changed over time. 

De Smedt posits there could be ­greater biodiversity in older starters, which could be linked to many factors such as fewer pesticides in use when they were created, more diverse grains, and less monoculture of microorganisms. It’s just one of the research questions he hopes to answer. 

Other studies, such as one involving Howell, demonstrate that sourdough breads benefit from mixed microbial communities. Not only can microbial diversity improve the taste and texture of the bread – it may also change the loaf’s health properties.

Kelly Wong is The Royal Institution of Australia’s social media manager. She has a background in biomedical science. This is an excerpt from her feature article in the latest edition of Cosmos magazine. You can subscribe to Cosmos here.

The Royal Institution of Australia has an Education resource based on this article. You can access it here.

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