Measuring IBUs: Part science, practice, and opinion

My view about how international bittering units or IBUs are used by the modern brewer is a blend of science, practice, and opinion. The science behind the IBU is something I have covered many times in past columns, so I will be brief. The original IBU method, and one that is still used by many breweries, begins by extracting hop compounds in beer using iso-octane (an organic solvent), measuring the absorbance of 275 nm light by this mixture of compounds, and multiplying the absorbance by 50. This method was originally developed as a rapid way to quantify the compounds in normal beer that relate to bitterness and the unit of measurement was standardized against 1 mg of iso-alpha-acids (IAAs) per liter of beer. This is why we assume that 1 IBU is the same as 1 ppm of iso-alpha-acids. The problem with the method is that it is not specific to IAAs and other compounds, including hop polyphenols that are not bitter, absorb 275 nm light. The HPLC (high performance liquid chromatography) method for specifically measuring IAA fractions is becoming more common, especially as larger breweries have begun brewing a greater volume of beer styles with large, late, and dry hop additions.

Measurement methods aside, the key to calculating hop bitterness is knowing how alpha acid additions to wort or beer translate to IAAs in finished beer. In other words, we need to know our hop utilization. Boiling duration, wort gravity, wort pH, hop type (cone, pellet, or extract), yeast behavior, and clarification method are all variables that influence hop utilization. But the most important variables to control in the pursuit of consistency are total thermal exposure and knowing how much hop goodies are added to the kettle. This is why the practical brewer part of me is less concerned about the numerical value of utilization and more concerned about a consistent brewing process and actually knowing what is being added to the kettle. The latter is extremely challenging and explains why practical brewers tend to assess hop specifics from a treetop level. I will get back to the practical side of hopping in a moment.

What does opinion have to do with this topic? Well, my opinion is that the numerical value of the IBU has taken on a life of its own among both brewers and consumers. For some, a high IBU value imparts some sort of physical prowess to beer; high IBU beers are strong, masculine, formidable, superior, and regal. Three Floyds’ Alpha King and the Alpha King Challenge are great examples of how highly hopped beers are personified by brewers and consumers. Recently, some consumers have recoiled from high-IBU beers because not everyone likes bitter beers, and we now see beers marketed as “zero IBU IPAs.” My opinion is that the IBU has been hijacked from the brewery lab and shoved into the lexicon of beer marketing terms, and as such cannot be trusted. Seriously, show me a beer with hops that registers zero.

Well, my opinion is that the numerical value of the IBU has taken on a life of its own among both brewers and consumers.

So here is the skinny on the 80 IBU recipe for Knotted Root’s Perpetually Unimpressed. If one assumes 12% alpha for the 6 ounces of Citra® hops in the recipe, 7.5% hop utilization for hop additions to 190 °F (88 °C) wort, and crunches the numbers the result is 80 IBU. But does this really make sense?
In order to dissect this information, the first question that comes to mind is what is meant by 80 IBUs; are we talking the iso-octane method or HPLC method that quantifies IAAs? Method aside, is it even reasonable to assume any isomerization at 190 °F (88 °C)? The answer to that question is a resounding “Yes!” (Mark Malowicki, 2004, Hop Bitter Acid Isomerization and Degradation Kinetics in a Model Wort-Boiling System). Is it reasonable to assume 7.5% utilization and 80 IBUs from a late hop addition to 190 °F (88 °C) wort? If I were a gambler, I would probably bet against this if we are talking IAAs measured by HPLC, but wouldn’t be surprised by a high value from the iso-octane method.

At the end of the day, many practical brewers don’t fret too much about the semantics because the IBU is just one way to gauge hop bitterness in beer. Many simply want to know what the beer tastes like because flavor rules for this beer style and the numbers follow. If brewers developed recipes only by calculating a brew, New England IPA probably would not exist as a style because conventional rules about brewing calculations don’t work with this style. This is a style that came out of throwing calculations out the window and lots of hops into the kettle, especially late in the boil. It didn’t take long for brewers to figure out that some bitterness, sometimes a little sometimes a lot, was coming from these additions, so they began backing off on hop additions during the boil.

You ask how brewers are performing bitterness calculations for this style and the answer is all of the above. The corollary is how they are controlling aroma additions. The truth is that many brewers are not calculating either, unless we include IBU calculations with wild guesses about hop utilization. Brewers have become bilingual when it comes to hop talk. If this conversation is about old school styles, it’s IBUs, multiple kettle additions, balance, and maybe some dry hops. Start talking hazies and the language shifts to “pounds-per-barrel,” biotransformation additions, and the minimum addition rate to provide a stable haze. Although not specifically discussed, many brewers have figured out that the best way to be relatively consistent with this style is to crank the volume control up to 120% because once the music is really, really loud, small adjustments in volume are imperceivable because the senses are maxed out.

Response by Ashton Lewis.