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How bitter is a beer?

Almost like asking ‘how long is a piece of string?’ this is one of life’s more ambiguous questions…

But for those of us who struggle with ambiguity, using Bittering Units (BU) provides some direction.

The BU scale is the measurement of the bitterness of a beer. Starting at zero, the scale is unrestricted, but generally beers don’t stray higher than 100 BUs. Anything above this is likely to be too bitter for most of us to taste much difference, and is certainly an acquired taste. 

Our recent story on extreme beers listed Alpha Fornication as the bitterest beer in the world at 2,500 BU. For comparison, here’s how some well-known beer styles and beer brands stack up...


So on the face of it, 2,500 BU is incredibly bitter – but does the beer have a stronger taste when you drink it?

Not necessarily. It’s important to remember that while the BU scale is the measurement of the bitterness of a beer, it is not a measurement of how hoppy a beer is, how bitter a beer tastes, or what the quality of the bitterness is. It’s simply a measurement of how many parts-per-million of something called isohumulone is present in the beer.

While BU is useful to brewers to give us an indication of the strength of the bitterness, bitterness can be complimented, masked, or increased, by other ingredients in the beer. Often described as ‘balance’, fermentable sugar, flavours such as herbs or spices, and malts can all change the taste of beer without altering its isohumulone content. 

A great example is Pilsner Urquell, which uses a combination of different malts and a carefully controlled brewing process to deliver caramel and buttery notes expected of a Czech Pilsner, which combined with the 39 BU provides the consumer with a very approachable bitterness.

Science of bitterness units

Isohumulones are chemical compounds that enter the brewing process through hops. They are largely, but not exclusively, responsible for a beer’s bitter taste and they are in a class of compounds known as iso-alpha acids. One Bitterness Unit corresponds to one part-per-million of isohumulone.

All sounds straightforward to far, but measuring BUs is not a simple process; in fact there is a great deal of technical knowhow and skill required to do so. More often than not, it is done in a laboratory. Isohumulones need to be extracted from a beer sample, separated and then a technique called ultraviolet-visible spectrophotometry (UV-Vis) is used to measure the parts per million. More advanced breweries will also use high-performance liquid chromatography (HPLC) to measure iso-alpha acids, as well as other specific bittering compounds.

The precursor to isohumulone is humulone, a bitter-tasting compound that is found in the resin of mature hops flavours. A hop’s humulone content can vary, even within a hop variety, depending on when it was harvested and whether it is whole leaf, pellet hop or a hop resin extract.

So how bitter is a beer? Well, most of us don’t have a UV-Vis or HPLC tucked away in our kitchens, so perhaps the traditional way is best; just use your taste buds.

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