Adapted from 1

Brewing beer is weird once you think about it. You feed yeast a bunch grains to make ethanol (a poison that makes us feel good), then throw in hops and anything else you can think of to make it palatable enough to drink. There should be an easier way make it, but thousands of craft breweries throughout the world have turned brewing into a complex art. Paradoxically, beer causes bad things (drunk driving, addiction, etc), but is so ingrained in society that many cultures celebrate and cherish it. Heck, even I’ll admit that beer tasted terrible when I first tried it, and I love the stuff now.

This contradictory nature of beer, that it is both good and bad on so many levels, was at the forefront of my mind when I read an article about genetically engineering yeast to replace hops. This paper is cool science, but trying to get rid of one of the four essential ingredients of beer seems fundamentally wrong. It’s also in an important journal (Nature communications!) and has major economic implications about brewing, but the work is all ultimately designed to make it easier to get us drunk. I don’t know if any of these contradictions are worth philosophizing over, but I do know that this is an excellent opportunity to discuss a paper that manages to be dry and humorless while discussing the taste profile of a recreational drug.

Brewing with Hops

Beer is traditionally made of four main ingredients: water, grains, yeast, and hops 2. The first three ingredients work in tandem to create alcohol. Water acts as a medium in which yeast consume grains like barley to produce ethanol. Hops, which are flowers from the plant Humulus lupulus 4, stand out from the other ingredients in that they aren’t needed for this critical process. Instead hops serve two distinct functions. They impart a unique flavor that makes beer much more palatable and they preserve the beer against bacterial contamination.

Hops are seen as this kind of wonder-flower by brewers (5). The antimicrobial activity is very effective, and the bitter hoppy flavor is highly sought after by beer aficionados. No known plant serves these two functions quite as well (personal communications with Aeronaut brewing) and Brewers often delight in experimenting with different hop strains or different methods of extracting hop organics into their beer to improve a beer’s flavor. Hops are arguably the most celebrated ingredient in beer and the hop flower is probably the most common image associated with brewing

The only real downside to hops for brewers is that they’re difficult to produce. It takes a lot of water to grow hops and is usually the most expensive ingredient in a beer 6. Its also a challenge to keep hops consistent from one crop to the next as they can vary in essential oil content depending on growing and storage conditions.

The scientists behind the paper “Industrial brewing yeast engineered for the production of primary flavor determinants in hopped beer” wanted to address these hop downsides by circumventing them entirely 7. Instead of relying on hops to impart that essential hoppy flavor, they wanted to make some yeast strains that could do the same thing.

These yeast wouldn’t replace the antimicrobial activity of hops, but if they could replicate the flavor then brewers might be able to reduce or replace the most costly ingredient in making beer.

Hopping hoppy genes into yeast

In order to get yeast to produce hoppy flavors, the authors first had to identify the components of hops that impart their unique flavor to beer. This is a rather complicated question that still hasn’t been completely answered, but past research agrees that two of the major components to hop flavors are the monoterpene molecules linalool and geraniol 8. Now hops add a complex bouquet of other flavors to beer, but the researchers hoped that if they could nail these two ingredients, they might be able to approximate the flavor of hops.

To get yeast to synthesize linalool and geraniol, researchers first needed to insert monoterpene synthases that would produce these two compounds from the yeast metabolite geranyl pyrophosphate. Multiple linalool and geraniol synthases have been identified in plants, but few are natively active in yeast. The authors found that they needed to sometimes truncate a portion of the gene responsible for protein transportation in plants to get the proteins to have some activity in yeast. They then tested a bunch of different monoterpene synthases from a variety of plant species and found the highest production of linalool from a truncated linalool synthase of M. citrate and the highest production of geraniol from a full length geraniol synthases of O. basilicum. The genes encoding these enzymes were put into their target yeast strain.

The next stage of genetic engineering involved iterative manipulation to optimize the monotepene synthesis pathways. Just because the researchers had inserted active monoterpene synthases into the yeast strain didn’t mean linalool and geraniol would suddenly start being produced. They had to also have the yeast constantly produce the substrate, geranyl pyrophosphate. The synthesis pathway for this substrate was well documented, so the authors were able to focus on manipulating this pathway. It still wasn’t a simple process. The authors made tons of changes to the expression levels of genes in this pathway and analyzed how they affected production of the monoterpenes. This took multiple rounds of iteration and was probably the bulk of the work on this project.

I wish I could learn a bit more about all the tricks they did to manipulate the synthesis pathway, but the authors kept their cards close to their chest. All I know for sure is that they used CRISPR to do the genetic manipulation and adhered to a cycle of designing changes, building new yeast strains, testing them out, and analyzing the data. They do have some concrete details in the figures and supplemental data, but I found it difficult to interpret. This could be because I’m not that great at analyzing papers (probably true :P) but it might also be due to conflicts of interest in this research. I’ll discuss that a bit more later. At any rate, the end result of all this genetic manipulation is that they had a strain of yeast that produced enough linalool and geraniol to match that found in hoppy beers.

With this “hoppy” strain of yeast, they brewed some beer. Again, the exact brewing conditions were kept under wraps, but they did measure the monoterpene content of the beer after it was brewed.

^{Linalool and geraniol levels in beer brewed wtih hoppy yeast vs traditionally dry hopped beer. Adapted from (7)
</sup)}

In comparison to a traditionally dry hopped beer, the beer brewed with “hoppy” yeast without any actual hops has higher content of linalool and geranol. The monoterpene content was also substantially more consistent between batches. However, just because the beer is full of monoterpenes monoterpenes doesn’t mean the taste is right. Tastings were performed to assess hop content of beer brewed with and without the hoppy yeast. Tastings were done by professionals from the Lagunita’s brewing company and used an established tasting protocol. In the end, the authors found that the hoppy yeast produced the hoppiest tasting beer

^{Hoppy taste of beer brewed with hoppy yeast compared to a dry hopped beer and a unhopped beer. Adapted from (7).
</sup)}

The only odd thing I noticed about this data was that dry hopped beer wasn’t significantly hoppier in taste than beer brewed without traditional dry hopping. You’d think it would be which makes me a bit less confident in the taste test. The authors don’t address this discrepancy which is a bit concerning. They also don’t mention whether or not the beer tastes any good. It’s possible that this “hoppy” yeast makes a bunch of secondary metabolites that taste terrible.

At any rate, the authors have successfully created a strain of yeast that creates hoppy beer so I guess you could call it mission accomplished!

Omissions and conflicts of interest

What really puzzled me about this paper was why the authors didn’t do more robust taste tests. With the professional tasters already trying their beer, you’d think they’d have them fill out a survey or something. Nevertheless, all they were instructed to do according to the methods was rate how hoppy the beer tasted. Now this doesn’t mean such a survey wasn’t performed. It could have been omitted because this is a Nature Communications paper. Articles in nature often have to go through a ton of information in a small amount of space and they consequently end up being skimpy on some details. However, it could also be because the beer tastes bad and the authors didn’t want to report on that finding. If this data was omitted, that wouldn’t be a bad thing by itself. After all, they succeeded in making a hoppy yeast strain and could worry about taste in later experiments. However, as I was looking into this paper, I noticed a conflict of interest worth mentioning.

It turns out that the first two authors of this paper launched a startup based on this work called Berkeley Brewing Science 11. Thus, they have a financial incentive to make this hoppy yeast strain look as promising as possible. They certainly generated some positive PR as a bunch of popular science articles on this paper came out soon after it was published 12, 13. Now conflicts of interest like this don’t mean the work is invalid. This is still peer-reviewed work after all and the science looks pretty good from what I can tell. I should also note that this conflict of interest is declared within the paper so everything is above board. Still, I would really like to see the follow up paper showing that hoppy yeast make good beer rather than just hoppy beer.

Time for a nightcap

Regardless of the conflict of interest, this is some cool work. Modern science has eliminated the need of one of the four main ingredients of beer. This might seem like a silly use of genetic engineering and may offend some beer purists. However, reducing the need for hops makes beer more environmentally friendly and the idea of using Science to make beer hoppy is awesome.

Cheers! 14

References

(1) https://pixabay.com/en/hops-leaf-nature-food-plant-3334334/
(2) https://en.wikipedia.org/wiki/Brewing
(3) https://en.wikipedia.org/wiki/Yeast
(4) https://en.wikipedia.org/wiki/Hops
(5) Personal communication with Aeronaut Brewing founders in Cambridge, MA
(6) https://ychhops.com/who-we-are/sustainability/annual-report
(7) https://www.nature.com/articles/s41467-018-03293-x
(8) https://pubs.acs.org/doi/abs/10.1021/jf00108a041
(9) https://en.wikipedia.org/wiki/Linalool
(10) https://en.wikipedia.org/wiki/Geraniol
(11) https://www.bbsbeer.com/
(12) https://www.insidescience.org/news/engineering-yeast-make-hoppy-beer
(13) http://news.berkeley.edu/2018/03/20/brewing-hoppy-beer-without-the-hops/
(14) commons.wikimedia.org/wiki/File:Two-pints-beer-main.jpg

Images

All images were taken either directly from the publication referenced or labeled for reuse on Google Images. If any image owner has an issue with this article, please contact me and I will address the issue.