/* Bittering Oils */ Improved clarity of resins and oils
Hop bitterness is measured in [[International Bitterness Units]] or ''IBU's''. One IBU is one part per million of ''isohumulone'' which is a bittering (alpha) acid. IBU's can be estimated when brewing a beer by several different formulas, the most popular of which are the ''Tinseth'', ''Rager'' and ''Garetz'' formulas. IBU's for light beers are generally in the 10-20 range, while dark flavorful beers such as stouts may have an IBU as high as 50. Some barley wines have IBU values of 100 or more to offset the extreme malty sweetness of the beer. See the [[Beer Styles]] BJCP guide for some typical IBU ranges for different styles of beer.
===Bittering Oils===Bittiness in beer is provided by
oils released by the hops. The bittering oils of the hops are isomerized (rearranged) during the boil. Insoluable alpha acids (α-acids) are isomerized by the boil into more soluble and stable alpha acids. As the boil time increases, the bitterness released also increases. These alpha acids provide the majority of the bitterness in finished beer, as estimated by the Alpha Acid Units (AAUs). A second component called beta acid also provides some bitterness. Additional compounds in hops provide both aroma and preservative qualities.