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. 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. In fact, humulinones are one of the most abundant oxidized hop acids in aged hops.

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. 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.” 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.” 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). 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!

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).

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!

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. 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. 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. 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.

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. 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. 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)

