I want to start-off by saying I am not a specialist in the topic; and what I thought I knew has only gotten deeper since I started writing this post (hence the length and delay).
In my educational/academic life, I have a University degree (University of Guelph) with a major in Biological Sciences, Honours in Marine and Freshwater Biology with an area of emphasis in Graphical Information Systems and Environmental Analysis. That’s all to say that I understand the science behind what I am about to say; but only on the surface level.
In my professional life, I have been a part of the same industry for over 20 years, served the public in multiple roles within my silo, but, no matter how much experience I bring to the table, I do not possess the “DVM” at the end of my moniker, therefore, my coaching, advice and caution on policy and operational changes is met by a somewhat-significant roadblock when it comes to making physiological dispositions (MAYBE one day I will expand on that a little further). Yet, individuals completing their university program are given greater clout in disposition decisions the day they walk across the commencement stage…Seems kinda backwards.
The purpose of me stating this is that I completely understand my Brewery and Brewing journey “pales” in comparison to others, does not come with a degree or diploma, and I recognize there are elements of what I type that may not be 100% and completely factual; but I am trying, I hope to spark an interest in others to learn, and I, myself, want to continue to learn.
With that, here is what I know about IBUs (as car as calculating them in your brewing process).
First off, I will start with something I read and think is the most interesting thing when calculating IBUs in relation to hop additions. Did you know that IBUs are typically (and from what I can read, ONLY) calculated based on when, how much, and what type (specifically the AA%) of hopes you add TO THE BOIL (ie: Dry Hopping a batch does not impact the IBUs)? I didn’t know that! The interesting part about it is, again, from what I have read (and I’ll stop adding that caveat to every additional detail from here forward), the addition of hops to the boil adds to the bitterness units (IBU = International Bitterness Units) and Dry Hopping adds to the flavouring of the beer; and both have a role in the preservation of the beer (however different the scale may be between their importance in preservation may be - maybe we will write an entry about that in the future???). Before we go much further, I should define what AA% is. AA% is a representation of the Alpha Acid rating (or percentage) the hop has versus the total weight of the hop itself. Year by year, the AA% of a specific plant can change (however slight), which could impact the AA% and, therefore, impact the IBU of your patch. For example, if the AA% of your hop increases by as little as 0.5%, and you slightly alter your addition-time of the hops to your boil, say by 10 minutes, this could alter the flavouring, bittering units, and overall profile of your batch; so it is something to be aware of. All that being said, I wouldn’t panic if you haven’t been paying attention to the AA% of your hops between batches (but it could be a reason you may be tasting minor differences between like-batches); many people who spend a respectable portion of their lives becoming (or working towards becoming) cicerones are likely the ones who can CLEARLY note the differences, but many untrained individuals may not notice anything significant.
Getting to the math and science of the determination of IBU, the formula I turn to is this:
IBU = U% x AA% x (Weight of Hops in oz) x 0.7489 / (Volume of Wort in Gal)
or
IBU = U% x AA% x (Weight of Hops in grams) x 1000 / (Volume of Wort in Litres)
NOTE: Some people add a gravity correction factor in the denominator of THIS calculation…I’m not certain “why”, but you can try (1/C) where C = 1 + ( (Gravity before Boil - 1.050) / 2)
Everything is pretty straight forward in the calculation based on your recipe. If you have multiple hop additions to your boil, at multiple drop times, you would need to calculate the IBU for each of the hops, AA% and drop-times. After you have done that, you will need to add them together to get your total IBU.
The most complicated component of the equation is the U%. In order to know where in the chart (referenced below/attached) your U% value comes from, you will first need to take a gravity reading of your wort post-sparge and before your boil (also used in the second calculation to determine the correction factor (C)). From the chart, you merely find your gravity reading before boil and move down the column (or row - depending on the version of the chart you reference) and align your reading (U) with the drop time of your specific hop within your recipe.
After you have done this for each hop-drop, you add those numbers together to get your IBU; and you’re done!
Give it a shot! Have fun with it! If you have any questions or comments about this calculation or the blog, email me and I can make any corrections you may be aware of that I am not.
https://blog.homebrewing.org/how-to-calculate-ibus/