Oxidized alpha acids called humulinones are found on hops and are extremely soluble in beer. Because humulinones are approximately 66% as bitter as iso-alpha-acids, they can potentially increase a beer’s bitterness during dry hopping. 

What Are Humulinones

The three alpha acids in hops that contribute to a beer’s bitterness are cohumulone, humulone, and adhumulone which isomerize when boiled into the major contributors of beer’s bitter taste. Specifically, these three acids isomerize into six iso alpha acids which are trans-isocohumulone, trans-isohumulone, trans-isoadhumulone, cis-isocohumulone, cis-isohumulone, and cis-isoadhumulone.1 However, hops also contain small amounts of oxidized alpha-acids known as humulinones (as well as oxidized beta-acids known as hulupones) which form via spontaneous peroxidation of the alpha acids and may also play a role in shaping beer bitterness.2 In fact, humulinones are one of the most abundant oxidized hop acids in aged hops.3

Humulinones are similar in molecular structure to iso-alpha-acids, but they have an extra hydroxyl group that makes them more polar than iso-alpha-acids.4 If something is more polar this means it is more water soluble, or in this case more soluble in beer. So it’s actually easier to get humulinones into beer than alpha acids.

Humulinones in Literature

Our first hint as to why any of this matters goes all the way back to 1955 when humulinones were described as being “bitter.”5 A follow-up study in 1965 described humuliones as a substance in hops which are not alpha-acids but which have bittering power and is present in beer in small amounts that will have a “marginal effect on flavor.” When introduced to beer (through hops), the study continues, it gives a “harsh lingering bitterness.”6 For brewing purposes, the first study to put an actual figure to the bittering potential of humuliones came in 1964, when it was found that humuliones are 35% as bitter as iso-alpha-acids (although this figure is later studied and increased).7 So over 50 years ago it was already established there was a measured bittering potential of humulinones which is found on hops in a form of oxidized alpha-acids. Although there wasn’t always complete agreement that humuliones was actually present in beer originally, some took the position that studying humulinoes is “purely academic since humulinone is hardly bitter and does not occur in beer or in hops,” which was clearly later found untrue, but still interesting to see how beer science evolves!8

An interesting study in 1957 looked specifically at adding humulinone directly to beers at various stages (not in hop form) and compared this to adding isohumulone directly to beers. The results also showed a lowering bittering potential of humulinone in comparison. As well as shed some light on how much humulinone is needed in beers to achieve different taste results (summarized results in chart below).9

Humulinone Iso-Humulone
Taste Perception
Sweet 27 15
Normal 30 27
Harsh 40 30

In final beer measured in mg. per kg. *1 mg. per kg is equivalent to 1 ppm

Humulinone Concentration in Hops

So how much humulinones are actually in hops? Baled hops contain less humulinone then pelletized hops, about 0.3% for baled to approximately 0.5% or more with pellets. While it doesn’t seem clear yet exactly why pellet hops contain more humulinones, there is an idea that because only about 10-20% of the lupulin glands are broken when baling hops this is in stark contrast to pelletized hops where 100% of the lupulin glands are broken, which may enhance the oxidation of the alpha-acids. The low overall concentration levels of humulinones will contribute very little to a beer’s bitterness when hops are utilized in the boil or whirlpool, but when added as dry hops is where we start to see why all of this matters!10

Because of the high water solubility of humulinones described above, when dry-hopping at relatively high rates nearly all of the humulinone will dissolve into the beer. Specifically, the Maye study mentioned above found that beers that are dry hopped at .5 to 2+ lbs/barrel, which is equivalent to 35 grams to 142 grams on the homebrew scale, will see high concentrations of humulinones in the final beer. On the other hand, beers that are solely kettle hopped contain little humulinone (less than 2ppm). To show the potential of humulinone concentration from dry hopping the Maye study looked 29 commercial IPAs and found they contained anywhere from 3 to 24 ppm!

Although I previously mentioned that humulinones were 35% as bitter as iso-alpha-acids, it was later determined that they are actually closer to 66% as bitter as iso-alpha-acids.11 So 1 ppm of humulinone would equal .66 IBUs. So you can see why it’s important to look closer at the dry hopping and the potential for increased IBUs when we know how soluble they are in beer!

The Maye study also suggests that the bitterness from the humulinones is likely “smoother” because they are more polar than iso-alpha-acids suggesting that humulinones should not “stick or linger on the tongue as long as iso-alpha-acids.” So not only are humulinones slightly less bitter in terms of IBUs, it seems their role in the bitterness is less intense.

An Oregon State University study using humulinone extracts dosed into unhopped lager beer at 28 mg/L and 21 mg/L and evaluated by a panel of 10 trained tasters found that humulinone bitterness was “lower in peak bitterness, and less medicinal, and shorter in duration” compared to beers dosed with hulupones.12 Although compared directly to hulupones (and not iso-alpha-acids), it does shed a little more light on the delicate bittering nature of humulinones.

Testing Humulinone Bitterness 

Although it appears easy to conclude that dry hopping leads to a pretty sizeable increase in IBUs through humulinones, this wasn’t necessarily always the case when tested. Looking more closely at the Maye study that I found fascinating, an experiment was performed with a low IBU beer (8.6 IBU) and high IBU beer (48 IBU) and dry hopped with centennial pellets for 5 days at rates of 0, 0.5, 1.0, and 2.0 lbs/bbbl. They found that the lower the dosage rate of hops the higher the utilization of humulinone with the 0.5 lbs/bll. beer had nearly 98% of the humulinone dissolved into both the low and high IBU beer! Where the highest dry hop dosage was still in the upper 80’s for both beers.

Amazingly in the high IBU beer, as the humulinone in the beer increased the iso-alpha-acid content actually decreased (at 2.0 lbs/bll there was a 38% reduction in iso-alpha-acids). So in the high IBU case, although the beer had increased IBUs from the humulinones the reduction in iso-alpha-acids basically canceled it out. Now, in the low IBU beer this wasn’t the case. Here they found as the humulinones increased from the dry hopping, the reduction in iso-alpha-acids was much smaller with only a 13% reduction in the 2.0 lbs/bll beer. So translating this to a homebrew 5 gallon batch terminology, when the low 8.6 IBU beer was dry hopped with 142 grams of centennial there was a net increase in IBUs of 18.5 IBUs!

I couldn’t find a precise reason why there was a decrease in iso-alpha-acids when the high IBU beer was dry hopped, thus increasing the humulinone content. However, it has been noted that the presence of hop particles in the boil apparently promoted the removal of iso-alpha-acids during wort cooling.13 Now, this is of course a completely different thing (boil vs. dry hopping), but maybe it’s possible the hop particles themselves in the dry hop have the potential to strip iso-alpha-acids in the higher IBU beer simply because there is more available for them to strip (just a thought).  

Dry Hopping and pH

There’s more than just humulinones that can have an impact on bitterness perception when it comes to dry hopping to keep in mind. The same Maye study found in all of their dry hopping experiments there was an increase in pH as the dry hops increased. Specifically, they saw a pH rise of about 0.14 pH units per pound of hop pellets used. So again in homebrew terms if you dry hop with 142 grams (about 5 ounces) you might see an increase in pH of 0.28. This is important because a it has been found that beers with the same iso-alpha-acid content actually tasted more bitter as the pH increased.14

Hop Storage Index & Humulinone Content

The Maye study also found a relationship between the humulinone concentration of hops and the hop storage index (HSI). The hop storage index is a used to estimate the losses of alpha and beta acids during aging. Specifically, the HSI is determined from the ultraviolet absorbance measured in the spectophotometric analysis of hops.15 Essentially, this is the measurement of the deterioration of hop bitter acids. Maye found that the higher the HSI the more humulinone content a hop had. This would seem to make sense that as the HSI increases in hops (less stable varieties) there is an increase in oxidized alpha acids, which we now know is what humulinones are.

My first thought after reading the information on HSI and humulinone concentration was, what if you aged the hops a little on purpose to increase the HSI content. However, the storage times would be long and conditions needed would be too detrimental to the hop to achieve a significant rise in HSI. It was found on one hop variety that when kept in a cold place under anaerobic conditions had little change in HSI. However, storage under the most adverse conditions tested (air access, temperature +20C) resulted in HSI increase of 0.13–0.19 after 6 months; after 1 year, the total HSI values increased 0.69–1.22.16 However, It doesn’t seem to take much time at all for humulinones to form on fresh hops. For example, in just 14 days fresh Galena hops (stored under vacuum and cold) the Maye study found had a concentration of 0.47% w/w humulinones!

Below is a list of hops listed by HSI. The closer to the top of the list (higher HSI or less stable hop), the higher the likely humulinone content and dry hop bittering potential.

 

Hops Varities by Hop Storage Index (HSI)17

Brewing a Beer Based on the Information

I find the academic literature in brewing fascinating, but I also like to try to take the great work done the authors and try to put the data into practice on the homebrew scale. For this post I did exactly this, I decided to try to brew a hop forward beer and see if I could create bitterness with humulinones through dry hopping! After brewing, I shipped a sample of the beer to a lab to get the IBUs tested. It is my understanding of the studies that spectrophotometric IBU test, which is what the lab used on my beer, will in fact detect an IBU increase from the humulinones. However, it would take a HPLC analysis to measure the iso-alpha-acids and humulinones concentrations separately.

I went with a sessionable hoppy beer recipe with plenty of oats, which I found has many possible benefits in a previous post looking at the brewing literature. I learned I needed to keep the IBU total low in order to try to get a bitterness benefit from the dry hopping, so I went with 10 IBUs. Because I still wanted a strong hop flavor in this beer, I decided to get all the IBUs from a post boil 90 minute steep hop addition. A recent 2010 study, which I’ll likely dissect a little more closely in a future post, found that beer’s brewed with “longer postboil residence of kettle hop additions led to more hop flavor and aroma” concluding that “longer postboil residence, approaching 90 min, is far better than times shorter than 60 min.”18 So because my total hop in the boil pot was so low (only 16 grams in a 5 gallon batch!), I decided this might be the best way to get my biggest hop flavor punch from the small addition.

Because the literature suggested that increased humulinone concentration in beer is required than iso-alpha-acids to achieve “normal” bittering, suggesting that lower concentrations might even come across as “sweet”, I decided to use a couple hop varieties I thought might take advantage of this sweet/lower bittered hoppy beer. Cluster, one of the oldest U.S. varieties grown in the U.S. and not often looked at as a for dry hopping, but on it’s own I find the descriptions very intriguing (strawberry, creamy, and apricot).19 Combining Cluster with Mandarina Bavaria, which has said to have strawberry and pineapple characteristics, I was hoping the two would combine for an interesting sweet fruit aroma for this creamy oat and chloride heavy low ABV base!20

Recipe Details

Batch Size Boil Time IBU Hop Stand SRM Mash pH Beer pH Original Gravity Final Gravity ABV
5.5 gal 45 min 10.1 IBUs 90 Minutes 3.0 SRM 5.51 4.45 1.049 1.013 4.70%

Water Profile

Gypsum (S04) Calcium Chloride (CaCl2) S04:CaCI2 Ratio
74.7 ppm 163.4 ppm 0.46:1

Fermentables

Amt Name %
7 lbs Organic Brewers Malt (Briess) (1.8 SRM) 56.00%
4 lbs Oats, Flaked (1.0 SRM) 32.00%
1 lbs 8.0 oz Malted Spelt (BESTMALZ) (2.4 SRM) 12.00%

Mash

Step Temperature Time
Mash In 152°F 60 min

Hops in Boil/Whirlpool

Amt Name %/IBU
16.00 g Citra Steep/Whirlpool 90.0 min 10.1 IBUs

Dry Hops

Amt Name
84.00 g Cluster – See Notes for Specifics
84.00 g Mandarina Bavaria – See Notes for Specifics

Misc

Amt Name Type
1.00 tsp Irish Moss (Boil 15.0 mins) Fining
0.50 tsp Yeast Nutrient (Boil 15.0 mins) Other

Yeast

Lab Yeast Name
RVA Manchester Ale (Boddington)

Brewing Notes

  • Fermentation was at a constant 68F
  • Day 2 of fermentation –  Top cropped yeast for further use and dry hopped with 56g Mandarina Bavaria and 28g Cluster (putting the higher total oil hop in the fermenter w/ active fermentation in hopes of more remaining in final beer).
  • Day 9 of fermentation – Kegged with 56g Cluster and 28g Mandarina Bavaria (left at room temperature for 24 hours then put in keezer). Hops remained in keg until it kicked.

IBU Lab Test Results

The Maye study tested the bittering of humulinones up to 2 lbs/barrel in dry hops (approx 142 grams), so I decided to go even higher and dry hopped with 168 grams (6 ounces) in a 5 gallon batch. I don’t know what the HSI of Mandarina Bavaria is, but in the list above I can see that Cluster’s HSI is 16, which means it likely bittering potential from humulinones is lower than most hops.

As you can see from the lab test below, this particular beer did see a fairly drastic increase in estimated IBUs from the dry hops of over 33 IBUs, which it seems likely is in part from the increase in humulinones. However, because humulinones are less bitter than iso-alpha-acids, it should taste less bitter than the 44 IBU figure implies. This is even more interesting when considering if I would have used a higher HSI hop like Columbus or Cascade the IBU number could of even been higher.

I find this incredibly interesting, not only can you get a substantial amount of bitterness from dry hops (when the initial beer’s IBU levels are low), you can also get a smoother less intense bitterness via the humulinone concentration. It’s also interesting to learn based on this post that it’s likely this beer’s final pH increased by more than 0.28 units from the heavy dry hop addition.

Estimated IBUs Tested IBUs Est. Increase In Part from Humulinones
10.1 44 +33.9

Tasting Notes

For an experiment beer, I really like how this turned out! The hop combination is definitely different than more traditional American IPA hop builds, as its much sweeter with a jam like condensed fruit aroma. Apricot and strawberries and red raspberries come to mind. The aroma reminds me a little of the jelly filling in donuts or even the apricot flavored syrup you can get at Perkins. For such a small hop steep addition, I definitely still get hop flavor in this beer. The mouthfeel is soft as expected with this yeast strain, oats, and chloride. At a DC Homebrewer’s meeting a few even thought it was a lactose IPA, along the lines of Milkshake IPA from Tired Hands, which I can totally see how you’d get this impression with a combination of the sweet fruit jam like aroma with a touch of vanilla that this yeast strains seems to produce and the creamy oat NEIPA style mouthfeel. It taste dryer than the final gravity would suggest. Great head on first pour, but dissipates fairly quickly (30% oats is a likely the culprit).

As far as the bitterness, it’s got a smooth low supporting bitterness. You can tell it’s a lower bittered beer, but I’d be surprised if you told me it only had 16 grams of hops total in the boil! Although it may just be in my head from the research, the bitterness does seem apparent and first but disappears quickly after swallowing, which would agree with the smoother less lingering bitterness from the humulinones. I had multiple people try this beer without them knowing anything about it and only one said the bitterness seemed low. Although I didn’t really know how this would turn out, for a low ABV session beer like this, the IBUs of 44 with help from the dry hops seem about right for an easy drinking flavorful beer. I’m wondering if this approach to bittereing isn’t boosting the perceived hop flavor a little. To me it seems higher IBU more aggressive bittered beers seem to hide the flavor a little (no data to back this up however).

Final Thoughts

  • Humulinones are found in greater amounts on pellets than leaf hops and seem to also be linked closely to a hops storage index (HSI).
  • Humulinones are about 66% as bitter as iso-alpha-acids (1 ppm of humulinone would equal .66 IBUs).
  • Humulinones are more polar than iso-alpha-acids, which means they more soluble in beer.
  • Humulinones may give a beer a smoother less intense bitterness.
  • Humulinones seem to only increase a beer’s IBU total if the original beer was low in IBUs as well.
  • Dry hopping raises the pH about 0.14 pH units per pound of hop pellets used and the higher pH actually increases the bitterness perception in beers.

Footnotes

  1. Intelmann, D.; Batram, C,;Kuhn, Ch.;Haseleu, G.;Meyerhof, M.;Hofmann, T. Three TAS2R bitter taste receptors mediate the psychophysical responses to bitter compounds of hops (Humulus lupulus L.) and beer. Chem. Percept. 2009
  2. 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
  3. Dusek, M., Olsovksa, J., Krofta, K., Jurkova, M., and Mikyska, A. Qualitative determination of Beta-acids and their transformation production in beer and hop using HR/AM-LC-MS/MS. J. Agric. Food Chem. 2014
  4. Preparation of Dicyclohexylamine Humulinones and Dicyclohexylamine Hulupones. (2016). Journal of the American Society of Brewing Chemists ASBC. doi:10.1094/asbcj-2016-1127-01

  5. Cook, A. H., Howard G.A., and Slater, C.A. The chemistry of hop constituents VIII. Oxidation of humulone and cohumulone. J. Inst. Brew. 1955
  6. Hudson, J. R. (1965). The Rationalization Of Hop Utilization-A Review. Journal of the Institute of Brewing, 71(6), 482-489. doi:10.1002/j.2050-0416.1965.tb02076.x

  7. Whitear, A.L., and Hudson, J.R. Hop resins and beer flavor III: Hop resins in beer J. Inst. Brew. 1964
  8. Verzele, M. (1986). 100 Years Of Hop Chemistry And Its Relevance To Brewing. Journal of the Institute of Brewing, 92(1), 32-48. doi:10.1002/j.2050-0416.1986.tb04372.x
  9. Howard, G. A., & Slater, C. A. (1957). Utilization Of Humulone And Cohumulone In Brewing. Journal of the Institute of Brewing, 63(6), 478-482. doi:10.1002/j.2050-0416.1957.tb06287.x

  10. Humulinone Formation in Hops and Hop Pellets and Its Implications for Dry Hopped Beers. (2016). Technical Quarterly TQ. doi:10.1094/tq-53-1-0227-01
  11. Algazzali, V., and Shellhammer, T. Bitterness intensity of oxidized hop acids: Humulinones and hulupones. J. Am. Soc. Brew. Chem. 2016
  12. Peltz, M., Algazzali, V., & Shellhammer, T. (2016). Sensory bitterness quality of oxidized hop acids: Humulinones and hulupones [Abstract].
  13. Applications of High-Performance Liquid Chromatography in the Control of Beer Bitterness. (1986). Journal of the American Society of Brewing Chemists ASBCJ, 44. doi:10.1094/asbcj-44-0101

  14. Meilgaard, M., and Trolle, B. The utilization of hops in the brewhouse. Proc. 6th Congr. Eur. Brew. Conve., Copenhagane. 1957
  15. Hop Storage Index. (1979). Journal of the American Society of Brewing Chemists ASBCJ, 37. doi:10.1094/asbcj-37-0184
  16. Mikyška, A., & Krofta, K. (2012). Assessment of changes in hop resins and polyphenols during long-term storage. J. Inst. Brew. Journal of the Institute of Brewing, 118(3), 269-279. doi:10.1002/jib.40
  17. Smith, B. (n.d.). BeerSmith (Version 2.2). Retrieved from http://beersmith.com/order/
  18. H. (2010). Maximizing Hop Aroma and Flavor Through Process Variables. Technical Quarterly TQ. doi:10.1094/tq-47-2-0623-01
  19. Barth, S. (2011). The hop aroma compendium: A flavour guide. Nürnberg: Carl.
  20. Barth, S. (2012). The hop aroma compendium: A flavour guide. Nürnberg: Carl.
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