The bitterness from hops is extracted during the wort boil in the brewing kettle. The process of boiling causes the alpha acid compounds in the hop flowers to experience a structural change called isomerization. The resulting compounds are detected as bitterness on the pallet. The factors that influence this process include Boil Length, Wort Gravity, Alpha Acid percent and the physical Form of the prepared hops. These factors are used to determine the Utilization percent, which is a measurement of the percent of available alpha acid isomerized in the finished beer. Utilization, Wort Volume, Alpha Acid and Hop Mass are then used to calculate IBU (International Bitterness Units) a measurement often given with international style specifications.
Calculating Utilization
Because the boiling of hops in wort is a chemically complex process, not all factors that affect bitterness in beer are practically accounted for. Predicting bitterness in beer is a reasonable approximation at best, especially since bitterness can come from other ingredients such as dark roasted malts.
There are several individuals who have researched formulas to mathematically reproduce emperical utilization data. The Formula selector lists various methods that have been used to calculate Utilization, and they are indicated by the name of the researcher. Out of respect for the hard work of these individuals, BeerTools Pro simulates their methods instead of using them verbatim. You see (simulated) after the researcher name to indicate this.
You have to determine for yourself which method best matches your brewing process. This may have to be decided with taste as your guide, since measuring actual IBUs in beer is economically beyond the reach of many brewers.
Kettle Environment
The conditions hops experience in the brewing kettle, such as Wort Gravity and Boil Length, affect how thoroughly the hops are utilized. Enter the gravity of the wort and the length of time the hops are boiled. The Final Volume should be the volume of wort transfered to the fermenter accounting for evaporation loss and the addition of any balance water to increase volume. Trub losses should not be subtracted however.
Hop Characteristics
Commercially grown hops are packaged with Alpha Acid information specific to the current harvest. It is best to use the alpha acid percent given on the packaging rather than published averages. Enter the alpha acid value and select the Form the hops are packaged in.
Matching Targets
Once the above values are entered, there are a couple ways to arrive at the correct Hop Mass to use for the recipe. First, you may already know the hop mass from a published recipe. In this case, enter that value and the IBU field changes accordingly. Second, you can enter your target IBU, apply Lock, and the hop mass will adjust to maintain the target IBU. With the lock applied, you can move the graph point with the mouse and the hop mass will dynamically update with your changes.