Survivables: Unpacking Hot-Side Hop Flavor

Why hops like Idaho 7 might be so good at imparting saturated hop flavor to beer

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ne of the things I obsess with at Sapwood is tasting our hop-forward beers pre-dry hop. After a tank has been soft crashed down to 58°F (14°C) for a few days, we’ll dump or harvest the yeast and then dry hop. Right before dry hopping is the best time to pull a sample to determine how much hot-side hop flavor the beer developed. We are always trying different hop varieties in the kettle and playing with varying temperatures of whirlpool and duration, usually using some of the latest hop science as our guide to our experimentation. The only way to see what is working (and what is not) is to taste the beer and take good notes! 

One of the varieties we experimented with early on in the whirlpool was Idaho 7. Tasting these particular beers pre-dry hop I was consistently amazed at how much saturated hot-side flavor they had. In fact, one beer that was dry-hopped with Sabro, the whirlpool addition of Idaho 7 was walking all over the dry hop. Rarely can a hot-side hop addition overshadow a dry hop! 

In March of last year, I tweeted that I thought Idaho 7 was the greatest hop-side hop I’ve used and guessed it was likely because of polyfunctional thiols or hop-derived esters, but had no idea why it was so effective. Luckily for us, Yakima Chief Hops has just put it to the test, and we now why Idaho 7 (and a few other varieties) are likely so great at imparting hot-side flavor! 

Before jumping right to the latest research from Yakima Chief, I thought it would be helpful to take a step back and look broader at some other research into hot-side hopping to help make sense of it all! So, in this post, I start by breaking down the various hop compounds into the classes that I believe are the most useful when trying to determine hop flavor potential in the kettle. The first class of compounds are oxygen-containing monoterpene alcohols like linalool and geraniol. The second class are hop-derived esters and fatty acids. Third are polyfunctional thiols like 3MH, 3MHA, and 4MMP. I then try to compile a list of hops that the science suggests may be good candidates to experiment with to try and push as much hot-side flavor as possible. 

You’re probably thinking, “why didn’t you include hydrocarbons in the list?” Nobody is actually thinking that, but in case you were, even though hydrocarbons can make up 40-80% of the total oil of hops, I decided to leave this class of compounds out of this hot-side discussion because they are so volatile. Myrcene (a hydrocarbon), for example, has been tested to be reduced 50% with just ten minutes of boiling and completely removed after a full 60-minute boil. 1 Whereas geraniol (part of the oxygen-containing monoterpene alcohol class) has a higher boiling point and more likely to find it’s way into the fermenter. Hydrocarbons likely play a more significant role in dry-hopping, especially post-fermentation dry hopping because even active fermentation can scrub these volatile compounds. 

Oxygen-Containing Monoterpene Alcohols 

The oxygenated components, which represent about 30% of the total oil, is a very complex mixture of alcohols, aldehydes, acids, ketones, epoxides, and esters. Most important to brewers looking for intense fruit-forward IPAs are the terpene alcohols located in the oxygenated fraction of hops (examples: linalool, nerol, geraniol). Despite representing a small fraction of the hop, they are more likely to remain in beer throughout the brewing process and are less volatile than terpene hydrocarbons such as myrcene.  

One paper found that hydrocarbons have very low solubility compared to the fruiter oxygenated monoterpenes. Specifically, the paper found that monoterpenes containing oxygen in the form of a ketone, alcohol, ether, or aldehyde had solubilities 10-100 times greater than hydrocarbons.2 However, it’s important to note that even though monoterpene alcohols have more staying power, they are still volatile. Linalool has been tested with losses as high as 80% in just five minutes of boiling, suggesting whirlpool hopping is your best bet to try to preserve these compounds. 

To put a finer point on this, a 2017 paper released by the Journal of the American Society of Brewing Chemists tested how whirlpool additions are impacting hoppy beers.3 Here, beers were brewed with Simcoe at either 60 minutes, 20-minute whirlpool (right after the boil, no cooling was done), or a dry hop addition only (for 48 hours). In terms of measured concentrations of hop-derived volatiles in the finished beer, there was a clear winner with the whirlpool beer, which was measured higher for linalool (flowery-fruity), α-terpineol (lilac), geraniol (rose), and β-citronellol (lemon, lime). A sensory panel could also detect a difference from the control beer in the study, which suggests the monoterpene alcohols introduced to beer in the whirlpool are surviving the kettle above their sensory thresholds. 

The temperature of the whirlpool is another factor to experiment with to encourage monoterpene alcohols to find their way into the fermenter. Takako Inui, a Specialist in the Beer Development Department at Suntory Beer Ltd., Japan showed that late hopping at 185°F (85°C) retained slightly more measured linalool compared to late hopping warmer at 203°F (95°C) or cooler at 167°F (75°C). 4 Because of the research (as well as intentionally trying to have less bitterness from significant whirlpool additions) at Sapwood we are generally targeting a whirlpool temperature around 180°F (82°C). 

The biggest reason to focus on these monoterpene alcohols is that they are being tested in significant concentrations in the finished beer and thus are likely to have an impact on final flavors and aromas. Again, it makes sense to focus on hop compounds that are surviving the entire brewing process. Geraniol is the one caveat in that it’s tested in beer pre-fermentation but can decrease during fermentation. The reason is it’s converted (biotransformed) into beta-citronellol (lime-like), which likely a topic worthy of its own post! 

Applying the research suggests that hops high in total oil and high in these oxygen-containing monoterpene alcohols could be good targets for whirlpool flavoring. Hops that fit this mold are Bravo, Brewers Gold, Centennial, Columbus, Ekuanot, Mosaic, and Simcoe. 

Hop-Derived Esters

Moving to another class of compounds that the research suggests is important when trying to be efficient at choosing hot-side hops are hop-derived esters (mainly isobutyric esters). Hop-derived isobutryic esters are found in high levels in dry-hopped beers (their transfer rate has been tested as high as 80%,5 but also from heavily whirlpooled beers. It’s important to note that beers that are only kettle hopped (early bittering addition) have very few of these compounds. The science has found that these isobutric esters likely contribute to the final flavor and aroma of late hopped beer, which is why it’s worth taking a closer look at.6

Isobutyl isobutyrate, isoamyl isobutyrate, and 2-methylbutyl isobutyrate (2MIB) are the three primary studied hop-derived esters in hop-forward beers. Together, these esters have a fruity apple and apricot-like flavors. Of the three, it’s likely of the three esters, 2MIB is the most dominant component. All three of these hop-derived esters have been found to decrease in concentration during wort boiling, suggesting that whirlpooling hops high in these esters is the best way to encourage them to get into the fermenter. 

In a study that looked at hop-derived esters concentrations in final beer from late hopped wort testing 42 different hop varieties (with a dosage rate of just 0.8 grams/liter (~0.25 lbs/bbl)) gives us a good look at which hops might be best used in the whirlpool. I wish the paper used higher late-hopping rates as we typically whirlpool closer to 7 grams/liter (~2 lbs/bbl), which is about nine times greater than the study. It seems logical to conclude that the results below would be increased at the higher hopping rates, likely getting above the threshold. 

Of all of the 42 hops tested, those used for late hopping with the most hop-derived esters from branch-chained alcohols of isobutyl isobutyrate, isoamyl isobutyrate in the finished beer were Nelson Sauvin, Amarillo, and Citra. The hops that tested the highest in the most dominant hop-derived ester 2MIB (in order of highest) Southern Cross, Pacific Jade, Vic Secret, Bravo, and Polaris. Of these, only Southern Cross and Pacific Jade were in concentrations above the threshold (78 ug/L), but the rest were very close. 7 

How might fermentation and yeast strains impact these hop-derived esters? In a 2020 paper, beers were brewed with Simcoe, Citra, and Mosaic in the whirlpool and tested for final concentrations of the various isobutryic esters across multiple yeast strains and found that SafAle S33 and SafAle K97 retained the highest concentrations (US-05 had the lowest levels) of isobutryic esters.8 None of the yeast strains appeared to produce beers with 2MIB above its threshold. An interesting follow-up would be heavily whirlpool hopping with varieties like Southern Cross or Huell Melon with SafAle S33 or K97. 

It’s possible the reduction in isobutyric esters during fermentation is the result of transesterifications of hop oil esters. In other words, these esters could be biotransformed into other aromatic compounds during fermentation, similar to geraniol converting beta-citronellol. I look forward to learning more about this area! 

If we want to try and push these fruity apricot-like hop-derived esters in our beer, we might want to experiment with hops that have shown to produce beers with higher concentrations and combine them with yeast strains that also test high in the final beer. For example, it might be worth trying out yeast strains such as SafAle 33 or Safale K97 for primary ferment (combined or even copitched with other strains) paired with heavy whirlpool additions with hops like Southern Cross, Pacific Jade, Vic Secret, Bravo, and Polaris (to push 2MIB specifically). 

Hop-Derived Branched-chain Fatty Acids 

Separate from the hop-derived esters looked at above coming from branch-chained alcohols, those coming from branched-chained fatty acids have also been found to be important compounds to keep in mind when considering hot-side hop varieties. During fermentation, hop-derived fatty acids can be esterified into three important esters (ethyl isobutyrate, ethyl isovalerate, and ethyl 2-methybutyrate) that can contribute to the flavor and aroma of beer because of their very low taste thresholds (as seen below). 

Thresholds in Beer:
ethyl isobutyrate: 6.3 ug/L
ethyl 2-methylbutrate: 1.1 ug/L
ethyl isovalerate: 2.0 ug/L 

Where do these fatty acids from hops come from? Age on hops can increase the concentration of fatty acids, as it’s been found that the oxidative-degradation of hop bitter acids results in the formation of branched-chain fatty acids. It’s assumed that hops with higher concentrations of alpha-acids contain higher levels of these fatty acids. 

During fermentation, these fatty acids can be esterified to fruity ethyl esters described above (isobutyl isobutyrate, isoamyl isobutyrate and 2-methylbutyl isobutyrate (2MIB)) through biotransformation. But the acids themselves that remain in the beer might also play a role, mainly through the synergy of the fatty acids with monoterpene alcohols. 

A 2019 paper found that hop-derived fatty acids in beer can strongly enhance the perception and intensity of monoterpene alcohols. Specifically, that paper found (through triangle test with a trained panel) that the addition of 2-methylbutric acid could enhance the “tropical” characteristic of beer and isovaleric acid could increase the “fruity” characterisic of beer. Here the acids were added to a solution designed to mimic a Citra hopped beer in terms of monoterpene alcohol concentrations.9

The above paper went on to test 12 different hop varieties for final beer concentrations of these hop-derived fatty acids. Of the hops tested, Apollo and Bravo both tested as some of the highest producers of these two important synergistic fatty acids (2-methylbutric acid and isovaleric). Because these acids can increase during aging, it might be worth whirlpooling with slightly aged Bravo or Apollo when trying to enhance the tropical and fruity characteristic of hot-side saturated flavor through synergy with monoterpene alcohols. 

To ensure I understood the research correctly and to try and get some applicable recommendations from the findings, I reached out to Kiyoshi Takoi, the sole author of the study and he agreed with the idea of experimenting with Apollo and Bravo specifically. Takoi also suggested that when the alpha-acid contents in hops increase, the amounts of these branched-chain fatty acids derived from hops also increase. So, high alpha/super alpha hops could increase the amounts of fatty acids in wort and finished beer.

Knowing that if we slightly age hops (like Bravo and Appollo) to increase the fatty acids, I asked how long and at what temperature brewers should keep them. Takoi suggested that to increase these fatty acids in hops; the storage condition should be 68°F-86°F(20-30°C) for about one year. He also noted that dried whole cone hops might age better than hop pellets for these purposes. Hop pellets should be stored at room temperature under open-air conditions. 

As hard as it might be for brewers to do, it might be worth cutting a few wholes in bags of Apollo or Bravo and leave them at ambient before using them in the whirlpool. However, I wouldn’t throw all your eggs in one basket as it makes sense to me to try and encourage as many diverse sets of hop compounds into the fermenter as possible. Because of this, I’d also pair the aged hops with other varieties mentioned in the study that are high in monoterpene alcohols, thiols, or hop-derived esters.  

Of the hop-derived fatty acids coming from hops, 2-methylbutyric acid has been tested to increase during fermentation, whereas isobutyric acid and isovaleric acid have been found to decrease. A paper looking at 40 different hop varieties and the final concentrations of 2-methylbutyric acid found that beers made with Huell Melon had the highest levels followed by Ekuanot, Apollo, and Polaris. 10

Before moving on to thiols – to recap the studies so far – whirlpooling hoppy beers with hops high in monoterpene alcohols (like linalool and geraniol) can increase hot-side hop flavor. These can be hop varieties like Bravo, Brewers Gold, Centennial, Columbus, Ekuanot, Mosaic, and Simcoe.  

Hop-derived esters coming from whirlpool hops can impact the final flavor of beer, and hops that have tested on the higher end were Southern Cross, Pacific Jade, Vic Secret, Bravo, and Polaris. Yeast strains can impact the concentrations of the important hop-derived esters, and two strains that may retain them the most are SafAle S33 or K97. 

Hop-derived fatty acids, which are formed on slightly aged hops, can have an important synergistic impact on the important monoterpene alcohols, by amplifying their perception. Slightly aging hops can increase these fatty acids. Hop-derived fatty acids can also impact beer if they are esterified during fermentation into fruity, low threshold esters. 

Polyfunctional Thiols 

Polyfunctional thiols are the portion of a hop’s sulfur compounds that can be desirable in beer because of their potential for intense fruity flavors. 

The main thiols studied in hops (and in wines) are 3-mercaptohexanol (3MH), 4-mercapto-4-methylpentan-2-one (4MMP) and 3-mercaptohexyl acetate (3MHA), which is converted from 3MH.  

4MMP has a blackcurrant and catty aroma (at high concentrations). Taste threshold 1.5 ng/L
3MH has a grapefruit/rhubarb flavor. Taste threshold 55 ng/L.
3MHA has a passionfruit flavor. Taste threshold 4 ng/L.11

Focusing on how these fruity hop-derived thiols are impacted by the boil, a study tested thiol levels throughout a boil using Simcoe. The authors found that during a 60-minute boil at 212ºF (100°C), 4MMP content decreased 91% and 23% after heating to just 194ºF (90°C). On the other hand, 3MH increased threefold at 212ºF (100°C) than at 90ºF (32°C), which suggests that not only is this fruity thiol preserved during the boil but enhanced! 

When it comes to trying to push hot-side saturated hot flavor, it makes sense to pay closer attention to the 3MH levels of hops and potentially use them in the whirlpool and even for bittering. If you are trying to push 4MMP levels, late hopping at cooler temperatures is likely best to encourage the thiol to get into the fermenter. 

Again, synergy appears to be an important part of why these fruity thiols are important hot-side compounds. A paper put 4MMP (blackcurrant) thiol to the test with monoterpene alcohols (linalool, geraniol, and β-citronellol). The results showed that 4MMP did have an additive impact with monoterpene alcohols and that only 1.2 ng/L was needed for this effect to occur (which is close to it’s threshold value). Specifically, the synergy between 4MMP and linalool, geraniol, and β-citronellol enhanced tropical characteristics. Using hops rich in both 4MMP and monoterpene alcohols could result in a more perceived intense tropical flavor. This process is similar to how the hop-derived fatty acids above intensified monoterpene alcohols, showing how complex hot-side hop flavor can be! 

Survivables

I did my best above to layout what hot-side compounds might be contributing the most to fruity flavors in beer (monoterpene alcohols, hop-derived esters and acids, and thiols). Whether it’s through synergy or being in the beer above a sensory threshold, the compounds above are being tested to be in packaged beer and key drivers to hot-side hop flavor. 

Finally, this brings us to the most recent research in this area from Yakima Chief Hops. If we know what compounds are likely the most important to hot-side flavor, why not test which hops have the highest combination of all these compounds? This is exactly what Yakima Chief did and coined these compounds as “survivables.” I like it! 

Below are the results of Yakima Chief’s study12 (and one of the more exciting hop charts I’ve seen in a while). Testing hops for a combination of monoterpene alcohols (linalool and geraniol), thiols (3MH), and hop-derived ester (2MIB), you can see why I was likely so bullish on Idaho 7! Idaho 7 leads the pack when it comes to having the most of these survivable compounds and why it’s likely such a great hot-side hop. Not far behind are Mosaic, Bravo, Citra, and Millennium, Mount Hood, Euaknot, and Simcoe. Yakima Chief is using the research to put together a blend of Cryo called TRI-2304CR, that will be a blend of hops that are loaded with these survivables! You can check out the full presentation on their webinars website

Conclusion

Hopefully, this post helps inspire some new experimentation when it comes to packing in as much hot-side hop flavor as possible into your beers! 

Below is a list of the hops that came up in the research that may be great to play with in the whirlpool. Because hot-side flavor is so complex, it makes sense to try and blend these varieties to push as much sensory explosion as possible! I’m curious to hear which hop, or combination of hops, you have the most luck with to push pre-dry-hop flavor. 

  • Idaho 7
  • Bravo
  • Brewers Gold
  • Centennial
  • Citra
  • Columbus 
  • Ekuanot
  • Mosaic
  • Simcoe
  • Southern Cross
  • Pacific Jade
  • Vic Secret
  • Polaris 
  • Huell Melon
  • Millennium
  • Mount Hood

 

Key Points 

  • Sampling your beer pre-dry hop is one of the best ways to determine how well your hot-side hops performed. I try to make this a habit.  
  • Monoterpene alcohols like linalool and geraniol are consistently mentioned as key components of determining hop flavor intensity. To push hop-derived monoterpene alcohols in beer, lowering the temperature of the whirlpool to 185°F (85°C) may be worth exploring. 
  • Hop-derived esters help to give beer fruity apple and apricot-like flavors and are likely retained at higher concentrations when added to the whirlpool. 
  • Hop-derived fatty acids can impact the flavor and aroma of beer through the esterification processes (which have low thresholds) and synergy. Specifically, the acids can amplify the sensory experience of monoterpene alcohols. 
  • Slightly aging your hops (especially Bravo and Apollo) and using them in a blend with other great hot-side hops may be a good way to build in hot-side flavor complexity. 
  • Hop-derived thiols have low sensory thresholds, but can contribute to hop fruity hop flavor synergistically and on their own. 
  • The thiol 3MH (grapefruit-like) can increase during the boil, but 4MMP is likely retained in higher concentrations during cooler whirlpool conditions.  
  • Hops high in a combination of all these “survivables” may be some of the best to experiment with in the whirlpool. 

    Footnotes

    1. Mitter, W., & Steiner, S. (2009). Annual fluctuations in hop quality – options for adjustment in the brewhouse. Brauwelt International, 36-37/
    2. Weidenhamer, J. D., Macias, F. A., Ficsher, N. H., & Williamson, G. B. (1993). Just how insoluble are monoterpenes? Journal of Chemical Ecology, 19(8), 1799-1807.
    3. Sharp, D., Qian, Y., Shellhammer, G., & Shellhammer, T. (2017). Contributions of Select Hopping Regimes to the Terpenoid Content and Hop Aroma Profile of Ale and Lager Beers. Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-2017-2144-01
    4. Inui, T. (n.d.). Study on the attractive hop aroma for beer. Speech presented at World Brewing Congress 2012, Portland, OR.
    5. Gahr, A., ; Forster, A; De Clippelleer, J. and Van Opstaele, F.: Reproducibility trials in a research brewery and effects on the evaluation of hop substances in beer. Part 3: transfer rates of aroma compounds from hops to beer and their aging behavior, BrewingScience, 72(2019), n. 11/12, pp. 217-227
    6. Takoi, K.; Itoga, Y,; Takayanagi, J.; Kosugi, T.; Shi oi, T.; Nakamura, T. and Watari, J.: Screening of geraniol-rich flavor hop and interesting behavior of beta-citronellol during fermentation under various hop-addition timings. J.Am. Soc. Brew. Chem., 72 (2014), no. 1, pp.22-29.
    7. Takoi,, K., et al. Behavior of Hop-Derived Branched-Chain Esters During Fermentation and Unique Characteristics of Huell Melon and Ekuanot (HBC3666) Hops, vol. 71, 2018.
    8. Opstaele, F. V., Rouck, G. D., Janssens, P., & Montandon, G. (2020). An exploratory study on the impact of the yeast strain on hop flavour expression in heavily hopped beers: New England IPA. BrewingScience, 73.
    9. Takoi, K. “Behavior of Hop-Derived Branched-Chain Fatty Acids during Fermentation and Their Sensory Effect on Hopped Beer Flavours.” BrewingScience, vol. 72, 2019.
    10. Takoi,, K., et al. Behavior of Hop-Derived Branched-Chain Esters During Fermentation and Unique Characteristics of Huell Melon and Ekuanot (HBC3666) Hops, vol. 71, 2018.
    11. Reglitz, K.; Lemke, N.; Hanke, S. and Steinhaus, M.: On the behavior of the important hop odorant 4-mercapto-4-methylpentan-2-one (4MMP) during dry Hopping and during storage of dry hopped beer, BrewingScience, 71 (2018), no. 11/12, pp. 96-99.
    12. Tielkemeier, Spencer. Sulfur: The Next Aroma Frontier, Yakima Chief Hops, 2020.

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