Over the past couple of years I’ve kept a file of different hop studies I’ve come across through various podcasts, presentations, and articles. I thought it would be beneficial (to me as least) to layout all of the different studies and pull out the highlights in an attempt to create a detailed look at different methods and concepts for achieving maximum hop flavor and aroma in beers. I’m not a scientist, I’m merely doing my best to evaluate each study and pull from them what I believe would be the most beneficial to homebrewers. If I’m misinterpreting any of the results, please feel free to let me know so I can adjust the article, I don’t want to mislead anybody! Now, let’s get into into it…

Hop Oils

Typically, discussions around hop oils tend to center around the following: Total oil amount, B-Pinene, Myrcene, Linalool, Caryophyllene, Farnesene, Humulene, and Geraniol. As an example, for the hop oils calculator I created last year, these are the oils being used in an attempt to estimate flavor and aroma descriptors of beers because these are the exact oils that Hopunion tested from the 2014 hop crop. The main hydrocarbons of hop oils are the terpenoids (like myrcene, humulene, caryophyllene etc.) which have been determined to be one of the main contributors to the hop aroma in beer. It makes sense then that most of the studies in this post center around these terpenoids. However, back in 2008 it was determined that there has been 485 hop compounds currently identified and evidence to suggest there are upwards of 1,000 hop oil compounds that may be present!1 So there is clearly way more research needed to even further understand the complexities of hop oils!

This post of common oil aroma and taste descriptors of hop oils and acids is a good quick reference guide for many of the hop oils mentioned throughout this post.

Hop Oil Behavior Throughout the Wort Boiling Process 

The Kishimoto study mentioned above did some great research examining how hop oil behavior is altered during the wort boiling process. As the following graphic shows, Kishimoto found that the majority of hop oils escape during the boil. This would help explain why there has been a major move towards late hop additions in an attempt to get the more of these oils into the fermenter. The research shows that myrcene and linalool levels fell rapidly during boiling because of their low boiling points. Humulene, farnesene, caryophyllene and geraniol have slightly higher boiling points and had more of their oils remain during the boil. The study also found that among the identified hop-derived ordorants, the most intense odor-active components included linalool and geraniol. These two particular oils were also the main focus of many of the other studies in the post.

Kishimoto Boiling Of Wort and Hop Oils

Figure 1-4 (from Kishimoto, 2008)

The research shows that to retain higher concentrations of these terpenoids, we should be adding hops later in the boil right? Well, this tells part of the story because the fermentation process also plays a big role!

Hop Oil Behavior During Fermentation

I have reluctantly participated in the huge whirlpool/steep additions when brewing hoppy beer.  This has been the new process undertaken by most homebrewers in an attempt to get a hop saturated flavor from beers and not just the hop aroma that comes with dry hopping. I have gone along with this process reluctantly because no matter how many hops I pack into the whirlpool when it comes time to taste the post fermented beer before dry hopping, I never really get much in the flavor or aroma that suggested this wort was once jam packed with hop material. So, where did it all go?

A 2013 study titled, “From Wort to Beer: The Evolution of Hoppy Aroma of Single Hop Beers produced by Early Kettle Hopping, Late Kettle Hopping and Dry Hopping” sheds some light on how the fermentation process affects hop volatiles. My understanding of their results shows a big decrease in hop oils studied throughout the brewing process, with the majority of hop oils decreasing after the fermentation process. 2

The researchers took wort and beer samples at different times throughout the brewing process and immediately stored them at -4F until they were examined. The first set of charts below shows that the floral oils compounds (like myrcene) “significantly” decreased as a result of fermentation. On the other hand, linalool seemed to show the greatest ability to stick around during fermentation, but still decreasing nearly in half. The second set of charts shows that the noble or spicy hop oil compounds also decreased significantly during fermentation and nearly disappeared after centrifugation (which suggests to me that the yeast seems to strip some of these volatile oils as it is separated from the beer). The overall conclusion from this analysis was that fermentation and centrifugation were identified as the crucial processes steps for decreasing hop oil compounds.

Hop Volatiles During Brewing Process

Fig 1A (from Dresel, Praet, Opastaele, Van Holle, Van Nieuwenhove, Naudts, Keukeleire, Aerts, and Cooman)

Monitoring Hop Volatiles noble spicy characters

Fig 2 A (from Dresel, Praet, Opastaele, Van Holle, Van Nieuwenhove, Naudts, Keukeleire, Aerts, and Cooman)

Abbreviations
w/o hops: after 15′ boiling – just before early hopping
after EH: after 75′ boiling – just before late hopping
after LH: after 5′ cooling
before ferm: after 10′ cooling – just before fermentation
after lag: after lagering – just before centrifugation
after past: after pasteurization
beer ND: bottled beer without dry hopping

Biotransformation of Hop Oils 

Biotransformation, which is the transformation of hop oils in the presence of yeast, is a fascinating processes. I was surprised to find a study titled, “Biotransformation of hop aroma terpenoids by ale and lager yeasts” dated all the way back to 2003, considering this is still not something you hear very often in the homebrew community today (at least I don’t). The study looked at brewing yeast (both lager and ale) ability to transform terpenoids. Specifically, they looked at geraniol, linalool, myrcene, caryophyllene, and humulene.

The study found that yeast does in fact have the ability to transform terpenoids. Specifically, geraniol was converted into citronellol (which is described as having a sweet, rose-like, citrus and fruity aroma) and linalool was converted into terpineol (which is described as having lilac like aromas). Remember that linalool had the greater ability to stick around the longest during the brewing process. Interestingly, they found that “most of the terpene alcohols were lost from the fermentations within the first few days of the fermentation, and after that, the decreases occurred at much lower and steadier rates…This indicates that the loss of terpenoids was associated with the increase in yeast biomass.”3 This might suggest that if dry hops are added after the first couple days of the most active fermentation, the biotransformation process would decrease. Maybe we should we should be pre-dry hopping our beer (adding dry hops to the fermenter before the wort and yeast) to take the greatest advantage of both the biotransformation process as well as the greater hop oil extraction due to not being introduced to the wort during the boil?

You can see from the two charts below the biotransformation by the ale yeast (S. cerevisiae). Although its cutoff in the charts, the bottom line represents time (days) during fermentation. It’s amazing to see geraniol basically disappear during fermentation and transform into citronellol!

geraniol to citronelloi

Figure 3 (from King and Dickinson, 2003)

linalool to terpineol

Figure 5 (from King and Dickinson, 2003)

Although probably not the point of the experiment, one item I found interesting was that the large doses of terpenoids used in their process seemed to slow down the fermentation time (see the chart below).  In other words, because the level of terpenoids was higher than you would typically find in hopped wort, the fermentation time was slower than you would typically expect (it took all of 15 days to reach final gravity). While it’s not clear to me what levels are considered normal in hopped wort, I’m curious if this would imply that pre-dry hopping wort might result in a slower ferment? In my few tries with the process, I haven’t noticed much of a different in fermentation time however.

Terpenoids and Fermentation Time

Figure 2 (from King and Dickinson, 2003)

It seems logical to conclude then that if you’re looking to get a more predictable “out of the bag aroma” from hops, it might be best to avoid biotransformation and to dry hop after fermentation or after the beer has been removed from yeast cake. This would also have the added benefit of reducing loss of oils by yeast cell membranes, which can “act as a fining agent” thus removing them from solution.

Introduction of Oxygen When Dry Hopping

I’m guessing that most homebrewers don’t think much about oxygen getting into their beer with the addition of dry hops, but a thesis by Peter Harold Woolfe, titled, “A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer,” notes that the introduction of dissolved oxygen is “inevitably introduced” when dry hops are added to beer “resulting from the multitude of crevices inherent to their anatomy.” We all know that oxygen is the enemy of fresh beer! So what is the best way to limit this pickup of oxygen? Woolfe suggests that dry hopping early in fermentation while the yeast is still present and active would allow the oxygen to be metabolized by the yeast before it could oxidize the beer.4 We also know this would also have the added effect of biotransformation of geraniol to citronellol and linalool to terpineol.

A second method suggested is to add the dry hops to a vessel and then fill the vessel with C02 to remove any oxygen. In the homebrewers case, this would mean adding the hops to a purged keg, before racking the beer into the keg, removing any oxygen present. You would then rack the beer into the keg from the fermenter, ideally with a closed transfer. It’s also worth noting an additional benefit of keg hops I have experienced, which is a strong correlation of increased hop aroma detectability when additional dry hopping is done in the keg.

Adding Used Dry Hops to the Boil for BitternessA quick side-note from the thesis that I found interesting was the idea of reusing hops that were used as dry hops. The thought goes that although the oils from the hops have been extracted during the dry hopping process, their alpha acid composition is still largely intact. We could then entertain the option of reusing dry hops in the boil as a way to get the bitterness and bitterness only into beer. The author couldn’t locate any brewers who actually did this however, so I just gave it a shot in my last beer! As the pictures shows, I gently squeezed the used dry hop bag a little to get the old beer out and then dumped them right into the boil. I can say the wort smelled and tasted very similar as to using fresh hops, so we’ll see how the finished beer comes out!

If you decide to give this a shot as well, don’t forget that your oil extraction would be very low if non-existent since the dry hop processes already took place. However, if you plan on adding your bittering hops early in the boil, you’d be losing most of these oils anyways (per the Kishimoto study), so why not give it a shot!

 

Dry Hopping Contact Time

Now that there is evidence to suggest that hop oils added to the boil are largely volatilized and the yeast further strips these oils during fermentation, its clear dry hopping it’s going to be a main method of getting these glorious hop oils into the finished beer. So what is the ideal dry hop contact time to get the most from our hops? Researchers looking at the extraction rate of aroma compounds during dry hopping took a solution consisting of filtered water and ethanol in flushed cornelius kegs with 1/3rd pound/barrel cascade hops placed in a mesh bag and sunk to the bottom of the keg with stainless steel weights (similar to how many homebrewers conduct dry hopping). They then measured the oil concentrations at various days throughout the dry hop (it’s important to note that the kegs were not agitated during the their test). They found that the concentrations of oils were either near the same level as day one or had even fallen slightly, with the exception of geraniol.5

Here we can see their results, which show the concentration of linalool tested at 3 different stages of the dry hop (day 1, day 4, and day 7). Interestingly, the oils were almost completely extracted on the first full day of dry hopping and even decreased after day 4! Nearly identical results were found when the myrcene concentration levels were tested (second graph), with myrcene dropping after day 1.

linalool concetration

Figure 5 (from Wolfe, Qian, and Shellhammer, 2012)

Myrcene Concentration

Figure 6 (from Wolfe, Qian, and Shellhammer, 2012)

The A-B-C-D groups in the above charts represent different diameter pellet hops from different producers.

Pellet Label | (mean diameter)
D | 1.72 mm
C | 1.37 mm
A | 1.09 mm
B | 0.95 mm

Dry Hopping – Pellets vs. Whole Cones 

Another study examined the extraction rate of beer dry hopped with pellets vs. whole hop cones. The researches found that “more rapid extraction and greater final amounts of hop aromatic compounds” with pellets as “compared to dry hopping with whole cone hops.”6

In this same study the researchers also looked at the extraction rate of hops by hour (not days) in both a stirred vs non-stirred vessel used for dry hopping. Stirred basically means the hops were added to a conical fermenter and a pump pushed the wort in and out of the tank creating a whirlpool. Below is a chart from the study showing the extraction percentages of the hops oils of pellets. You can see that circulating the beer greatly increased the extraction percentage of the hop oils (within hours) but with a greater increase in polyphenols (which may have a potential downsides like increased “bitterness, astringency, and possible oxidizing agent”). It’s interesting to see that the unstirred method of dry hopping, which is more practical to homebrewers, suggests nearly full extraction potential of the hops is basically reached in just 6 hours!

Extraction of Oils

Table 14 (from Wolfe, Qian, and Shellhammer, 2012)

A study by Hopsteiner titled, “Dry Hopping – A Study of Various Parameters” also looked at the issue of pellets vs. leaf hops in dry hopping. They furthered the notion that pellet hops have increased extraction potential of hop oils compared to leaf hops. In this case the researches looked at linalool and you can see from the chart below that pellets achieved nearly 50% greater extraction of linalool and within just a couple days. 7

linalool pellet vs leaf

Figure 5 (from Mitter, Cocuzza, 2013)

This Hopsteiner study also examined the extraction of linalool in beer dry hopped with pellets loosely vs. contained in a finely woven sack. The beer dry hopped loosely had nearly 50% greater extraction than the beer dry hopped with the use of a hop sack! This suggested that it’s possible we could get by dry hopping with far less hops if we don’t contain them in a bag. For this reason, I’m going to begin experimenting with loose dry hopping in a keg with the method described in detail here. In addition, I don’t use a bag when adding hops to primary either to hopefully achieved more extraction.

loose vs mesh bag dry hopping

Figure 4 (from Mitter, Cocuzza, 2013)

Dry Hopping and Alpha Acid Extraction

The Hopsteiner study referenced above found that dry hopping does increase the alpha acid content in the finished beer, but there was no change in iso-alpha acid (which occurs during the boil). They found the increase in alpha acids by dry hopping was around 6%, which I interpret as meaning a large alpha acid portion of the hop still has bittering potential. If this interpretation is correct and if you choose to try bittering your beer with used dry hops as discussed above, you would have to slightly adjust your bittering hops to reflect the slight drop in bittering potential due to the small extraction of alpha acids during the dry hop.

increases in acids after dry hopping

Figure 4 (from Mitter, Cocuzza, 2013)

It seems logical to me that dry hopping beer could influence the perception of bitterness in a beer, but does this increase in alpha acids to the beer actually add bitterness? The W. M., & S. C. study mentioned above found that alpha acid concentration dose not correlate to increased bitterness beer. Specifically, the researchers found that “alpha acid concentrations as high as 28mg/L in beer were not detected as being bitter by beer drinking consumers as well as a trained panel.” So you can see that although the very small amounts of alpha acid (averaged 3.7 mg/L) introduced in the beer by dry hopping found by Hopsteiner does not actually translate into increased bitterness (it may help with head retention however).

Final Thoughts

  • A large portion of hop oils are removed or altered during the boil and during during fermentation.
  • Do your best to avoid any oxygen pickup in your beer when dry hopping. Since oxygen is “inevitably introduced” when dry hopping, dry hop early in the fermentation process to scrub this oxygen pickup. Or try adding the hops to a C02 purged keg to remove any oxygen present on or in the hops.
  • If you want to take advantage of biotransformation, which can lead to more of a rose/citrus/fruity aroma from the conversion of geraniol to citronellol, add dry hops early in the fermentation process while the yeast are still present and active.
  • If you’re looking to get an “out of the bag” aroma with your dry hops, it might be best to add these after the yeast is removed or at least after the bulk of fermentation.
  • It looks like long dry hop times aren’t necessary and may actually reduce the amount of oil extraction in your beers! Even without agitating your beer during dry hopping, most of the oil extraction is done in just a day.
  • Don’t contain your hops, let them free! With 50% more extraction found in certain oils when not contained in a hop bag it makes sense to come up with ways to deal with hop material (like a stainless steel filter in your keg).
  • Based on the data above on pellets ability to extract at a higher rate than whole cones, it might make sense to experiment with blending up or breaking apart the whole hops to improve extraction rates.
  • Dry hopping doesn’t appear to increase the actual bitterness of beers, at least to the human palate. This is not to say it won’t influence the bittereing perception in a dry hopped beer.
  • If the increase in yeast biomass has the ability to strip terpenoids, it would make sense to avoid over-pitching your hoppy beers. [Update: Further research showed it is actually possible to increase bitterness by dry hopping]

Footnotes

  1. T. K. (2008, July 23). Hop-Derived Odorants Contributing to the Aroma Characteristics of Beer. Retrieved from http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/66109/4/D_Kishimoto_Toru.pdf
  2. D. (2013, May). From Wort to Beer: The Evolution of Hoppy Aroma of Single Hop Beers produced by Early Kettle Hopping, Late Kettle Hopping and Dry Hopping. Retrieved from https://www.researchgate.net/publication/271848239_From_wort_to_beer_The_evolution_of_hoppy_aroma_of_single_hopped_beers_produced_by_early_kettle_hopping_late_kettle_hopping_and_dry_hopping
  3. Andrew King, Richard Dickinson (2003, March). Biotransformation of hop aroma terpenoids by ale and lager yeasts. Retrieved from http://onlinelibrary.wiley.com/doi/10.1111/j.1567-1364.2003.tb00138.x/full
  4. P. H. (2012, August 7). A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer. Retrieved from http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/34093/Wolfe_thesis.pdf
  5. P. H., M. Q., & T. S. (2012, August 7). The Effect of Pellet Processing and Exposure Time on Dry Hop Aroma Extraction. Retrieved fromhttp://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/34093/Wolfe_thesis.pdf
  6. P. H., M. Q., & T. S. (2012, August 7). Brewery Scale Dry Hopping: Aroma, Hop Acids, and Polyphenols. Retrieved from http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/34093/Wolfe_thesis.pdf
  7. W. M., & S. C. (2013). DRY HOPPING – A STUDY OF VARIOUS PARAMETERS. Retrieved from http://hopsteiner.com/wp-content/uploads/2014/03/Dry-Hopping-A-Study-of-Various-Parameters.pdf
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