The International Bitterness Units scale, or simply IBU scale, provides a measure of the bitterness of beer, which is provided by the hops used during brewing. An IBU is measured through the use of a spectrophotometer and solvent extraction. It is not equivalent to one part per million of isohumulone or isomerized alpha acid as is sometimes reported. Instead it is the result of an empirical formula whose development was based on tasting beer samples and correlating the perceived bitterness to a measured value which represented the total concentration of bitter compounds in the beer.
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.
- 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 like Guinness: 25–60
A formula craft brewers use to calculate 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.
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