Utilization is the percentage measurement of hop's alpha acids "utilized" during the boiling process. This gives you a formulated number to measure the bitterness of a final beer known as IBU's (International Bittering units), AAU's (Alpha Acid Units), or HBU's (Home brew Bitterness Units).
Equations for determining utilization
Utilization increases with your hops boil time, so the longer you boil your hops the more bitterness you will have. Boiling hop additions in the 0-15 minutes range or less add very little bitterness, and are used for aroma hop additions. Flavor hops are usually boiled anywhere from 10-45 minutes and add hop flavor with a few IBU's of bitterness to the wort. Bittering hops are added for the full boil time of 60-90 minutes and provide the most bittering of all your hop additions.
Hop utilization also increases as you lower the gravity of your boil. If you are brewing a high gravity beer, or an extract brewer using a partial batch boil (small pot) you will get much lower utilization. This is why extract brewing requires more hops (in general) than all-grain brewing. Since different brewers use different equipment, it is important to take into account your own boil size and boil gravity when estimating the bitterness of your beer.[Smith,2008]
Three equations to estimate utilization and IBU's are most often used: Rager, Tinseth and Garetz. The equations differ in the way that they estimate the utilization percentage. Rager is most often associated with extract and partial mash brewers. The Rager equation takes original gravity of the boil into account, and tends to produce IBU estimates that are on the high side of the three equations. It’s important to understand that the three estimation methods will provide widely varying results in some cases. Each provides only an estimate of bitterness, and none are perfect or all encompassing. I would not spend too much time worrying the differences. Choose a single estimation method and stick with it.[Smith,2008]
Here is a page with all three methods on it and their equations.[Pyle,1995]
Sources
[Smith,2008] Brad Smith author of BeerSmith Brewing Software, BeerSmith Forum Blog
[Pyle,1995] Norm Pyle, Realbeer.com Hops FAQ's page
--By WortMonger, member of HBT.com 18:55, 18 August 2008 (CDT)
