Bitterness & The IBU – What’s It All About?

In the earlier days of the American India pale ale, bitterness was king. The more Bitterness Units (BU), the better. We didn’t have the hazy-juicys, we had bitter and hoppy, piney, ashy, resinous, maybe dank. Predominant West Coast IPAs began to evolve the palates of every cool craft beer junky into a bitter-beer-ophile. Bitterness addicts were made. One pound (0.45 kg) of hops per barrel not satisfying? Six to seven pounds (2.7–3.2 kg) of hops per barrel equated heaven on earth.

In my hop chemist — er, “Hop Queen” — days, a beer sample claiming to be over 500 BUs was sent to my attention. Was it possible to achieve this in beer? This beer was touted to be 658 BUs. In the myopic, well trained, chemist’s view, this brewer had achieved the impossible. 

If you internet search “Bitterness Unit” you will find a myriad of definitions. It is largely synonymous with “IBU” and often defined like this: “IBU is an abbreviation for the International Bitterness Units scale. IBUs measure the parts per million of isohumulone (isomerized alpha acids) found in a beer.”

Could this beer really be 650 ppm iso-alpha acid? Fact or fairy tale? This beer seemed to defeat the laws of solubility chemistry.

Under typical brewing conditions, at pH 5–5.5 and 212 °F (100 °C), alpha acid (AA) solubility maxes out at approximately 60 mg/L — that’s 60 ppm that could solubilize into beer and become isomerized-alpha acids (IAA) and only in an ideal world would this lead to 60 BUs. It would require 100% hop utilization (solubilized and converted).

Figure 1:  Isomerization of alpha acids to iso-alpha acids

Conversion of alpha acids to isomerized-alpha acids, ‘iso,’ (see Figure 1, below) requires that alpha acids be in their anionic form. Alpha acid pKas (a method used to indicate the strength of an acid) range from 5–5.5 (dependent on the analogue). This means that under typical wort conditions (pH 5–5.5) alpha acids are only partially in their anionic form. Another way to say it is that they are still partially protonated or “acidic” and they prefer to be in a more oil-type media. It’s possible to achieve 90% utilization under ideal conditions where wort pH is above pH 6 when alpha acids are in their anionic form and more soluble in water. In the presence of divalent metal cations (Mg2+, Ca2+, Zn2+) this reaction can be catalyzed to achieve maximum utilization. This is how pre-isomerized extracts are made, in a vessel, under ideal isomerization conditions. But for this to happen in the absolute correct ratio of metal cation to alpha acid in a polar wort medium is not likely.

Under typical brewing conditions a brewer must challenge two major hurdles: Extraction of alpha acids from the hops and losses due to hot break, fermentation, filtration, etc. Taking these two factors into account, a brewer is likely to achieve only about 50% extraction of available alpha acids, and will suffer another 10–15% loss due to hot break, trub, fermentation, and filtration losses. So, does 30% sound reliable? Maybe 40%? According to Kunze et al. (2004) when using whole leaf hops the relative distribution of bitter substances during brewing results in a transfer of just about 20% of the bittering compounds in the finished beer. Round it up to 30% for pellets? Maybe.

Considering limitations to alpha acid solubility, isomerization kinetics at typical wort pH and interactions with other wort constituents — is it really possible to get 658 IBUs? And if so, how much hops would that equate to? If you do the straight math that is roughly 6 lbs. (2.7 kg) per barrel of whole leaf high alpha hops (18% alpha and 20% utilization) going into the kettle. OK. That sounds . . . feasible? Yeah. Feasible. Good luck with that boil and have fun cleaning out that kettle. 

If you move to a CO2 hop extract as high as 55% alpha acids, 35–40% utilization, then about 1 lb. (0.45 kg) of CO2 extract per barrel would suffice. This sounds reasonable, but it still defeats published solubility limits, right? I actually wasn’t able to find a published number beyond formulation chemistry. I have written down in some old notes that say 84 ppm of iso after isomerization, but only under ideal conditions, high pH, high heat, divalent cation inclusion. Logically, higher alcohol could get you closer. Maybe alter your residual extract, throw in more zinc, but not too much to be toxic to yeast. Stressed that I couldn’t find an actual number, I contacted longtime hop scientist for MillerCoors Dr. Patrick Ting, because, if anyone has done anything with hops and never published it, it’s Dr. Ting. I asked him, what in his experience is the iso-alpha acid solubility limit in beer? 

“About 70–80 ppm,” he guessed. “At Miller I tried up to 100 ppm in beer and was not successful.”

I believe that this is where my number in my notes came from. But this is only a fraction of 658 BU. What the heck is going on? Was it just a lie? 

Here’s the grim ending to this fairy tale, a BU is not always equivalent to ppm iso-alpha acid. In fact in all of the research we conducted, and in any published literature, it is nearly never a 1:1 correlation. Depending on the hop products used and how they are used, the BU starts to diverge from ppm due to the presence of other molecules. This is true even for purified hop extracts such as Iso, Rho, and Tetra. 

So, what is the IBU and what’s it all about? And how bitter is an IBU? Because that’s the most important thing, right? 

Today the American Society of Brewing Chemists defines the BU (IBU) as a calculation: 

ABS @275 nm x 50 = BU

The BU method is a non-polar liquid-liquid solvent extraction method. The bittering compounds of hops or hop extracts and those dissolved in beer are extracted in a solvent, then acidified and the absorbance measured at 275 nm via a spectrophotometer. The absorbance is then multiplied by 50. In this method we assume that an IBU is about equivalent to ppm iso if about 70% of the bittering substances are iso.

How did we get here?

Long ago in 1885, Hayduck demonstrated that hop soft resins could be precipitated with lead acetate in methyl alcoholic solution (Pyman). He discovered two fractions. The first fraction that precipitated with lead as a salt was called the alpha-fraction, because it precipitated first. The second fraction was called beta because, you guessed it, it precipitated second.  Hayduck was mostly concerned with the antiseptic properties of hops, of which he had many theories, but his interest subsequently lead to the discovery of the alpha fraction of the soft resins. Thus we call them “alpha” because they were the first precipitated. The alpha acids were renamed humulones a few decades later based on the species name of Humulus lupulus.

Finally, in 1925, Wieland proposed a chemical structure that was fairly adequate. However, it wasn’t until 1952 when F.L. Rigby and J.L. Bethune reported the occurrence of alpha acid analogues as well as stereo-isomers of alpha acids (cis and trans – iso). They both also proposed methods to quantify these acids. In 1955, Rigby extracted the soft resins with solvent and measured their absorbance at 255 nm under basic conditions to approximate the bittering power of iso-alpha acids. Also in 1955, Morten Meilgaard (inventor of the flavor wheel) analyzed similar extracts at 275 nm, under neutral chemical conditions, to allow for approximation of all derived bittering compounds. You see, hop acids can absorb light at multiple wavelengths and depending on the pH of the media they are in, they will show maximum absorbance at specific, yet distinct, wavelengths.

The current ASBC Beer 23-A Beer Bitterness method was adapted in 1967 from Meilgaard’s method, but modified for analysis under acidic conditions. Under acidic conditions, all of the alpha and iso-alpha acids are protonated. Absorbance is read at 275 nm, which is one of the max absorbance wavelengths of the spectra at low pH. This method, over 50 years old, predates pelletization, CO2 extraction, as well as production of most commercially-available solvent extracts (Miller had already developed a version of Rho — but that’s another tale to tell), and was developed for analysis of beers that were typically less than 30 BUs that were not dry hopped. These facts are cause for concern to some brewers. Is the IBU still relevant? Is it still predictive of beer bitterness? 

Figure  2:  The ASBC Acidic Bitterness Unit method adapted from Meilgaard’s method at 275nm

Concern is warranted. Because this method is an exhaustive extraction of non-polar compounds, any component in the beer that is hydrophobic and absorbs light near 275 nm under acidic conditions will give a positive BU response. This includes iso-alpha acids, alpha acids, beta acids, oxidized alpha and beta products, phenolic acids and polyphenols, essential oils, and flavor components, as well as some amino acids. 

Overall the method is a compromise. Many components of hops and beer contribute to the IBU. Hop variety, kilning practice, time in storage, hop product form, addition rate, and timing each alter the chemical component contribution to the BU and more importantly perceived beer bitterness and bitterness quality. 

Surprisingly, very oxidized hops may give comparable BU levels as fresh hops, yet yield qualitative differences — as well as inferior foam — but maybe some positive flavor attributes (Peacock). Val Peacock, retired Anheuser-Busch hop guru and technical advisor to the Hop Quality Group, demonstrated this effect by aging fresh hops for 18 months under different storage conditions. As the hops aged under higher temperatures, their alpha acid content declined due to oxidation. However, resultant beers showed that brewing with aged hops with less than 1% alpha acid content produced a beer with 11 IBUs, and only 2.9 ppm iso-alpha acids, and were still bitter. Research that I have done concurs with this. Beers were brewed with pellets, extract, and spent hops (hop vegetative matter left over from CO2 extraction) from the same hop lot. Beers made with equivalent mass of spent hops (2% AA or less) and pellet hops (14%) yielded a beer of similar BUs yet different levels of IAA. The spent hop beer measured at 13 BUs and 3.5 ppm iso, vs. the pellet hopped beer at 14 BUs and 11 ppm iso. It turns out that brewing with old hops and spent hops can still yield “bitter” beer (they can also denote interesting and desirable flavors) and thus are more useful than some may believe. 

So, if the AA is so low in aged hops, why is the BU so high? And is the beer still bitter? Why?

What is Bitterness?

Let’s take a step back and look at “bitterness” and what affects bitterness in beer. Bitterness is a taste response that occurs when molecules hit Type II taste cell receptors on the tongue. A signal is sent to the brain and the brain acknowledges “bitter.” A person’s ability to perceive bitterness is affected by genetics as there are at least 25 different taste Type II receptors. Some of these receptors are specific while others are promiscuous. Each person’s tongue has a different number of these receptors and a person’s genetic sex type may play a role. A good pick up line for bitter-o-philes might be, “How promiscuous are your T2 cells?” 

Generally, women are better tasters than men. And young women are usually much more discerning. As we age, our response to bitterness declines and our diet can also influence or “train” our brain response to bitterness tolerance. There is also a heritage effect that is seen across populations where Caucasians are generally less sensitive to bitterness than Asians and African Americans. Although, there is a subgroup of super tasters who are literally disgusted by bitterness.

Add to this the effect of suppressants or enhancers in food matrices. Other molecules in food and beer can affect binding and thus perceived bitterness. In beer, increased malt color due to roast and caramelization can augment perceived bitterness. Alcohol may enhance or reduce the effect of bitterness depending on the matrix. Higher beer pH enhances bitterness and mineral content plays a role in perceived quality. Burtonization, or the addition of sulfate to brewing water, can yield a beer with crisp, clean bitterness. After all, Burton-on-Trent is the birthplace of the original IPA — the perfect example of a beer style derived from water conditions. 

Carbonation can broaden and enhance bitterness, however it can also act as a solvent in highly-carbonated, low bitterness beers to scrub the palate of oils. Aging of beer and oxidation generally leads to reduced bitterness perception or increased sweetness. High levels of hop oil may enhance bitterness, leading toward ”hop burn,” which can further be enhanced by the bitterness and astringency denoted by some polyphenols, especially due to prolonged dry hopping. 

The contribution of polyphenols from hops due to dry hopping could impact overall bitterness measurement and bitterness perception. Researchers at Oregon State looked at the contribution of polyphenols to bitterness, specifically the bitterness modifying properties of hop polyphenols derived from spent hop material (Shellhammer). Their 2008 study demonstrated that total polyphenol contents of 300 mg/L or less can influence lager beer BU values: A one unit BU increase was observed for every 15–20 mg/L total polyphenols. 

Oladukan et al. (2016) analyzed 34 commercial lager beers for their hop bitter acid, phenolic acid, and polyphenol contents. Beers expressed a range of phenolic content, with those high in polyphenolic profiles having “harsh” and “progressive” bitterness. While monomeric polyphenolics may be perceived as bitter, beers brewed with high hop-derived polyphenol content do not always display an equivalently augmented bitterness. This is likely due to polyphenol type as well as polymer chain length. 

Figure 3:  Structural difference between iso-alpha acids and humulinone

Actually, the biggest bitter impact derived from spent or aged hops comes from oxidized hop compounds, namely humulinones and hulupones (Intelmann et. el.). Humulinones are oxidized alpha acids that have a similar structure to iso-alpha acids (Figure 3) yet have an extra oxygen molecule. With about 60–70% bitterness as iso (Agazalli, et al.), humulinones are slightly less hydrophobic than iso, which makes them prone to higher extraction rates than iso in beer, especially in late and dry hopped beer. Humulinones are found in leaf hops as well as pellet hops 0.2–0.5% weight for weight and in my past life I would routinely run analysis on their content in spent hops, not just because they are bitter, but because they can lead to light struck character. 

Heavily hopped (dry and kettle) beers can have as much as 4 ppm humulinones. Typical lagers have 1 ppm or less, but may vary by hopping regime and hop product usage. Dry hopped beers have been measured to reach 24 ppm humulinones. Work by the research team at Hopsteiner suggests that as much as 87–98% of the humulinones found in hops can result in finished beer (Maye, Smith). 

Hahn, et al. (2018) concurs. Their work at Oregon State University examined the bitterness of more than 120 commercial beers using both sensorial and analytical techniques. Their results indicate that the BU measurement may be used to predict sensory bitterness, yet with a non-linear response. Their group has thus proposed an alternative approach to predict bitterness that includes factors such as isohumulone, humulinone, and ethanol concentration. However, humulinone quantity is not part of a hop COA (certificate of analysis), so this unknown variability in hop quality may throw a rather large wrench for this model of predicted hop-forward beer bitterness consistency. Unfortunately HPLC is required to analyze humulinone content in hops and in beers and not all processors or suppliers analyze for humulinone and most brewers do not have HPLC capability in their labs. The happy ending however, according to the Shellhammer group, is that the BU analysis does a relatively good job at predicting overall beer bitterness intensity, and I will add, as long as you understand the impact of other beer parameters on bitterness quality.

What is bitterness quality?

You may hear it referred to as “harmony” by German brewers. Bitterness quality is generally described in terms of bitterness duration, intensity, harsh vs. round, medicinal vs. sharp, bright vs. cloying, etc. That brings us back to those modifiers of bitterness. To understand a beer’s bitterness potential as well as how the overall bitterness may be perceived by a general group of beer consumers we must also take into account residual extract (sugar), alcohol, mineral content, haze contributors, as well as hopping practice. Hop variety, timing, form (e.g., pellet, extract, advanced hop product), as well as dose can all affect the balance of molecules in the beer matrix.

Even volatile components such as terpenes can affect bitterness quality. Yes, high hoppy aroma has shown to modify perceived bitterness in beers (Oladokun). A research group out of the UK hopped beers with three distinct hop varieties (Hersbrucker, East Kent Golding, and Zeus) to achieve beers of equivalent bitter levels. The beers had differing bitterness character. Hop aroma alters hop variety-derived beer bitterness character. A follow-up study by this same group at Nottingham looked at the effect of hop aroma top notes on bitterness quality of low (13 BU), medium (25 BU), and high (42 BU) bitter beers (2016). The beers dosed with 0, 245, or 490 mg/L of hop aroma extract were analyzed with and without nose clips in order to differentiate taste and aroma. Results indicate that the added aroma modified the intensity, character, as well as temporal profile of bitterness in beer. Depending on the dosing rate, nose clips or no nose clips, bitterness perception could be altered. The authors suggest that the addition of a hop aroma as an “over add” in beer can evoke trigeminal sensations in the oral cavity causing taste-aroma interactions that modify perceived bitterness: Increased lingering, intensity, and harshness. It’s a balance of hop aroma and taste bitterness levels that determine a beer’s perceived bitterness character. 

Other beer flavors (secondary metabolites) and aromas can alter perceived bitterness. Take for example a Pilsner Urquell that has a bitterness of about 39 BU. This is roughly the same as some infamous American pale ales, however Pilsner Urquell is generally perceived as lower in bitterness due to its all-malt backbone, soft brewing water, and the presence of diacetyl, which is part of the brand footprint. 

Did we crush your BU is king, go big or go home fairy tale yet? No fret, the good news is that despite the fact that an analytical BU isn’t always the same “BU,” the analysis is relatively good at predicting bitterness potential. Beyond potential, brewers must rely on both science and artistic expression to navigate the sensorial effects of brewing material and process variations to craft their iteration of a harmoniously hopped brew. 

The BU is still king in my book, despite the haze craze going on in the East, because as you now understand, creating a harmoniously bitter brew consistently is no small task.  

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Issue: October 2020