This week I cover the formal definition of an International Bitterness Unit (IBU) widely used in beer recipe design, but also present some of its weaknesses.
IBUS in Beer Defined
The International Bitterness Unit (IBU) is very widely used in beer recipe design, as it gives a good overall measure of how bitter the finished beer. The modern method for measuring IBUs attempts to measure the milligrams per liter of iso-alpha acids, so a 20 IBU beer would have 20 mg of isomerized alpha acids per liter of beer. The The European Brewery Convention also introduced the European Bitterness Unit (EBU) scale, which numerically should be identical to the IBU scale, but in practice can differ very slightly in measurement with EBUs running a bit less than IBUs in many cases.
It is important to understand that IBUs measure only the five major alpha acids, and that these are all isomerized (the molecular structure rearranged) in the boil and provide the bulk of the bitterness. Hops contain some 500 flavor compounds, so obviously IBUs don’t tell the entire story in terms of perceived bitterness.
The most common method for accurately measuring IBUs in a sample of beer is spectrophotometry. Since the isomerized alpha acids are hydrophobic, they are drawn out of solution and then measured in a spectrophotometer by shining light through them at 275 nm, which gives a good measurement of the concentration of these bitter compounds. (Ref: Wikipedia). There are alternate methods for measurement as well such as the IAA method.
IBUs in Practical Beer Recipe Design
While the above methods provide a clear definition and method for measuring IBUs, in practice IBUs are actually quite a bit different from the measured and perceived bitterness in the finished beer. In reality, relatively few home and small craft brewers have their own laboratory with precise IBU measuring equipment. So instead we’ve adopted the practice of estimating the IBUs using one of several equations with the aid of software like BeerSmith to design our beers. The main equations used are Tinseth, Rager, and Garetz, though Tinseth is by far the most common used for all grain brewing.
A surprising fact I stumbled upon several years ago is that the equations in use today for beer recipe all estimate the IBU level pre-fermentation. Since IBUs are always reduced in both fermentation and aging, its fairly safe to assume that the IBUs used for designing your recipe can be higher than what you would find if you sent your finished beer to a lab for measurement. However, the “estimated” IBUs are so widely used in beer recipe design that they are often posted on the signs at your local pub. I also found that most beer style guides like the BJCP style guide reflect the estimated IBUs rather than the measured IBUs. So to a large degree, the estimated IBUs are more important than the actual measured IBUs. If you want to learn more you can read my article on this phenomena here.
When designing beers to target a given IBU, there are several factors that come into play. The first is the malt-hop balance which can be estimated using the bitterness ratio. The bitterness ratio is simply the ratio of estimated bitterness in IBUs divided by the original gravity points (take the original gravity, subtract the leading 1.0 and multiply by 1000, so a 1.046 gravity would be 46 gravity points). Generally the more malty the beer, the more bitterness you will need to balance the sweetness from the malt.
Another consideration is the color of the finished beer. Dark beers like stouts require more bitterness to balance the strong flavor from the dark malts. So an Irish Stout might not seem all that bitter even at 40+ IBUs while a light lager would be quite bitter at that same hop level. Water also comes into play as different water profiles can affect the perception of bitterness. This can be estimated using the sulfite to chloride ratio which is simply a ratio of the sulfite to chloride ions in parts per million in your water profile.
And of course the style of the beer is critically important in your recipe design. As mentioned above, the style guides all make use of estimated IBUs, so you want to design your beer to be roughly “in style” for the particular beer style you are brewing. An IPA is going to have a lot more bitterness than a American Adjunct lager, and your Irish stout is going to target a higher IBU level than your English mild. Fortunately most brewing software like BeerSmith have the style guides built in so its easy to compare the estimated IBUs against the chosen beer style guide as you create your recipe.
Some Practical Limitations of IBUs
While estimated IBUs are widely used in recipe design, the reality is that if you send your beer sample to a lab, it probably won’t measure out at the same bitterness level. Additional factors come into play that are not included in the IBU estimates. For example, different alpha acids actually isomerize at different rates, so a cohumulone heavy hop would be perceived as more bitter than one with low cohumulones. Also some bitterness is lost both in fermentation and aging, so the measured IBU level will likely decline over time.
In addition, since IBUs only measure isomerized alpha acids, they really say nothing about the hundreds of other flavor compounds in hops that can affect the perception of bitterness. Chief among these are the aromatic oils in hops that are infused during whirlpool and dry hopping. While whirlpool hopping does isomerize some additional alpha acids, dry hopping does not. Yet the aromatic oils have a significant impact on the perceived bitterness of the beer, with some commercial examples made using whirlpool and dry hopping alone.
Despite these limitations, IBUs are still a great tool to use in recipe design especially since the estimated IBUs calculate by software are closely related to those in the style guides. So the estimated IBUs are still an amazing tool to get in the ballpark on bitterness when you design a new beer. However being aware of the limitations of IBUs and how whirlpool and dry hops impact perceived bitterness.
I hope you enjoyed this week’s article on IBUs. Thank you for joining me this week on the BeersSmith blog – please subscribe to the newsletter or listen to my podcast for more great material on homebrewing.