Human Domesticated Yeast by Bread Baking: Study


Ost is usually not considered a domesticated organism. Dogs, cats, and cows may come to mind first, but humans nonetheless played an important role in the development of the fungus. A study in Current biology published yesterday (December 9th) reports that humans caused most of the bread loaf strains, Saccharomyces cerevisiaeto fall into two distinct groups: one for large-scale industrial bread making and one for artisanal sourdough bread. While industrial strains begin to ferment faster, sourdough yeasts have more copies of the genes responsible for metabolizing maltose. This process usually takes place later in the fermentation process and helps the dough rise.

“It’s a good paper,” says Aimée Dudley, a senior investigator at the Pacific Northwest Research Institute who studies yeast genetics and was not involved in the study. “It makes sense what we know about how people have baked bread in the past,” and points out the priority of speed in commercial breadmaking over artisanal bakers.

Bread is universal and has been around for thousands of years, explains Sicard. With a dramatic shift towards mass-produced yeast starters in the industrial age, her team wanted to understand how these changes affected yeast evolution. The domestication of yeast has been studied in other foods and beverages – such as cheese, wine, and beer – but not in bread. Delphine Sicard, co-author of the study by the French National Research Institute for Agriculture, says bread fats have rarely been studied because many of these strains of yeast are tetraploids, cells that contain four homologous sets of chromosomes, which makes studying their population genetics more complicated than studying diploids or haploids.

Historians and scientists have long theorized that brewers and bakers swapped their yeasts in the past, and the results of the study give the idea some weight.

The Sicard team received 198 sourdough strains from France, Belgium and Italy, as well as 31 commercial strains from starter kits or international yeast collections. They analyzed the number of chromosome sets in each of these strains using flow cytometry and examined the genetic diversity of diploids and tetraploids using microsatellite marker analysis. Sicard and her colleagues then analyzed the newly sequenced genomes of 17 baker’s yeast strains and 1,011 others S. cerevisiae Tribes described in a 2018 nature Study involving 51 baking trunks to create a phylogenetic tree and map variations in copy number along this map.

They found that sourdough strains typically contained higher copy numbers of genes that encode proteins that transport and regulate maltase and isomaltase, as well as the enzymes themselves that break down the sugars maltose and isomaltose. Copy number irregularities can be detrimental to cells, but the authors explain that they can also allow rapid adaptation in a period of rigorous selection, for example in the toxic, high-maltose and isomaltose environment of wheat flour, which is used to make sourdough bread becomes.

The researchers found that in a synthetic sourdough environment with only maltose as the carbon source, yeast with more copies of these genes at the end of fermentation was more common than yeast with fewer copies. Whether bakers purposely passed on sourdough starters that they believed worked best, or whether the yeast adapted to their environment, the bottom line is that sourdough yeast is well suited for making specialty sourdough bread, explains Sicard. “This is a fine example of a domestication genetic signature,” says Sicard.

Meanwhile, industrial bakers seem to have selected yeasts that begin fermentation faster – they produce 1 g of carbon dioxide after inoculation about half an hour earlier than sourdough yeasts on average in the study – and prioritize the breakdown of glucose over more complex sugars such as maltose and isomaltose which would take more time to dismantle. Both industrial and sourdough strains started fermenting faster than any other yeast analyzed, including beer and wine strains.

Dudley points out that sourdough starters contain other microbes like lactic acid bacteria and other yeasts that form complex interactions with the S. cerevisiaeShe hopes for future work that examines these relationships to understand how they can contribute to domestication of S. cerevisiae.

With this renewed interest in sourdough bread, we may be able to preserve more microbial diversity in the future, at least in this food chain.

—Delphine Sicard, French National Institute for Agriculture, Food and Environment

Historians and scientists have long theorized that brewers and bakers swapped their yeasts in the past, and the study’s results add some weight to the idea: a group of yeast strains that researchers found in the nature The study related to African beer includes sourdough strains isolated from Ghanaian corn dough, suggesting that the same strains were historically used for both fermenting corn dough and brewing beer. Sicard and her colleagues also report that another group is in the nature The study includes beers, commercial bakery strains from around the world, and some of the newly sequenced sourdoughs from Belgium and France. In the meantime, all but three bakery tribes grouped themselves separately from wine and sake lines, which suggests different histories of development.

Yeast can be found almost everywhere, making categorizing yeast strains a daunting task, says Edward Louis, director of the Center for Genetic Architecture of Complex Traits at the University of Leicester, who was not involved in the latest study. Only about two-thirds of the yeast strains analyzed, which the Sicard team analyzed, could be divided into the industrial and artisanal sourdough clades – and even among these there were many “mosaic strains” that contained characteristics of both.

“Is there really domestication, or are we just using the natural variations that exist, or are we moving yeast to create new varieties that just suit whatever environment we throw at them?” Asks Louis. Still, he says, the finding about the rapid rate of fermentation among industrial strains is compelling evidence as these properties would most likely not bring yeast any benefit on oak bark or soil wherever S. cerevisiae is found in the wild, for example.

Sicard fears that industrial breadmaking could reduce the diversity of yeast strains, but that it is being encouraged by the resurgence of home-made bread-making over the past decade and particularly during the COVID-19 pandemic. “With this renewed interest in sourdough bread, we may be able to preserve more microbial diversity in the future, at least in this food chain,” she says. “It is difficult to make bread, and if we as scientists can add value to the work of the bakers, that would be great.”

According to Louis, this study will help capture the genetic diversity of bread yeasts, which is an important step in maintaining these yeasts. “If we go the commercial route and replace everything with a brand or two, we still have good bread and beer, but we probably lose a lot of the nice things about it,” he says.

F. Bigey et al., “Indications of Two Main Routes of Domestication in Saccharomyces cerevisiae connected with different bread-making processes ” Current biology, doi: 10.1016 / j.cub.2020.11.016, 2020.

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