The International Bittering Units scale, or simply IBU scale, provides a measure of the bitterness of beer, which is provided by the hops used during brewing. Bittering units are measured through the use of a spectrophotometer and solvent extraction. [1]
This technique was adopted at the same time as another method based on measuring the concentration (in milligrams per liter; parts per million w/v) of isomerized α acids in a beer, causing some confusion among small-scale brewers. [2] The American Society of Brewing Chemists, in the introduction to its methods on measuring bitterness, points out some differences between the results of the two methods:
While the results of the IAA [isomerized α acids] methods are practically identical to those obtained by the BU method for beer brewed with fresh hops, the IAAs of beer brewed with old or poorly stored hops, and with certain special hop extracts, can be significantly lower than the BU figure. [1]
The bittering effect is less noticeable in beers with a high quantity of malt, so a higher IBU is needed in heavier beers to balance the flavor. For example, an Imperial Stout may have an IBU of 50, but will taste less bitter than an English Bitter with an IBU of 30, because the latter beer uses much less malt than the former. The technical limit for IBU's is around 100; some have tried to surpass this number, but there is no real gauge after 100 IBUs when it comes to taste threshold.
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Typical measurements This section may contain original research. Please improve it by verifying the claims made and adding references. Statements consisting only of original research may be removed. More details may be available on the talk page. (May 2009)
A light American lager might be as low as 5 on the scale, while barley wines, India pale ales, or imperial stouts can range up to 100 or higher. Measures for other beverages include:
Belgian Lambics: 11–23
Blonde ale: 15–30
Kölsch: 18–25
Märzen/Oktoberfest: 18–25
Ordinary English bitter: 20–35
Porter: 20–40
Brown ale: 15–25, with North American styles higher, 25–45
Bohemian-style Pilsener: 30–45, sometimes it can range up to 100 (e.g. German Bitterpils)
India pale ale: 40 or higher
An Irish stout: 25–60 (eg. Guinness ~45 IBU)
A formula craft brewers use to estimate IBU is:
Wh × AA% × Uaa ⁄ ( Vw × 1.34 ), where
Wh refers to the weight of the hops used, in ounces
AA% refers to the alpha acid percentage, which is influenced by many factors, including cultivation method, species, and time of year — hops are often sold labeled with this percentage
Uaa is the percentage of alpha acid that is actually used during the boiling process
Vw means the volume of the wort, in gallons
1.34 is a constant factor that adjusts the measurement to account for the use of U.S. customary units
There are several different methods for finding Uaa, which can yield very different results. Generally, Uaa increases with longer boiling times and decreases with higher boil density.
