There are many websites touting the virtues of hops and the qualities that gives them their great flavours. This article digs deep into the bowels of hop science to discover what makes them tick.
It’s no secret that alpha acids are what gives hops their bitter qualities, and that alpha acids are really a grouping of three primary acids: humulone, cohumulone and adhumulone. (There are about 250 oils in lesser degrees that do not contribute much to the flavours of the hops.) A little known fact is that cohumulone is the alpha of the three.
As you may already know alpha acids isomerize to form isohumulone acids that give the bitter taste in beer and provide antibacterial properties that prolong beer shelf-life. Isomerization is simply the process of rearranging the molecular structure of a molecule but that otherwise keeps the same chemical formula. Each alpha acids are about equally bitter, but the utilization of cohumulone is about twice that of the humulone and adhumulone acids. This is why you never see humulone and adhumulone listed individually, but you will see the percentage of cohumulone. (Hop Union does this.) So, hops that have a high concentration of cohumulone as opposed to the other two will taste quite a bit more bitter.
Between humulone and admumulone, the utilization of the two are virtually the same and the margin of error is small enough that it doesn’t give a drastically different result when combining them, but it is found that cohumulone accounts for a 21% difference in the bittering power of a given hop when accounting for the cohumulone factor. It is for this reason that hops with high cohumulone give the impression of having a more harsh bitterness than hops with lower levels. So, when calculating the bittering power of a hop, it is important to take into consideration the percentage of cohumulone and apply the following calculations to obtain the real bittering power.
Cohumulone Bittering Formula:
Thus, a hop that has 15% alpha acids with 40% cohumulone:
Hop Bitterness Degrades
As most brewers know already, hop bittering power degrades over time at the rate of about 7% per year. So, to make matters worse, you have to factor in the age of the hop from the time it was harvested and dried to the time of your brew. Thus, this equation is handy:
Therefore, using the same hop in the example above, but that is 10 months (304 days) old:
Putting It All Together
Precision becomes more important for those beer styles that require a lot of hops like IPAs and even more so with double IPAs. Without factoring in the cohumulone percentage and age of the hops, there is no realistic way to calculate IBUs. So, to get a better equation to calculate hop bitterness, you’ll need to have an idea of how much cohumulone that your hop has. Then you can employ this equation: