The Science of Hop Glycosides

When I started homebrewing in 2005, the American hop assault was beginning in earnest. IPAs and Double IPAs were gaining popularity among beer nerds but weren’t nearly as ubiquitous as they are today. Hoppy beers seemed to be becoming more bitter without a commensurate rise in hop aroma. Ten years later, that trend has thankfully been reversed. Many breweries are increasing their beers’ juicy hop punch without relying on recipes that contribute 200 (theoretical) IBUs. Drinking an IPA doesn’t need to send your tongue into spasms for you to smell the citrusy and piney hop aroma from five feet away!

When I fell in love with IPAs my favorites had a raw “nose in the hop bag” aroma that comes from dry hopping fully-fermented bright beer. Many thanks to Vinnie Cilurzo of Russian River Brewing Co., who in addition to inventing double IPA was generous enough to share his tips on brewing hoppy beers with us mortals. #8 from his 10 Factors to Making Better Hoppy Beers is: “The more yeast you remove, the more beer surface area you’ll have exposed to the dry hops.” This is the best advice for imparting a “true” hop aroma. Without yeast cells in the way, alcohol extracts hop oils largely unchanged. However, be careful not to introduce any air if you choose this route; without yeast to scavenge oxygen, the hops aromatics will oxidize quickly, muddying their fresh aroma within weeks.

Not all brewers are pursuing beers that smell like hops straight from the freezer, there are those who want pineapple, tangerine, melon, or pine sap aromatics without the “green/pellet” character. One technique that achieves this effect is to steep a large dose of hops in the hot wort for 20 to 60 minutes after flameout before chilling (for more on this, read Dave Green’s article “Hop Stands” in the March-April 2013 issue of BYO). Another is to dry hop before the end of fermentation. In both cases, this gives the yeast an opportunity to enzymatically liberate aromatics and convert others. As an added benefit, adding dry hops while fermentables remain ensures that yeast will metabolize most of the oxygen introduced. The first time I heard this technique suggested was by Matt Brynildson, Brewmaster of Firestone Walker Brewing Co., but many other professional brewers have advanced the topic in the years since.

What are Glycosides?

Plants in general, and hops specifically, are the chemists of the natural world. While animals have the options of flight and fight, plants stand and endure. Accordingly, plant life has evolved a deep bag of chemical tricks for almost any situation. The humble rice plant has more than twice as many genes as we humans do! One plant trick is to attach a sugar molecule to an aglycone (this can be any functional group, for hops these molecules often happen to be aromatic) as a way to make it water-soluble for transport or inactive for storage. The combined molecule is called a glycoside. These are relatively common, with one study of 150 plant species finding glycosides two to five times more concentrated than their volatile aromatic molecule.¹ No specific studies exist for hops, but we can assume it is within this range.
In hops, rather than being located in the lupulin glands with the various acids and oils brewers usually focus on, glycosides are found in the vegetative material. As a result, extracting them is not an issue if you are adding hops in their more traditional forms (whole or pellets). However, if you are adding only hop extract your beers are missing out because the super-critical CO₂ extraction process targets hydrophobic molecules. Miller Brewing Co. has done extensive research on glycosides because many of their beers are brewed with hop extract to avoid skunking in clear bottles.

Hops don’t produce aromatic molecules to make your IPA smell nice, but rather to fulfill a biological need such as defense from insects (although as hop breeders continue their work, that isn’t the case as much as it is for wild hops; one could reasonably argue that all domesticated plant crops have been selected to express traits that really have nothing to do with Darwinism). The decreased reactivity and increased water-solubility of glycosides make them easier to convey into the wort or beer. However, if the aglycone is still bound to its sugar molecule by the time you pour a pint, it won’t be free to stimulate your olfactory receptors. On the other hand, in Stan Hieronymus’ For the Love of Hops, Miller hop chemist Pat Ting suggests that even the enzymes and microbes in the mouth of the beer drinker may be able to free some aglycones.

The amount of glycosides in hops differs by variety. Extensive research into the actual amounts in specific hops was done by Miller Brewing Co., whose scientists released a study with results that looked at treating unnamed hops with the enzyme beta-glucosidase. This procedure released: “Benzaldehyde (almond, maraschino cherry), vanillin (vanilla), raspberry ketone, geraniol (floral, rose), linalool (floral), phenylacetaldehyde (honey, floral), and many other primary alcohols, ketones, and aldehydes that are also aromatic.”²

Glycosides aren’t only contributed by hops; they can come from fruit and spices as well. Research focusing on the addition of Schaerbeek cherries to Belgian kriek showed that beta-glucosidase positive yeast strains are able to release “important contributors to sour cherry aroma such as benzaldehyde, linalool and eugenol” during refermentation compared to beta-glucosidase negative strains.³

How are Agylcones Freed?

Beer pH plays a role in non-enzymatic freeing of aglycones. As all beers are acidic (even non- “sour” beers usually have a final pH of 4.5-4.0) some of these glycosidic bonds will be broken even without yeast present. The lower the pH, the more effective this will be. There is also an enzymatic route if a yeast is capable of freeing and fermenting the sugar molecule. Unfortunately, the bond holding glycosides together requires a different enzyme than the one used to ferment maltose. What we need is a yeast strain that can break a Beta bond (i.e., those that produce beta-glucosidase). This is the same bond that holds glucose and galactose together to form lactose (making this a test you could theoretically perform yourself).

While there are a few Saccharomyces strains capable of breaking glycosidic bonds (e.g., LD40), it is more common for Brettanomyces to possess the ability to release aromatic aglycone. Very few Saccharomyces strains can release aglycones, and those that do tend to have a lower rate than the most effective Brettanomyces.⁴ Another reason to try dry hopping beers fermented with Brett like New Belgium Le Terroir, Almanac Devil’s Advocate, and Pizza Boy Eternal Sunshine!

Biotransformation

In addition to freeing up trapped aroma molecules, yeast also have the ability to convert one aromatic compound into another. For example, yeast can convert floral geraniol provided by hop (especially pronounced in Citra®) into citrusy beta-citronellol.⁵ Coriander is another source of geraniol, which is why this spice contributes much of the citrusy flavor in Belgian wits, rather than the pithy dried orange peel that often steals credit. When I want a beer that has a subtle spice flavor that is more integrated, I’ll add the spices late in the boil. Spicing post-fermentation (directly, tea, or tincture) is ideal for a true spice aroma, as you might want in a pumpkin or gingerbread ale. Biotransformation is another area where some yeast strains seem to be more effective than others. It may partly explain why certain otherwise clean yeast strains leave muted hop character in their wake, while others produce beers that burst with hop aroma.

Best Practices

If you want to taste the interaction of yeast and hops for yourself, re-brew a tweaked version of your favorite IPA recipe. Monitor fermentation closely. With the right pitching rate and fermentation temperature, two to four days after brewing you should see fermentation wane. The kräusen will begin to deflate, the swirl of yeast will slow, and airlock burps will grow less frequent. If you are monitoring the gravity, wait until 80-90% of your expected apparent attenuation is achieved (e.g., 1.024–1.020 if you are expecting a 1.060 original gravity (OG) beer to finish at 1.015). At this point, add your standard dose of dry hops, maintaining fermentation temperature near the top of the strain’s suggested range. After fermentation stops, the hops will settle out with the yeast and you can package the beer as usual.
Dry hopping during fermentation is an especially valuable technique for homebrewers who do not have a handy source of carbon dioxide, because it is otherwise difficult to introduce dry hops without oxidizing the beer. If you keg (or have a CO₂ tank), place the hops in a sanitized keg or fermenter, purge with CO₂, and then transfer the beer onto the hops. Don’t worry, racking the beer off of the yeast cake will not stall fermentation (the yeast at the bottom of the fermenter is mostly dead or dormant).

If you are looking for some raw/fresh hop character as well, you can dry hop a second time after the yeast has flocculated. I often bag and weight whole hops, place them into the serving keg, and purge with CO₂ before racking the beer in. Whole hops can be left in cold beer for several months without developing a “grassy” flavor to my palate. For hoppy beers I add hops at only four points: start of the boil, hop stand, as primary fermentation slows, and in the keg. As a homebrewer, don’t be too concerned about how efficient your dry hopping process is. This is a hobby after all! While large flame-out additions and earlier dry hopping may sacrifice some hop character to absorption by yeast and scrubbing by CO₂, the quality of the aroma is worth an extra ounce or two of hops!

Conclusion
While the research on the interaction between plant compounds and yeast is still in its infancy, it is another consideration for brewers looking to refine their process. It is especially powerful for those brewers who bottle hoppy beers and find that the aromatics taste dull and muted by the time their beer is ready to drink. Hopefully more studies looking into the hop variety and yeast strain specific contribution to glycosides and biotransformation will come out soon, but until then, let your fermenter be your lab!

References
¹ Winterhalter, P., Skouroumounis, G. K. Glycoconjugated aroma compounds: occurrence, role and biotechnological transformation. Advances in Biochemical/Engineering/ Biotechnology 1997
² Beer Sensory Science “Glycosides: The Hidden Flavors.” https://beersensoryscience.wordpress.com/2010/11/30/glycosides/
³ Luk Daenen, Femke Sterckx, Freddy R Delvaux, Hubert Verachtert, Guy Derdelinckx, “Evaluation of the Glycoside Hydrolase Activity of a Brettanomyces Strain on Glycosides from Sour Cherry (Prunus cerasus L.) Used in the Production of Special Fruit Beers.” http://www.ncbi.nlm.nih.gov/pubmed/18673394
⁴ Luk Daenen, Daan Saison, Femke Sterckx, Freddy R. Delvaux, Hubert Verachtert and Guy Derdelinckx, “Screening and Evaluation of the Glucoside Hydrolase Activity in Saccharomyces and Brettanomyces Brewing Yeasts.” Journal of Applied Microbiology. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.2007.03566.x/full
⁵ Kiyoshi Takoi, Yutaka Itoga, Koichiro Koie, Takayuki Kosugi, Masayuki Shimase, Yuta Katayama, Yasuyuki Nakayama Junji Watari, “The Contribution of Geraniol Metabolism to the Citrus Flavour of Beer: Synergy of Geraniol and β-Citronellol Under Coexistence with Excess Linalool.” Journal of the Institute of Brewing. http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2010.tb00428.x/abstract

Related Links:
• Ask eight brewers the key to crafting a great IPA and you will probably get nine different answers. One technique that is well-established in the professional brewing world and has recently gained traction with homebrewers is hop standing or whirlpool hopping. Read about it in this article from the BYO archives: https://byo.com/story2808

Written by Michael Tonsmeire