<Tr> <Td> India pale ale </Td> <Td> 40 - 120 </Td> </Tr> <P> Since the quantities of alpha and beta acids range among hops, the variety of hop should be considered when targeting a specific amount of bitterness in the beer . To maximize bitterness, hops with large alpha acid concentrations should be used . Such varieties include Chinook, Galena, Horizon, Tomahawk, and Warrior hops, and these contain alpha acid concentrations up to 16% by mass . Since the bitterness is not influenced by beta acids, beta acids are not considered when selecting the variety of hop . Also, the amount of time that the hops are boiled impacts the bitterness of the beer . Since heat is needed to isomerize alpha acids, applying heat for longer amounts of time increases the conversion to the isomerized form . </P> <P> 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 . 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: </P> <P> While the results of the IAA methods are practically identical to those obtained by the (I) 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 (I) BU figure . </P>

What does ibu stand for in beer making