International Bittering Units

From Wikipedia:

The International Bittering Units scale, or simply IBU scale, is used to approximately quantify the bitterness of beer. This scale is not measured on the perceived bitterness of the beer, but rather the amount of iso-alpha acids. There are several methods to measure IBU. The most common and widely used way is through spectrophotometry. In this process, hops are boiled in wort to promote isomerization. Since the iso-alpha acids are slightly hydrophobic, a reduction of the pH by adding acid increases the hydrophobicity of the iso-alpha acids. At this point, an organic solution is added and the iso-alpha acids shift to the organic layer out of the aqueous wort. This new solution is then placed in a spectrophotometer and the absorbance is read at 275 nm. At this wavelength, the iso-alpha acids have their highest absorbance which allows for the calculation of the concentration of these bittering molecules. This technique was adopted at the same time as another method based on measuring the concentration (in milligrams per litre; parts per million w/v) of isomerized α acids (IAA) in a beer, causing some confusion among small-scale brewers.