People often ask me “what are the IBUs” in a particular beer. The standard answer is that I don’t really know and I’m not willing to describe my beers by talking about their IBU levels.

Lots of breweries reveal IBUs, so why won’t we? To answer that question, first we need to ask: What is an IBU?

IBU stands for International Bitterness Unit, and is a measure of a specific bitter compound in a particular beer. Specifically IBUs represent the concentration of isomerised (iso) alpha acids in a sample of beer. Iso-alpha acids are bitter compounds that are formed during the boiling process of wort in the brew kettle. As the wort boils, the alpha acid content of added hops (the primary chemical in the hops that are responsible for creating bitterness but in their original form are not particularly bitter) isomerise, or rearrange their molecular structure and become iso-alpha acids. Iso-alpha acids are quite bitter and are the primary compound responsible for beer’s bitterness. Isomerisation of alpha acids takes both heat and time. The longer an alpha acid is kept hot and in solution, the higher the conversion to an iso-alpha acid.

A standardized test method exists for measuring IBUs and this method is performed in a laboratory on a finished beer sample.

Without getting into boring analytical and technical detail, the test involves performing the following test procedure:

A beer sample is mixed with hydrochloric acid and iso-octane, which is an organic solvent. Since iso-alpha acids are organic compounds they are much more solvent in iso-octane than in water. To make iso-alpha acids less soluble in the beer (near zero solubility) the lab tech will acidify it with hydrochloric acid. Therefore the iso-alpha acids migrate to the organic (iso-octane) phase of the mixture. Iso-octane also has a very low solubility in water so it easily separates out of the beer, similar to the way oil and vinegar separate out from each other after you mix them together. This makes for a convenient method for stripping out the iso-alpha acids from the beer and driving them into the iso-octane. After a period of mixing and settling in a mixing and separating tube, the iso-octane solution containing the dissolved alpha acids is transferred to a special test tube called a cuvette and it is placed in a measuring device called a spectrophotometer.

A spectrophotometer is a lab device that shines a particular wavelength of light (in this case the wavelength is 275 nanometres) through the cuvette, and then measures the light’s absorbance through the cuvette. The absorption of the light at 275 nm is directly proportional to the concentration of dissolved iso-alpha acids in the sample.

If this sounds a lot like laboratory chemistry, that’s because it is. But this standardized method is the definition of the International Bitterness Unit.

Now there are several ways to calculate (rather than measure) what the potential bitterness of a beer will be and these calculations involve formulae developed over the years by a few notable brewing scientists. One of the most popular calculation methods was developed by Glenn Tinseth and is referred to as the Tinseth IBU formula. Many homebrewers and pro brewers alike use the Tinseth formula to calculate the potential IBUs in a beer. Without going into gory mathematical detail, the Tinseth formula calculates the IBUs by calculating other parameters, like the alpha acid content of the hops, the boil time, the hop utilization factor, and a parameter called the “bigness factor” (I’m not kidding!). Most of these are derived empirically through experimentation and observation, and the formula was constructed to try and best fit Glenn Tinseth’s observations. Many homebrewers and pro brewers use brewing software like ProMash and BeerSmith to develop recipes, and these readily available software packages have these bittering calculations built into them.

There are at least two other popular formulae for calculation IBUs: the equations developed by Jackie Rager and Mark Garetz. Often the Rager formula and the Garetz formula are also installed into brewing software and users can pick their favorite. There are other methods of calculating IBUs as well, but are generally less popular than the Tinseth, Rager and Garetz formulae.

The problem lies in the fact that there is very little agreement between the Rager formula, the Garetz formula and the Tinseth formula.

For instance, let’s say a particular IPA contains 54 IBU according to the Tinseth formula.
That same beer will have 70 IBU according to Rager and 40 IBU according to Garetz.
See the problem? Which is correct?

Also, the laboratory method, measures just one bitter compound – the isomerized alpha acid iso-humulone. IBU measurement doesn’t take into account other bitter ingredients in beer. Non-isomerised alpha acids are bitter, but to a much lesser extent than iso-humulone. Beta-acids in hops are also bitter, and so are some of the essential oils present in the hops. Dark roasted malts can impart bitterness to beer, and so can added ingredients like coffee, cocoa nibs, or popular bittering herbs such as sweet gale (bog myrtle), and even heather flowers that are often added to Scottish Ales.

IBU calculations don’t take into account the role of yeasts. Some yeast varietals have “sticky” outer skins that readily absorb and stick to iso-humulone and other bitter compounds. Once those yeasts are removed from the beer, a lot of those bitter compounds are carried away with the yeast.

So we’re left with two problems. One is that the laboratory measurement method doesn’t take into account other bitter compounds. The other is that yeast carries away some of the iso-humulone along with other compounds rendering the Tinseth (and Garetz and Rager) formula inaccurate.

So what good are the IBU measurements? Well all other things being equal (boil time, yeast application, malt consistency, water chemistry) IBU measurement is a good a indicator of the bittering potential of the hops that you are using in a particular brew. Hop alpha acid levels deteriorate in storage, so older hops have less bittering potential than freshly harvested and processed hops. Measuring (rather than calculating) IBUs is one method of checking for these changes and adjusting hop application rates.

IBU calculations also have practical uses. They allow us to stay relatively consistent in our hopping rates to make beer that tastes the same as the previous batch when hop parameters change. Alpha acid content changes every year for a given hop varietal, and changes from hop supplier to hop supplier so it’s important to account for these changes to keep beers tasting consistent.

But two things are certain: IBU measurement, doesn’t reflect the true or perceived bitterness of a particular beer, and IBU calculation doesn’t either. So while IBU determining methods, both measuring and calculating, are useful tools for the brewer, they’re very poor indicators of how a beer will taste.

Back to the question: Why don’t we reveal IBUs?

  1. We don’t reveal them because we haven’t currently got a convenient method for measuring them, so we don’t really know.
  2. We believe that there are numerous other factors at play that modify the true IBU level in beer.
  3. We think that the best indicator of beer bitterness is the perceived overall bitterness, which is something that can’t be measured.

So relax and stop worrying about IBUs.


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