The beer world is exploding with diversity. All you have to do is stroll into any store that sells beer and chances are you will be greeted by a wide range of choices. Different styles of beer boast different features, which means that there is a good chance that the right beer for you is out there – it just takes a little trial and error to find it. Beer has a wide range of flavor profiles, from sweet, malty beers to bitter, hoppy beers. And it turns out that the bitterness of your beer is actually a delightful mix of chemistry and flavor.
Why is my beer bitter?
Before getting into the nuts and bolts of how beer bitterness is calculated, some readers out there might be wondering why it is bitter in the first place. Beer is a simple combination of water, hops, malt and yeast. Malt is sweet and hops are bitter. A beer with a strong malt backbone will taste sweet, while one with a heavier hop profile will come across as more bitter. The exact kind of hops are also import for things like aroma, with some hops for example giving nice earthy, piney notes, while others might give more citrus like notes.
So does hops = bitterness?
Yes and no. Hops will make a beer more bitter, but they also make a beer more aromatic and flavorful. The difference between bitterness and aroma is often determined by when in the brewing process the hops are added. Hops that are added early will make the beer more bitter, while hops that are added later will add more aroma, but not impact the bitterness as much. Also, the bitterness of the hops can, and often is, balanced out by the amount of malt used. So you can have a beer whose "hoppiness" is not reflected by its bitterness.
The bitterness unit
So, now we come to the question of how the bitterness of beer is measured. The common unit you will see on the back of your beer bottle is the IBU or International Bitterness Unit. One bitterness unit is roughly equal to one part per million of isohumulones, alpha-acids that enter the beer from the hops and are responsible for a beer's bitterness. The way breweries will measure this is by taking a sample of beer, and examining it for isohumulones. The two common ways of doing this are by measuring with ultraviolet-visible spectrophotometry or with liquid chromatography.
The IBU measurement is standardized between many different brewing organizations, including The American Society of Brewing Chemists, the European Brewery Convention, the Institute & Guild of Brewing, and the Brewery Convention of Japan. Sometimes, you will see a beer's EBU or European Bitterness Unit listed, but most sources consider the IBU and EBU to be essentially interchangeable.
IBUs of homebrewers
Another growing trend is homebrewing. Since the mechanics of making beer is not drastically more difficult than making pasta, there are many who dabble in homebrewing. These people may wonder how to find the IBU of their beer. Since most homebrewers cannot afford the equipment necessary to do ultraviolet-visible spectrophotometry or liquid chromatography, there has to be another way. Luckily, anyone with a calculator can figure out their home brew's IBUs by using a simple formula. For 5 gallon batches, for example, you would take the ounces of hops used, multiplied by the alpha acid percentage (typically found on the hop packaging), multiplied by the percentage used (aka boil time), then divide that number by 7.25. If you do smaller than 5 gallon batches, be sure to adjust the formula accordingly.
So now you know that the bitterness in your beer comes from hops and is found by measuring the isohumulones in the beer. You even know how to measure the bitterness of your own homebrew. The last thing that is worth stressing is that bitterness is ultimately subjective. Two people might consider the same beer to be more or less bitter depending on their personal tastes. Also, keep in mind when looking at the IBU count how much malt is used. A beer with a lot of malt, like a stout for example, might have the same IBU as a pale ale with a low hop profile, but would not have the same bitterness. The subjective nature of people's taste and the fact that the IBU measurement does not include malts have led some to criticize the term bitterness unit, pointing out that it is not truly the most accurate reflection of bitterness.
In the end, people do not usually choose their beers by IBUs, but rather by personal taste. So, with that said, go out and try some different beers and find what works for you.