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, https://commons.wikimedia.org/w/index.php?curid=23240798)

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.

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