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Why Beer Tastes Good to Us and to Flies

Beer yeasts produce chemicals that mimic the aroma of fruits in order to attract flies that can transport the yeast cells to new niches, report scientists from VIB, KU Leuven and NERF in the reputed journal Cell Reports. Interestingly, these volatile compounds are also essential for the flavor of beverages such as beer and wine.

by VIB (the Flanders Institute for Biotechnology)
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Yeast on the leg of a flyVIB, 2014Kevin Verstrepen (VIB/KU Leuven): “The importance of yeast in beer brewing has long been underestimated. But recent research shows that the choice of a particular yeast strain or variety explains differences in taste between different beers and wines. In fact, yeasts may even be responsible for much of the “terroir”, the connection between a particular growing area and wine flavor, which previously often was attributed to differences in the soil”.

Yeast is essential in production of bread, beer and wine
Humans have been using yeast for thousands of years to produce bread, beer and wine. The reason for this is simple – the microbes eat sugars and convert them into carbon dioxide gas and alcohol. In bread, the gas causes leavening of the dough while the alcohol evaporates during baking. In beer and sparkling wines, both the alcohol and carbon dioxide gas are retained; whereas in wine the gas is allowed to escape.

However, the role of yeast cells is more complex than the conversion of sugars into alcohol and carbon dioxide. Yeast cells also produce several aroma compounds that are key for the taste, flavor and overall quality of beer and wine. In fact, different yeasts are producing different amounts of these aroma compounds. Whereas the importance of yeast aroma production is now fully appreciated, the reason why yeast cells would make these special, volatile chemicals remains mysterious. Surely, they don’t do it to please humans…. 

Aroma compounds to attract flies
A new collaborative study led by scientists from VIB, KU Leuven and NERF shows that the fruity volatiles produced by yeast cells are highly appealing to fruit flies. This attraction allows some yeast cells to hitch a ride with the insects, who carry the otherwise immobile microbes to new food sources.  Moreover, deleting ATF1, the key yeast gene driving aroma synthesis, all but abolishes the attraction of flies to the mutants. Moreover, the brain activity in flies that are exposed to such aroma-mutants is very different from that in flies exposed to normal, fruity yeasts. 

“Flies are strongly attracted to normal yeast cells, when compared to mutant yeasts that don’t produce esters. Knowing that esters make beer taste good, it seems that the same flavors that allow us to enjoy our beer, probably evolved to attract flies and to help yeast disperse into broader ecosystems” says Emre Yaksi (NERF – VIB/KU Leuven), the neuroscientist who led the experiments on flies. Bassem Hassan (VIB/KU Leuven): “This ground-breaking study was only possible because we were able to combine the know-how of different research groups, with expertises ranging from yeast genetics to animal behavior and neurobiology.  It was quite fun to learn from each other”.

Far-reaching implications
The team believes that their findings have far-reaching implications. “We all know that flowers attract insects by producing aromas. But there’s also a lot of microbes living inside flowers, and the chemicals they produce may also play an important role” says Joaquin Christiaens (VIB/KU Leuven), who performed the experiments with yeast cells. Luis Franco (NERF - VIB/KU Leuven), who performed the fly assays, agrees “There’s a lot to be learnt about the mutualism between insects and microbes, and some of what we find may have implications in agriculture and medicine. Don’t forget that insects also carry disease-causing microbes…”