Alpha acid resins only contribute to the bitterness of the beer. Occurring naturally in the lupulin gland on the hop cone they are not very soluble in water. The reason that beer hops must be boiled is because the heat of boiling water creates a condition that allows a chemical reaction, called isomerization, to occur which makes the alpha acid resins soluble in water.
AAU stands for Alpha Acid Units, and is a measurement of potential bitterness in hops. The percentage of alpha acid in a given sample of hops multiplied by the weight in ounces of that sample. One ounce of hops with an alpha acid content of 1 percent contains 1 AAU, or .01 ounce of alpha acid.
HBU stands for Homebrew Bitterness Units and is another method by which home brewers can determine how much hops to use. Homebrew Bitterness Units equals the percent alpha acid of hops multiplied by the quantity in ounces of hops used. If, for example, a recipe of a given volume of beer calls for 2 ounces of 5 percent alpha acid Cascade hops, it is equal to 10 HBU's.
If your Cascade hops are only 4 percent alpha acid you will know to use: 10 HBU ¸ 4% = 2.5 oz of hops. Or, if you wish to use another variety of hops, say Centennial hops at 10 percent alpha acid, you know to use: 10 HBU ¸ 10% = 1 oz of Centennial hops.
IBU stands for International Bitterness Units. One Bitterness Unit is equal to 1 milligram of isomerized alpha acid in 1 liter of wort or beer or 1 part per million isomerized alpha acid. This is a system of measuring bitterness devised by brewing scientists and is an accepted standard throughout the world. Home brewers usually do not have the sophisticated equipment to measure actual BU's and often use the system of HBU's to closely approximate the desired bitterness in their beer.
The simplest formula for attempting to predict IBU is to use a sliding scale (depending on boil time) as a best-guess for utilization, and use a constant to convert AAU into parts per million. A fairly accurate ballpark figure to use for your homebrewing hop utilization value based on a one-hour boil would be 23-25%, .23-.25 respectively. This translates into:
IBU = (AAU x 74.4) ¸ (wort Volume in gallons x Utilization percentage).
IBU = (AAU x 74.4) ¸ (V x U). IBU = (5 x 74.4) ¸ (5 x .25) = 18.6 IBU.