The taste of beer is thanks to flies

When yeast grows it consumes sugars and turns them into alcohol, carbon dioxide and a range of what are called aromatic compounds. These chemicals are identical to those given off by ripening fruit, and are key to giving beer its distinctive flavour. But why would yeast expend precious energy making these flavours? To find out, researchers in Belgium have been genetically modifying yeasts, turning on and off the genes responsible for making these compounds, to investigate what they do. And just as penicillin - another fungus - was discovered thanks to a lucky lab accident 76 years ago, a similar mishap set University of Leuven scientist Kevin Verstrepen on a similar trail of discovery, as he explained to Georgia Mills...

Kevin -  I left the lab on a Friday night and I was a bit lazy, I didn't clean-up.  When I came back on Monday, there was an extraordinary sight.  I had a few flasks with different yeasts on my bench.  One yeast was producing way more of this aroma compounds because we genetically modified it so that we over activate the aroma gene.  One didn't produce any aroma compounds at all and the other one was a normal yeast.  There's a fly genetics lab right next door and somehow, some of these flies had made their way into my flasks and they were quite picky.  50 flies made their way in the high aroma flask and only two of them made their way in the normal yeast and none of them chose the flask that contained this mutant that doesn't produce any aromas.  So, that of course got me thinking.  Maybe the yeast makes these aromas to attract flies.  But that's a nice hypothesis, but it's not so easy to really scientifically prove it.  And it's taken quite a few years more specifically until I met two neurobiologists here working at the University of Leuven.  So, they had all the expertise to look at this problem from the fly side.  So, we worked together to setup experiments where we could really study how flies respond to a different yeast and their aromas.  In a nutshell, what comes out of it is that, clearly, the yeast's aromas are attracting flies and the flies then, when they're attracted to the yeast, they will eat some of this yeast but they will also carry a few of the yeast cells to different places.  They will fly around and yeast cells sort of hitch a ride to the - they're kind of using the flies as a taxi.

Georgia -  Are they tricking the flies or is there something to be gained for the flies as well?

Kevin -  They're luring the flies with aromas.  So in one way, you can say, "Well, that's very smart for yeast cells."  But of course, the flies also like yeast cells because they eat them as a protein.  So, as fruit flies, we think usually eat fruit, but fruit doesn't necessarily contain enough protein for them especially when they need to lay eggs.  For the yeast cells, it's really important to go to other places.  I mean, they don't have any legs or wings so they can really move around.  So you have to imagine being a poor yeast cell sitting there on maybe a grape where you pretty much depleted all the sugars and there's now millions of yeast cells, but they don't have anywhere to go to, or they don't have any means to go there.  So, maybe you want to sacrifice a few of your brothers and sisters just to go to a new grape where you can start eating again and growing it.

Georgia -  These aromas that the yeast is making are ideally, you said the flies will come and transport them around so they can continue to grow and survive.  And then humans come along and decide we rather like them too and start drinking lots of beer.  Are we sort of ruining this process for the yeast?

Kevin -  Well, not really because beer is not really a natural product.  I don't think we're disturbing the ecosystem too much.  But it's true that we really love these flavours and I know that most consumers won't necessarily think of bananas when they're drinking their beer.  But trust me when you would take away these flavour compounds, these yeast derived flavour compounds from beer, from wine, you would end up with a very bland and not a very tasteful product, one that frankly, you probably wouldn't want to drink unless you're a desperate PhD student perhaps.  They drink anything.  I've seen it in my lab.  So, it is true that we like these flavours and it's not a coincidence because these flavour compounds are literally chemically identical to the flavour of compounds that fruits produce when they're ripening.  So, it's kind of maybe logical that we as humans are also attracted to them because they're also nice food for us.

Georgia -  Now, you found these genes and you can turn them off and turn them on.  Are there any practical applications for what you've discovered here?

Kevin -  With those, we can already make quite good beer, but we can make it even better.  One of the things we're doing is we're breeding yeasts now to make even more flavours because in the process of this research, we effectively slaughtered quite a number of flies and we've isolate the microbes that are growing on them.  So, there's lots of different yeasts on there.  All the microbes you find on these flies produce aromas.  Now, some of them produce way more even than your typical wine yeast.  So, one of the things we're doing now and have been doing is taking some of these and breeding them and seeing if they're useful for beer making.  And we've also been taking normal beer yeast and literally crossing it with some of this more exotic yeasts to get better yeasts that make more aromas.  It's working out quite well.  We've actually sold some of these yeast to breweries and they're making really nice beer with them.

Kat -  That's Georgia Mills talking with Kevin Vestrepen who's presumably going out for beers with his desperate PhD students to celebrate their publication in Cell Reports.

Chris -  Kat, got a tweet here from Chris Coney who's tweeted @nakedscientists, "Why does fungus grow so close together?"  What's the answer?

Kat -  I don't know.

Chris -  Because they don't need mush-room.