IBUs: Modern Beers with Old Formulas – Advanced Brewing

The International Bittering Unit (IBU) is a measurable property of wort and beer. A common misconception is that the IBUs equal parts per million of isomerized alpha acids (isohumulones), but that isn’t quite accurate. Actually, the American Society for Brewing Chemists (ASBC) IBU test uses a spectrometer to measure the concentration of bitter hop resins. The test methodology then applies an adjustment factor of 5/7 to approximate the typical proportion of iso-alpha acids to total hop resins in a beer, as calculated in the 1960s. Although the IBU scale generally is a reasonable indicator for parts per million (PPM) of iso-alpha acids, it can be misleading; an extreme example is lambic, which often measure at 20-30 IBUs despite being brewed with aged low-alpha acid hops.

Although simplified tests without expensive lab equipment exist and labs are available to test your beer, most homebrewers rely on simple formulas to estimate wort IBUs. These formulas rely on rough utilization factors fit to boil time and wort density. While these approximations are reliable enough for classic hopping techniques and amounts, hopping has evolved to be more varied and aggressive. Calculated IBUs of 200? No hops until the whirlpool? 2 oz. per gallon (15 g/L) of dry hops? How does a measurement devised to handle adjunct lager 50 years ago stand-up to modern West Coast DIPAs, New England IPAs, and milkshake IPAs?

Too Many IBUs

I brewed my first double IPA in 2006 based on a recipe released by Russian River Brewing Co. of Santa Rosa, California for their Pliny the Elder. According to the Tinseth formula the resulting beer was 269 IBUs (and that’s factoring in 0 IBUs for the flame-out hops). So then why did Pliny the Elder measure only 68 IBUs?¹ The simple answer is that there is a saturation point for hop bittering compounds. If you’re adding more hops after a certain point, around 90 IBUs at a pH of 5.22, the wort can’t absorb them and they settle out. The IBUs lower further through fermentation and dry hopping as discussed below.

These days, I go light on the “bittering” additions for IPAs knowing that late-boil hops will contribute plenty of IBUs along with aromatics. Most craft breweries are of a similar mind as it has become increasingly common for them to hype stratospheric dry hop quantities rather than IBUs.

Isomerization Rate

IBU formulas take into account the total amount of alpha acids in the hops, but not the ratios of the individual alpha acids (e.g., humulone, adhumulone, or cohumulone). For decades brewers preferred hops low in cohumulone to avoid the supposedly “harsh” bitterness iso-cohumulone imparts. When starting out I noticed the difference as well; I found that Columbus (whose alpha acids are approximately 30% cohumulone) added a nice “grippy” bitterness on the tongue at the same calculated IBUs compared to low-cohumulone Magnum (23% cohumulone).

In recent years however, blind taste tests have indicated that cohumulone contributes a similar or even slightly smoother bitterness in beers with equal IBU measurements.² The explanation is that cohumulone isomerizes more readily than either humulone or adhumulone. This results in a greater concentration of IBUs than formulas suggest with high-cohumulone varieties. There is no issue if you want to bitter a delicate beer with a small dose of Chinook (another 30%-cohumulone variety), but you should aim for calculated IBUs 10-20% under your usual target to compensate for higher utilization.

Whirlpools and Hopstands

One of the biggest quality improvements to hoppy beer brewing technique in the last decade is the shift from rapid chilling after flameout to extended hop-steeping in the near-boiling wort. The hops infuse in the 160-211 °F (71-99 °C) wort for 20-60 minutes before chilling to pitching temperature. This is a more effective approach to extract aromatic compounds from the hops, but while the wort is above 180 °F (82 °C) isomerization is ongoing. Exactly how much isomerization is difficult to estimate because most IBU tests focus on the boil.

Interestingly, one experiment suggested that the temperature did not have a linear effect on the utilization rate. A 30-minute hopstand at 200 °F (93 °C) imparted the most IBUs, followed by 190 °F (88 °C), 210 °F (99 °C), and 180 °F (82 °C).³ However, an academic study using “model wort” found that each 18 °F (10 °C) increase from 194-266 °F (90-130 °C) increased isomerization by 223%.4 Granted not many brewers boil their wort well in excess of 212 °F (100 °C).

For 30-minute hopstands beginning at flame-out, my palate suggests the utilization rate is similar to a 10-minute boil. My wort loses 1 °F (0.5 °C) per minute, unlike commercial brewing where the massive volume assures the wort in the kettle will still be around 210 °F (99 °C) after an hour of whirlpooling and settling. At home the longer the hopstand, the lower the correlation with boiling. A hopstand also increases the IBU contribution of late-boil additions. A 5-minute boil addition will continue contributing IBUs as it is held for 30 minutes at near-boiling temperatures.

Many homebrewers chill wort below 180 °F (82 °C) before starting their hopstand. I haven’t found this to more effectively preserve volatile aromatics, but it does make IBU calculation easier. With this approach you can hit an IBU target with reliable boil additions: partially-chill, and then add aromatics with an extended hopstand without the worry of over-bittering.

Fermentation and Dry Hopping

IBU targets for wort and finished beer are not equal. If a brewery measures 60 IBUs in their finished, packaged IPA, a clone recipe needs to aim for more than 60 IBUs in the wort. This dichotomy reflects the fact that as beer ferments, iso-alpha acids come out of solution through a variety of processes. To add confusion, it is often unclear whether a label or website lists estimated wort IBUs or measured finished beer IBUs. Homebrew recipes tend to focus on IBUs in the wort.

As yeast ferment, they lower the pH to 4.2-4.6 (5 to 10 times more acidic than post-boil wort). This natural acidification causes isomerized alpha acids to drop out of solution. In fact, one step in the ASBC IBU test is to add acid to cause the iso-alpha acids to come out of solution so they can be absorbed by a solvent. The yeast themselves also pull hop compounds from the beer as they flocculate. The cumulative effect is a 20-40% reduction in the IBUs from wort to beer, but the precise amount will differ by recipe.

It is often repeated that dry hopping adds aromatics without increasing IBUs. This simply isn’t true, while it is accurate that significant iso-alpha acids are not added, that doesn’t mean that dry hopping doesn’t change measured IBU or perceived bitterness. Actually, dry hops absorb iso-alpha acids from the beer and also contribute humulinones (oxidized alpha acids) which add bitterness (although only 2⁄3 as much mg-for-mg as iso-alpha acids). The amount of humulinones contained in specific hops depends on processing (with pelletized hops having significantly more humulinones than whole hops [.5 ppm compared to .3 ppm]). The level also depends on age; hops develop humulinones during initial processing, but the amount increases with storage. The cumulative result is that low bitterness beers increase in perceived bitterness and measured IBUs when dry hopped, while high bitterness beers decrease. Dry hopping loosely has the tendency to gravitate the beer towards 25 IBUs of perceived bitterness. Anecdotally, these are not subtle changes, IBU reductions of 40% and increases over 100% have been measured!5


Even when precisely measured with High Pressure Liquid Chromatography (HPLC), the IBU scale isn’t a perfect indication of perceived bitterness or hop character. Factoring in variances introduced by imprecise formulas, inconsistent hops, and untested brewing processes makes it clear the difference of a few calculated IBUs is insignificant. Also the human tongue only has a resolution of 5-10 IBUs.

So, how does the homebrewer deal with IBUs? The first and most important advice is select and then consistently apply an IBU formula. Next, learn how bitter a 30 IBU recipe on your system tastes. Realize that a “30 IBU” New England pale ale and Kölsch will have a different perceived bitterness that existing formulas don’t account for.

Use your formula as a tool for recipe development and hop substitution, not as gospel. If the style guide says 50 IBUs but a beer brewed to that target tastes too bitter compared to examples of the style, lower your target IBUs for the next batch! If you are entering a competition the judges won’t know if your calculated IBUs are outside of the guidelines, as long as your beer tastes like it is within the range.

1. The Jamil Show Imperial IPA (9/11/06)
2. Karnowski, Cohumulone Friend or Foe

Issue: March-April 2018