The myth of the 100+ IBU IPA

The myth of the 100+ IBU IPA
by Will Deutschman

Check out Will’s article on Beer Aging and Oxidation (tastes like cardboard)

We are a country that is all agog about IPAs right now. Regular IPAs has become the norm. No brewery worth its salt is without one, and for the craft beer consumer it has very nearly become the Bud Lite of the day. Now it seems people compete to stand at the top of the heap of the biggest, baddest, hoppiest IPA out there. It may have started out as a joke in a video, but octuple IPAs are a thing.

But, here’s the thing, they’re not. In fact, the whole IBU measure is grossly misused and if you walk into a pub/brewery that is advertising a 100+ IBU beer, you can be pretty certain they are full of it. Yes, there is a small chance they are for real, but unless they are using hop extracts to brew their beer, they’re full of it.

This seems to be a pretty bold claim to make. After all, who am I to tell them they are presenting ‘alternate facts’ about their beer, given that I have never been to their fine establishment, nor tasted their fine octuple IPAs? Nobody, really. Except that I know a thing or two about chemistry and where the IBU measurement comes from. Brace yourself – a bit of brewing science is coming your way!

So, what is an IBU? Well, it’s an acronym for International Bittering Unit. It’s a measure of the concentration of bittering compounds that are found in beer. One IBU is equivalent to 1 milligram of bittering compounds per liter of beer. So, where do these bittering compounds come from? Hops, of course. But, here’s the sticky part. Hops don’t actually have the compounds that make beer bitter. They only have the precursors (building blocks) of those. For those in the homebrew scene, you already know this. This is why you put in hops and boil them for 45-60 minutes when making your lovingly crafted homebrew. The heat in the boil kettle converts the precursor molecules into the bitterness producing compounds over time. For the more chemically inclined, an example of this reaction is shown below. It’s known as an ‘isomerization’ reaction, where compounds known as ‘alpha acids’ are converted into ‘iso-alpha acids’.

(By GolemXIV – Own work, CC BY-SA 3.0,

Names, complicated pictures, and scientific mumbo-jumbo aside, the concentration of iso-alpha acids is directly related to the bitterness of a beer. The more hops you put in a beer, the more alpha acids there are. The more alpha acids you start with, the more iso-alpha acids you can produce. The more iso-alpha acids, the more bitter the beer and the bigger IBU number you get to put on that chalkboard over the taps.

But, here’s where the rub comes. Most brewers estimate their bitterness based on the amount of hops they put in, ASSUMING that all the alpha acids get converted to iso-alpha acids. For regular beers, this is a pretty good assumption. But, for big, hop heavy, would-be IBU kings, that assumption comes crashing down faster than a backyard jenga game played by drunk hipsters.

Two things kill this association: solubility and degradation. (Chemistry jargon alert!!!!)

First up, solubility. Solubility is the measure of how much of any chemical you can add to a specific volume of solvent. Here, the ‘solvent’ is our beer. (Technically, it’s the wort before fermentation, but let’s not get picky.)  It turns out alpha acids are really poorly soluble. So, I don’t care how many hops you throw in your beer, you will only dissolve a certain amount of alpha acids in it. If that is limited, then the amount you can make into the bitter iso-alpha acids is also limited. Guess what? That limit is well below the amount it takes to get to 100 IBU. Estimates from  literature place it at a mere 6-10 milligrams of alpha acid per liter.1   That’s WELL below the 100 IBU limit.

But, wait! Doesn’t that mean that we could only produce beers with 6-10 IBU? No, as it turns out. If that were true, IPAs would be only a dream. Thank goodness that’s not the case!

While we can only have a few milligrams of alpha acids per liter, once we heat them and convert them to iso-alpha acids, then we can extract more alpha acids from our hops. Thankfully for us, the iso-alpha acid solubility limit is WAY higher than 6-10 milligrams per liter. So, then, what is that limit and can it get us to the 100 IBU holy grail? Yes and no.

In a strictly chemical system (not in a real-life brewery!), it turns out that iso-alpha acids are soluble to over 300 milligrams per liter.1 So, in theory you could have a 300 IBU beer. (Does Amazon have overnight shipping on tongue scrapers? Because I’d need those in bulk for a 300 IBU beer!)

In practice, there is a different story. That story depends on the chemistry phenomenon known as kinetics. Kinetics tells you how fast a reaction goes and how fast you can produce a chemical you want – or one you don’t want.

So here, we have a race going on. We are dissolving alpha acids and they are being made into iso-alpha acids. That’s good! Our IPA is becoming more bitter. But, as soon as we make iso-alpha acids, the heat that is part of our brewing process also starts breaking the iso-alpha acids apart into things that are not bitter any more. That’s bad. Our IPA is becoming less IPA-like.

Holy hell, this is getting complicated fast! Bear with me, I’ll make this all better soon. I promise.

At the end of the day, we essentially have two competing reactions. One that makes bitter compounds and one that removes them. The interesting thing is that each of those reactions goes faster as more of the chemical involved is present. So, add more hops and you have more alpha acids and you make more iso-alpha acids, you make them faster, and your beer gets more bitter. Yay! IPA here we come!

But, make more iso-alpha acids during the brew process and then those degrade faster into a non-bitter form. Boo! No octuple IPA.

At some point, these two reactions have a standoff and equal one another in rate. The ‘race’ becomes a tie. Neither one can win and you are at the limit of how bitter your beer can become.

Wanna guess what that limit is? Based on the premise at the start of this rambling diatribe, the smart money would say about 100 IBU. And, the smart money would be correct.1 To even get there, however, you have to add WAY more hops to the brew kettle than you would normally think, or than the traditional brewer’s rules of thumb would predict. (Take that statement on faith, please, or the chemistry and math get really ugly, really fast.)

So, for any brewer using traditional brewing techniques, I don’t care how many bushels of hops they throw in their brew kettle, they will be stuck with something right around 100 IBU. Why? Chemistry!

Now, there is one way around this. Instead of depending on doing the chemistry in one’s own brew pot, you can decide to buy chemically extracted and pre-isomerized iso-alpha acids. Then, you can toss those in with an Emiril Lagasse-like “Bam!”. Presto, you’re over the 100 IBU limit. Doing that, the only practical limit for the IBU level of your IPA is how long you want to spend scraping the bitterness off your taste buds. But, for most brewers (especially at the craft/micro level), buying and using pre-isomerized hop extracts is not a thing.

So, next time you wander into your favorite pub and they are advertising an 100+ IBU beer, rest assured that it will still be a hop bomb. However, don’t think that the 110 IBU beer is any different than the 100 IBU or 90 IBU beer either. They all are at the limit of what a brewer can do and they will all effectively be the same level of bitterness. You may perceive the bitterness differently, though, based on  other factors like the malt level, salt concentration, age and whatnot. That’s another topic for another day, though.

1) Malowicki, Mark G. Hop bitter acid isomerization and degradation kinetics in a model wort-boiling system. Diss. 2004.

Hillbilly Hops?

17 thoughts on “The myth of the 100+ IBU IPA”

  1. Thanks for the great article! If memory serves, there’s a limit to how much bitterness humans can perceive in beer, such that 100 IBUs is indistinguishable from (some value I can’t remember quite a bit less than 100 IBUs). I loved the commenter who said super-IPAs are a test of manhood–couldn’t agree more. It seems like (until recently, anyway) brewers have been trying to smack us over the head with extreme IPAs. There is surely a better way: Make a beer I want to drink, and one I can drink a few of without needing to call a taxi (or an ambulance).

  2. Tom,

    I appreciate the advice on the disclaimers and writing issues. I’m new to this blogging thing, so the writing is definitely a work in progress. There is a tricky balance to be struck with completeness and trying to keep it light and readable. I apparently missed that by a bit this time. I’ll do better next time, i hope! 🙂

    Thanks for the feedback!

  3. This whole argument presumes that all hops are intended to do is bitter, which is not at all true. Aromatics, flavoring – the tsunami of flavors possible with today’s hybrid hops – and even textural effects are all very much a part of the hops phenomenon. Yeah, there comes a saturation point at which a liquid cannot subsume any more solids but that’s true with every liquid. The proof is simple; keep adding Kool Aid to water and you’ll eventually end up with slurry.

    The whole idea of 100+ IBUs – or those puerile claims of “2,000+ IBUs” – are just frat-boy nonsense, existing on the same level as someone daring somebody else to chug a bottle of Tabasco. The whole thing is essentially meaningless and that’s easy to prove, too: pour Ninkasi “Tricerahops” , a 100+ IBU Double IPA, and then Stone Double Bastard. Taste. Not even remotely the same beer. HopHeads who genuflect before Double Bastard often dislike Tricerehops because they don’t perceive it as 100 IBU. Actual comment from BeerAdvocate: “This one is listed at 100 IBU’s and I really do not get that.” Jamie Floyd set out to make a IIPA that normal humans can drink and enjoy; no test-of-manhood nonsense involved. So he used aromatic and flavoring hops with subdued bittering. Voila: 100+ IBU but without flesh-eating bitterness.

    People who buy IPAs solely on the basis of their IBU rating are known by a technical term: “Chumps”. Ultimately, the chemistry explains something but determines nothing of our perception of the beer. That’s down to taste and drinkability, no matter what claims the brewery makes to pander to the Hopheads.

    1. Totally agree with you on the fact that high IBU beers tend to be a lot about chest pounding and not a lot about crafting a well rounded hop experience. I intentionally did not speak to flavor and aroma, as those are not part of the IBU or the bitterness of a beer. (Hence that beer you mention that Jamie Floyd made is likely not even close to 100 IBU, but it is probably full of amazing hop *flavor*.) The rainbow of wonderful hop flavors and aromas come from totally different compounds than does bitterness. As I commented below, they can totally contribute to *perceived* bitterness because we are cued in to think that if we smell huge hop aroma, we will also taste huge hop flavor and bitterness. So, we can trick ourselves into a 100+ IBU experience, even if it is not realistically possible.

      I think we’re pretty much on the same page with well flavored and nicely balanced IPA’s. I was trying to point that out through the fact that the IBU number is not often used correctly in making IIPAs and so folks should stop resting the ‘quality’ of their IPA on that.

  4. Meant to say dry hopping does NOT add bitterness. It adds flavor and aroma.

  5. I am curious, at what temperatures does iso-alpha acid production start, and at which temperature does iso-alpha acid degredation begin? Would it be possible to produce more IBU by finding a sweet spot for iso-alpha acid production and avoidance/slowing of degredation? If so, then only time and temp control would be needed to make a 120ibu beer right?

    Thanks for the article in anycase.!

    1. Travis,
      You have an interesting idea there, in that playing with temperature definitely affects reaction rates, and each reaction responds a bit differently. You can produce iso-alpha acids at lower temperatures (I’ve seen papers looking at generation as low as 80 degrees C). However it occurs much more slowly. So too does degradation.

      As it turns out, the chemical kinetics of the iso-alpha acid generation and degradation reactions are close enough in key ways, however, that they slow at very close to the same rate, so you are back to the place where solubility and concentration play a bigger role in the differing rates of the reaction than temperature does.

      As an aside, there are also really good reasons to NOT boil at lower temperatures. A big one is that you need a vigorous boil to drive off some volatile chemicals that, if left in your wort, will lead to production of a chemical called DMS that gives beer a cooked corn flavor. So, maybe you can squeeze out a couple more IBU by boiling at a lower temp, but at what overall cost to the flavor of your beer?

    1. Dry hopping is great for adding aroma to your beer, but does not really affect bitterness at all. The reason is that you need the heat of the boiling wort to make the production of iso-alpha acids possible. So, in a dry hop application, you can extract some alpha acids into the beer. (Not many, though because of low temperature and limited solubility.) But, those are not flavor active and will impart no real bitterness to your beer.

      That said, there *is* a whole body of literature about how our perceptions play tricks on us. So, if you pick up a beer with a huge dry-hop aroma, you will *expect* it to be bitter and so will perceive it that way, even if it is not actually as bitter as you think it will be. So, brewers can trick you into thinking their beer is wicked bitter, even if they have bumped up on the 100 IBU limit.

        1. Babbo,

          That’s interesting stuff to be sure. I did not have that discussion in mind when I wrote this article. It is true what they are pointing at, though. There are a TON of factors that influence the perceived bitterness of a beer, and some of the things that come out of dry hopping definitely do that. (There’s no way in hell I’m going to dispute what Shellhammer says. That man knows his hop chemistry!) When you get right down to it, even your water chemistry changes the perceived bitterness – that’s why New England IPAs have such a low concentration of sulfates and high concentration of chlorides in the water.

          Perhaps I need to write a broader article (or set?) on some of the many factors that influence sensory bitterness of a beer. My main point here was that throwing bushel after bushel of hops at a beer won’t do what most brewers think it will. Your link shows another weakness in our current system: The IBU measurement is overly simple, as it only looks at 1 compound and 1 way of generating it.

          Thanks for the link, though! That was an interesting read and good reminder of some stuff I had nearly forgotten.

        2. Also this:

          I hope this doesn’t come off the wrong way, but there really needs to be an edit at the top of this article, as you only deal with IBUs generated through the isomerisation of alpha acids. John Palmer himself points out that IBUs are calculated with reference to more compounds than just iso-alpha (such as polyphenols), and in fact perceived bitterness is most definitely affected by the presence of polyphenols.

          With an amount of polyphenols in your beer typical from a normal US IPA dry hopping dose (200mg/L) you can attain 40 IBUs with only 12.5mg/L of iso-alpha. That’s a huge gulf, and a massive omission from your article. The world of homebrewing doesn’t need the “no IBUs from dry-hopping” being propped up any longer by articles based on half-truths. You are setting yourself up as an authority with articles like this and letting down the community by an apparent total lack of in-depth research, and then answering people’s questions with more misinformation. Literally the top two results from Google alone contradict your answer to his question.

          1. Tom,

            Your comment totally does not come off the wrong way – it’s just a matter of how technical you want to get and how fast you want to get there. I was aiming my article at the average craft brew connoisseur, not the homebrew/brewing science pro.

            That said, if you want to geek out over bitterness, let’s talk!!! 🙂

            You are absolutely correct that if you *measure* your BU numbers, using spectroscopy and the ASBC methods, you will definitely see a contribution from polyphenols. You’re totally right that those (and other compounds like isomerized beta acids, sulfates, chlorides, and many other things) will contribute to sensory (or measured) bitterness scores. However, the average craft brewery who is posting their IBU next to their beers is taking *none* of that into account. They are doing a traditional calculation of IBU based on alpha acid content of the hops, boil time, and (probably) gravity of the original wort. If you do that, the number becomes meaningless as you approach 90-100 – probably even less than that. That’s the point I was trying to make.

            Now, to add context to the link you posted, what Palmer says is right on, of course (he’s a bigger fish than I am, no doubt), but pay close attention to the details. 1) He is talking about *measured* BU numbers. Again, most small to medium operations are not doing solvent extractions and UV/VIS measurements according to the ASBC methods in order to determine their BU numbers. 2) When he talks about polyphenol contributions, he also mentions that that only becomes significant if your hops are in dry-hop for a week or more. That fits most homebrewers who wait for hops to settle out before racking, but few small/medium brew operations, who tend to dry hop and filter within a couple of days. The big boys use things like hop rockets and dry hop on the order of hours, not days. (Time is money and fermenter/bright tank space is *gold*.) So, again, that has to be taken with a grain of salt.

            Bottom line, this is a wonderful conversation about all the things that lead to sensory bitterness. You are right to call into question the “dry hops do not contribute to bitterness” statement. It all just depends on how nerdy we want to get about all this. Hence, the *general* rules tend to work, but it gets complex, quickly, when we try to go deeper than that.

            Thanks for the comment. I’ll try to be more clear about the limitations of the claims in the future!

          2. Oh, and as an addendum, even Palmer is taking some shortcuts. If you want to start thinking about how polyphenols contribute to bitterness, you have to know the form of hop used (whole flower, pellet, etc…) and the length of time it was in contact with the beer. There’s some quite interesting data showing that dry hopping with different hop forms leads to VERY different contributions of aromatic and polyphenol concentrations. One example: (page 59 deals with polyphenols. Scroll up for flavor and aroma compounds.)

            This gets very complicated, very quickly, alas.

          3. Thank you. Your comment is well-appreciated and you make good points. I think keeping things light can be important, but it should definitely come with some sort of disclaimer that there is much more to know so people don’t take the gist of what you say and start bandying around generalisations and creating more misinformation. I am just a novice homebrewer who happens to have read a few articles on the subject recently – I’m exactly the target audience for this sort of article! I just feel the point needs to be made that the 100+ IBU beer, technically, might not be a myth – even within the realms of regular brewing practices (though I don’t know if the limit is much beyond 110? There seems to be so much more to know in this area.).

            The science in the article stands for itself – it doesn’t need to concern itself with taking a stand against silly trends or trying to make some overarching statement.


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