CRISPR Yeast and Hop Compounds

There is a lot of discussion (or there was for a few weeks) about a modified yeast that produces hop flavor compounds. Specifically, a yeast strain of Saccharomyces cerevisiae was modified to express the genes responsible for linalool and geraniol synthesis. It was reported by Denby et. al, that beers produced using these strains are perceived as hoppier than traditionally hopped beers by a sensory panel in a double-blind tasting. That’s impressive.

Cards on the table, for almost 20 years I worked as a molecular geneticist. I studied the CRISPR system, I’ve done yeast and bacterial genetics. What has been done here is cool for commercial and practical reasons but this is not a complicated thing to have done. As a brewer and a scientist, I find this claim to be incredible honestly. Not the claim that the yeast can make linalool and geraniol, or that the quantities made are comparable or even more reproducible than the hop crops around the world. I don’t even find it hard to believe that this beer tastes “hoppy” or “hoppier” than a traditionally made beer. What I find hard to believe that this is sufficient to capture the complexity of hops in beer.

As I tried to articulate to myself what I didn’t like about the press coverage of this breakthrough in brewing science, I decided to put fingers to keyboard to flesh it out. It’s probably obvious to everyone who reads this blog that hops are one of the four critical ingredients of beer and that I’m a fan. I get experimental varieties and test them out. I grow wild hops in my front yard (to varying success), I “grew up” in the first IPA revolution that seems to happen every 5 years or so with a new beer style that uses more hops than the one before. It is a requirement codified into law that beer must contain hops in the United States. If it says beer, it was made with hops.

The reason I don’t like the way this story has been reported has nothing to do with some odd nostalgic loyalty I must the use of hops in beer…. or the adventure of finding a new hop or a new combination. I think my biggest objection is with the lack of understanding for the complexity of what happens to hops in the brewing, fermentation and aging process beer goes though. I’m not saying we won’t get there, but there is a lot of variables to consider when thinking about what makes a hop a hop.

Hop To Hop Variability

First thing first, not all hops are the same. I know you know this. A mosaic dry-hopped IPA is not the same as a Saaz dry-hopped IPA. Why not though? Hop flavors and aromas are a combination of dozens if not hundreds of compounds all present simulatenously in the beer all at different concentrations. The scientific and analytical beer / brewing community has only just now begun to scratch the surface of this. In 2015, Cibaka et. al published a paper profiling some terpenoids and mercaptans in seven different hop varieties.

 

The table above summarizes the relative concentrations of 14 different non-sulfur containing compounds found in 5 different hops, including linalool and geraniol. The concentrations of both of those compounds vary from hop to hop by an order of magnitude…and that observation says nothing about the significant and obvious variation of the rest of the aroma and flavor active compounds in the table. Cibaka et. al also presented data for additional hop varieties and some more common sulfur containing compounds that are also aromatic and/or flavor active for humans. The below spider diagrams are from the paper (see references).

From this graphical representation of the relative concentrations in the samples, you can see that in the case of linalool, Tomahawk and Nelson Sauvin have significantly higher concentrations of that compound while a hop like Sorachi ace is dominant in the β-citronellol and β-farnesene while having almost no linalool or geraniol.

Similar analyses can be made for the biodiversity of the sulfur containing flavor and aromatic compounds from hops.

The phenotypic diversity in the sulfur compounds seem to be far more strain specific than even the oils in the above examples. Each sulfur containing compound (mercaptans) in this study is heavily favored by one of these hop varieties and there does not appear from this data to be a lot of overlap.

Obviously, this is a small sample and just an example, but these varieties and limited studies show that different hop varieties are creating dozens of flavor and aromatic compounds and at different concentrations.

BIOTRANSFORMATION OF HOP COMPOUNDS

One of the things that has come out of the New England IPA “controversy” is the practice of dry hopping earlier, during the primary fermentation before the yeast drop out of solution. Conventional wisdom was that the greater contact time from the hops would create a “grassy” flavor in the final product and prolonged dry-hop times were never done. As of the time of this post, it is common to dry hop a day or two after pitching and letting it ride. The relative lack of the presence of the “grassy” character feared for so long could be a combination of a lot of things. For example, maybe the popular varieties of hops used in this new dry hopping protocol just don’t have that compound expressed in those strains, maybe the development of the grassiness may be an oxidation product and the active yeast are scavenging all the available oxygen in the tank during fermentation, or maybe the grassiness is partially converted by the yeast in solution to a different compound that does not have that flavor.

I’m going to post a thorough review of the chemical processes that flavor active compounds in hops undergo during the brewing process in a few weeks (I plan to write a much longer post about that in a few weeks), today I just want to provide a few examples illustrating how the normal biological and metabolic activity of yeasts can create new and interesting flavors from the secondary metabolites (some listed above) found in different strains of hops. There are several well-known biotransformation activities done by brewer’s yeast. For example, some strains of Saccharomyces cerevisiae can convert geraniol to β-citronellol. The effect of this activity would be to take a Tomahawk hop and make it taste more like Sorachi Ace. Geraniol can also be converted into linalool and nerol as well. The lager yeast Saccharomyces bayanus (or whatever we call it now) can produce certain terpenoid esters – both geranyl and citronellyl acetate. Both geranyl and citronellyl esters at higher concentrations in late‐hopped lagers than in lagers hopped at other stages in the brewing process. In addition, some Saccharomyces can use α‐humulene to make to small amounts of β‐carophyllene and carophyllene oxide.

My plan is to write up all the published biotransformation pathways that are currently known, look for that in about a month.

CONCLUSION

I’m excited about the possibility that yeasts can be used to increase or control the levels of some of the more important flavor compounds in hops. I just wouldn’t bulldoze all the hop farms quite yet. This is a field at the beginning of something special. Better understanding the genetics and phenotypic variation of hops and the chemical processes that compounds undergo in the brewing and fermentation steps will only serve to further our craft and improve beer quality. I can’t wait to see what’s next and for commercial batches of beer to be made with these yeasts.

REFERENCES

Charles M. Denby, Rachel A. Li, Van T. Vu, Zak Costello, Weiyin Lin, Leanne Jade G. Chan, Joseph Williams, Bryan Donaldson, Charles W. Bamforth, Christopher J. Petzold, Henrik V. Scheller, Hector Garcia Martin & Jay D. Keasling. “Industrial brewing yeast engineered for the production of primary flavor determinants in hopped beer” Nature Communications volume 9, Article number: 965 (2018).

Marie-Lucie Kankolongo Cibaka, Jacques Gros, Sabrina Nizet, and Sonia Collin. “Quantitation of Selected Terpenoids and Mercaptans in the Dual-Purpose Hop Varieties Amarillo, Citra, Hallertau Blanc, Mosaic, and Sorachi Ace”, J. Agric. Food Chem. 2015, 63, 3022−3030.

King AJ, Dickinson JR. “Biotransformation of hop aroma terpenoids by ale and lager yeasts.” FEMS Yeast Res. 2003 Mar;3(1):53-62.