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Thursday, 06 September 2018 13:28

Using yeast to generate new chocolate and beer flavors with Kevin Verstrepen

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You may know that beer is fermented, but did you know making chocolate requires a fermentation step? Kevin Verstrepen discusses how his lab optimizes flavor profiles of the yeast used in this fermentation step, and explains how yeast was domesticated before microorganisms had been discovered.

Host: Julie Wolf Kevin Verstrepen

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Julie’s Biggest Takeaways:

Microbes are used to ferment foods, but they do more than just add ethanol or carbon dioxide: their metabolic byproducts add flavors and aromas that are an essential part of the fermented food.

In cocoa bean fermentation, the yeast that are part of the initial fermentative microbial population control the development of the subsequent microbial populations and the quality of the final product.

How the volatile flavor compounds generated during fermentation survive the roasting step remains unclear. Heat can destroy these labile compounds, but Kevin thinks the compounds were able to survive roasting because they become embedded in lipids (fat) of the cocoa beans. Similar compounds produced during bread rising are destroyed during baking, possibly because there is less fat to protect these molecules.

Mixing data science and beer: a computer scientist in the Verstrepen lab analyzed the flavor profiles of several hundred beers, which were also analyzed by a trained tasting panel. The goal is to link the chemistry to the aroma, which requires complex algorithms due to the integration of hundreds of flavor molecules.

A spontaneous hybridization between Saccharomyces cerevisiae, the normal fermentative yeast, and S. eubayanus, a cold-tolerant yeast, resulting in a hybrid that can ferment at colder temperatures, as is required for brewing lager beers. There are 2 lineages that are used by most breweries, and while different characteristics have evolved over time, the genetic bottleneck limits characteristic diversity. The Verstrepen lab made several crosses between these two species and selected for hybrids that generated those with desirable characteristics. Molecular means can determine the offspring that are most likely to confer desired characteristics, but the commercial yeasts are not specifically genetically manipulated to this end.

Domesticated yeast have different characteristics than their wild counterparts. Domesticated yeasts have lost the ability to use certain sugars, but have gained abilities associated with their use; beer yeasts use maltose at much higher rates, for example. When the origins are traced using molecular methods, it goes back to medieval times. How did brewers domesticate an organism before it was even identified? Brewers have long transferred sediment from one batch of beer into new batches, which is how selection for human-desired characteristics began. Wine yeasts, which are not passaged but are likely inoculated from the same vineyard annually, show less domestication than the beer yeast.

 

Featured Quotes:

“The funny thing is, I didn’t know [chocolate] is fermented. I received a call from a very large chocolate producer who said, hey, we’re interested in working with you, and my response was, “why? We’re a yeast lab - you have the wrong guys!” They explained that chocolate is fermented - or at least the beans are fermented."

“With brewing, at least nowadays, it’s more industrialized. It’s almost sterile; we of course teach our students that there’s no such thing as almost sterile, but it is almost sterile. The beer medium is boiled, the breweries are really clean, so if you want bacteria, you have to add them. Of course in medieval times when they first brewed beer, trust me that there were always acetic acid or lactic acid bacteria in there. But nowadays if you want them, you need to add them.”

“We know the key [molecular] players and how they smell in pure form. Isoamyl acetate, for example, is one of these compounds formed by fermenting yeast cells and it smells of banana. It’s exactly the same compound produced by ripening bananas. However, when you start mixing a few hundred of these, as are found in beer or wine, there are interactions in the flavors and what comes out is more than just simple banana.”

On whether his lab imbibes beer derived from genetically modified yeast: “With genetic engineering, you kind of know what you’re doing - or at least, you think you know what you’re doing - and then of course we test. It’s one of the major points in the debate of genetic modification: there’s no such thing as proving that everything is safe other than drinking it for 50 years and not seeing any difficulties. TheVerstrepen lab group absence of something is very difficult to prove. We do everything we can to assure ourselves that it is absolutely what we think it is and then we go for it.”

In terms of their domestication, “Wine yeasts are more like cats and beer yeasts are more like dogs.”

"It’s the thing I enjoy most about being a PI. I always tell them, you’re a bit like my children. It is kind of fun to see the people you helped educate - they mostly do it themselves, to be honest - to see them go out into the world and to do well. It’s the most fun part of our profession."

Links for This Episode:

Do you think you can address Kevin’s question about how negative selection occurred during yeast domestication? Let us know if you have a good hypothesis by tweeting at us @ASMicrobiology or leaving a comment on facebook.com/asmfan.

History of Microbiology tidbit:

Emil Christian Hansen was the first scientist to cultivate pure yeast cultures. Hansen was a Danish scientist born in 1842. He was hired by the Carlsberg Laboratory, a lab funded by the Carlsberg brewery in Copenhagen dedicated to the science of brewing. Hansen isolated and described the lager-brewing yeast Saccharomyces carlsbergensis, now known as Saccharomyces pastorianus, the lager-brewing yeast discussed on the show, in 1883, when he described the yeast he was able to isolate as “Unterhefe Nummer et”, or bottom fermenting yeast number 1. This became the industrial yeast known as Carlsberg yeast no. 1, even though Hansen himself recognized that the sample was identical to one donated to Carlsberg 40 years earlier by the Spaeten Brewery of Munich.

The suggestion that S. carlsbergensis was a separate species, or at least different lineage from Saccharomyces cerevisiae, came in a 1904 publication from Hansen, but the same species had been described three decades earlier in a German lab and named S. pastorianus. Once the isolates were recognized as the same species, the pastorianus nomenclature won out, a move I think is pretty all right, since now the species is named after a famous scientist who contributed to fermentation science, Louis Pasteur, instead of a brewery.

Many narratives exist online summarizing Hansen’s role in developing pure yeast cultures and how brewing industry processes were thus changed, and some of these stories highlight the acrimonious nature of scientific agreements. Hansen himself wrote a book, titled Practical Studies in Fermentation, which is available online as an English translation. He covers his historical findings, summarizing his famous isolation techniques and adding context to the problem that the beer industry had been facing - here’s an excerpt:

  1. Wherein the new advance consists

It will be known to many readers that in the year 1883 I succeeded in introducing pure cultures of systematically selected yeasts in the brewing industry. My first experiments were carried out in the famous Old Carlsberg Brewery at Copenhagen. When I commenced work in this direction, the yeast question was everywhere a perfect enigma; it was the weakest point in brewing. When difficulties occurred, a change of yeast was introduced from another brewery, and frequently the yeasts from several breweries were mixed. Sometimes a good result was obtained in this way, sometimes a bad one, and often the result was worse than that which induced the brewer to try a change of yeast.

Hansen goes on to discuss his methods for isolating yeast, citing well-known scientists like Lister and Koch, who were active around this same time.

 

 

Send your stories about our guests and/or your comments to jwolf@asmusa.org.

 

Last modified on Thursday, 06 September 2018 14:14
Julie Wolf

Julie Wolf is the ASM Science Communications Specialist. She contributes to the ASM social media and blog network and hosts the Meet the Microbiologist podcast. She also runs workshops at ASM conferences to help scientists improve their own communication skills. Follow Julie on Twitter for more ASM and microbiology highlights at @JulieMarieWolf.

Julie earned her Ph.D. from the University of Minnesota, focusing on medical mycology and infectious disease. Outside of her work at ASM, she maintains a strong commitment to scientific education and teaches molecular biology at the community biolab, Genspace. She lives in beautiful New York City.

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