By David Cordrey
(This post was restored from the old site, and contains some really good and knowledgeable information, and deserves to be kept up. If you are reading this David and would like it removed just contact us) Props to David for the awesome write-up.
How hop bitterness is determined:
The bittering agents in hops are found in the soft resins of the hop flower. These chemical agents; Humulone, Cohumulone and Adhumulone collectively are refered to as Alpha Acids. Alpha acids themselves are not soluable in water or beer wort, however, at elevated temperatures (~212°F) Alpha Acids go through a chemical change called isomerzation. These Iso-Alpha Acids are partially soluable in water and beer wort.
When you buy hops the Alpha Acid content is usually stated in percent by weight. Typical values are 4 to 12%, depending on the hop variety. The amount of iso-alpha acids that are actually dissolved in the wort are typically only 15 to 20% of the total alpha acids available in the hops, depending on the boil time and specific gravity of the wort.
The bitterness of beer is commonly measured in International Bittering Units (IBUs) representing the the amount in parts per million of dissolved iso-alpha acids. Both the AHA and BJCP publish guidelines for hop bitterness in IBUs for the various beer styles. Since the equipment and methods to actually measure iso-alpha acid concentration is beyond the reach of homebrewers, a method to estimate IBUs must be employed by homebrewers. Fortunately, this is a straight forward calculation which can be applied when formulating a recipe in which a target IBU range is sought.
W * A * U * .7489
B = ————————–
V * (1+(G-1.050)/0.2)
B = bitterness in IBUs
W = weight of hops used in ounces
A = the weight percentage of alpha acids in the hops
U = the percent utilization of alpha acids
V = the volume of wort
G = the specific gravity of the wort
The alpha acid utilization is based primarily on boil time, but is also affected by specific gravity of the wort and whether the hops is in pelletized or whole form. Typical Utilazation factors are in the range of 15 to 25%. Pelletized hops have about 20% more bittering potential than whole hops because the soft resins have been upset and made more available during the pelletizing process. Thus, it can be seen that the the bittering potential of hops in beer is a function of the amount of hops used, the amount of alpha acids in the hops, length of the boil time, the volume of wort and the specific gravity of the wort.
Estimating the Utilization factor:
In his book Designing Great Beers (Brewers Publications, 1996), Ray Daniels presents tabulated data on hop utilization which I have used to come up with the following formulas based on boil time and whether or not pelletized hops is used:
For Whole Hops use:
U = -(.0041 * Time^2) + (0.6261 * Time) + 1.5779
For Pelletized Hops use:
U = -(.0051 * Time^2) + (0.7835 * Time) + 1.9348
Where Time is in minutes.
The above formulas assume a 1.050 specific gravity wort. Correction for other specific gravities is made in the denominator of the first formula.
These formulas can be easily input into a programmable calculator or spread sheet for ease of use. Otherwise this graph can be used to estimate hop utilization.
It is important to remember that these formulas are only estimates for predicting the IBUs in a given beer. Only laboratory analysis can tell you exactly how many IBUs are in a given beer. These formulas are useful tools for predicting the bitterness of a recipe or for determining how much hops of a given Alpha Acid content to use to hit a target bitterness level.