As we know by now, the basic premise of brewing beer is to extract sugar from malted barley and convert those sugars to alcohol through fermentation. To balance the sweet malt flavors that would otherwise dominate the beer, bittering hops are added during the boil.
Hops are a cone-like flower that grow on a climbing vine plant. Originally used for their ability to act as a natural preservative, hops are now a vital ingredient in the flavor profile of beer. Hops – often taking the form of pellets – are added to the boil at various times to contribute to the characteristic bitter flavors and aromas of beer.
Hops on the vine
Naturally occurring resins in hops are responsible for their bitter flavors. These resins are made up of two acids, alpha acids and beta acids, that are insoluble in water until they are boiled. Alpha acids are responsible for bitter flavors while beta acids add to a beer’s aroma.
Hops with high concentrations of alpha acids are added at the beginning of the boil giving them a full hour to impart their flavors into the beer. Additional hops are often added at various intervals during the boil that contribute to the final flavor and aroma. Hops added near the end of the boil do not have enough time to contribute bitter flavors but add pleasant smelling aeromatic oils to the beer.
Pellet hops
The bitterness of beer is commonly measured in International Bittering Units, or IBUs. IBUs allow brewers to compare the bitterness of beer with a constant measure. A typical ale will have 20-50 IBUs, although there are many exceptions to that range. Several factors need to be taken into consideration when calculating how bitter a beer will be. These include the amount of hops used, the concentration of alpha acids, the boil time, and how saturated the wort is with sugar (gravity).
(Science content ahead)
To calculate the IBUs of a beer, a brewer must first determine the alpha acid units (AAUs) from the hops being used. AAUs are dependant on the amount of hops being used (in ounces) and the percentage of alpha acids present in hop variety (%AA). A simple calculation is used:
AAU = weight (oz) x %AA
This number gives us the potential bitterness from the hops, but does not take the boil time or gravity of the wort into consideration. The formula for IBUs includes a term to account for these factors called utilization. Utilization describes how efficiently alpha acids are absorbed into the young beer and is a function of how long hops are boiled and the gravity of the wort. The formula for IBUs is:
IBU = AAU x U x 75/recipe volume
U = utilization
In the formula 75 is a constant to convert english units to metric and U is the utilization. Below is a chart to determine the utilization based on boil time and gravity. Knowing the number of IBUs in a recipe is useful in making changes in the amount and varietal of hops to tweak a beer’s flavor profile.
Utilization as a function of boil time and gravity
