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. A second component called beta acid also provides some bitterness. Additional compounds in hops provide both aroma and preservative qualities.