When making beer it helps to have certain guidelines that can help the brewer shape their overall goal. For example, with Malts we have the Lovibond scale which directly correlates to the SRM scale, allowing you to have a pretty good idea of what your beer will look like. Knowing the color of your beer can give you some idea of what your beer may taste like. What about bitterness though? How can we determine how bitter our beer might be? For hop bitterness we use the IBU scale. IBU stands for International Bittering Units (even though there is nothing “international” about it) and is based on the amount of isomerized alpha acids that get into your beer from the hops. When hops are boiled, the alpha acids in the hops become mirrored isomers and become soluble in wort. The longer you boil the hops, the higher the isomerized alpha acids (and therefore bitterness). In an ideal world, more hops = higher IBUs, right? Not quite. Although the iso-alpha acids are bitter, the scale doesn’t truly measure the bitterness and taste of a beer since a sweet beer (i.e. High Final Gravity) with the same IBU as a dry beer (i.e. Low Final Gravity) will not taste as bitter. Nor does it accurately reflect the amount of iso-alpha acid in the finished beer. IBUs are based on spectrophotometer readings that also read non-iso-alpha compounds within the same scale. IBUs are most important to professional brewers because if the IBU of their beers start to diminish even with the same hops at the same sizes, then they know their hop batch is beginning to oxidize. This is a big variable as hop alpha acids degrade with time and temperature based on storage conditions, the AA% will decrease over time - don’t worry though there’s math for that too.Since breweries use fancy spectrophotometers to measure IBUs, what is a homebrewer to do? Well, luckily there have been some math loving homebrewers out there that have worked backwards and figured out formulas to determine IBUs during the boil. The most widely used formula is from a gentleman named Glenn Tinseth. His formula takes the length and gravity of the boil to figure out the utilization of the hop iso-alpha-acid, which is the amount that actually gets into the wort. The higher the gravity, the lower the utilization, the longer the boil the higher the utilization. This is coupled with the amount and the alpha acid percentage of the hops.
To calculate IBUs with Tinseth’s method we would use the following formula:
IBUs = decimal alpha acid utilization * mg/l of added alpha acidsMetric:
mg/l of added alpha acids = decimal AA rating * grams hops * 1000
volume of finished beer in liters
mg/l of added alpha acids = decimal AA rating * ozs hops * 7490
volume of finished beer in gallons
This formula gets us pretty close to the spectrophotometer’s results. Luckily for us most homebrew recipe calculators use Tinseth’s math so we don’t have to calculate. One thing Glenn did not take into account though was the amount of iso-alpha-acids utilized during flame out or whirlpool. Homebrewers simply didn’t add massive amounts of dry hops or whirlpool additions back when Glenn wrote his original formula in the same way they do now. Using Tinseth's original calculations would mean that flame out or whirlpool additions add no IBUs, but in reality there is still some isomerization of alpha acids at any temperature above 180˚F. For example, a ten minute flame out addition will still have a certain amount of utilization and thus add a small amount of IBUs to the wort.Recently we’ve come across a Modified IBU calculator (mibu) that takes all aspects of hop additions into effect. Paul Hosom put this spreadsheet together so the IBUs from flame out hops can be calculated accurately. This is useful if you are making a North East IPA without any bittering additions (as many brewers now do). Adding all of the hops at flame out is likely to get you to the bitterness you desire and leave all of those delicious volatile oils behind. There are a lot of variables he takes into consideration like kettle diameter, boil off rate, cooling method, Krausen level and Post-Boil Wort Temperature Decay among others, all variables that will affect your IBUs. Luckily the calculator has links to the math that went into the making of it, if you like brewing science rabbit holes as much as we do.
Takeaways for the Homebrewer (TLDR)
How should a homebrewer use all this information? The answer is to combine the awesome power of math and brewing research with our senses and contextual clues to determine what level of bitterness we’re aiming for. It’s better to think of IBUs as a planning tool, rather than as a hard and fast rule that must be adhered to. Using IBUs as a sensory tool allows you to create or replicate recipes within the bitterness range you desire. For example, you calculate your beer to have 60 IBUs (based on all of the formulas and hop additions), ferment it and taste the finished beer. You now have an idea of what 60 IBU tastes like. It might be too much, it might be too little, but now you can plan accordingly for your next batch based on the info you’ve already gathered. Another way to look at bitterness is to look at the ratio between IBU and Original Gravity. Let’s take a look at one of our favorite recipes - Other Half’s All Together. This is a NEIPA that’s heavily dependent on flameout/whirlpool hops and includes over 14 ounces of hops for a 5 gallon batch. A majority of the bittering hops are added late in the boil or at flameout which leads to a relatively low (given the amount of hops being used) calculated IBU of 34. What’s the ratio of bitterness to gravity in this beer? We’d use this formula: BU/GU Ratio = IBU / ((OG - 1) * 1000)
All Together Ratio = 34 / ((1.064 -1) * 1000) = 0.53When we use the ratio of BU/GU (Bittering Units / Gravity Units) we see that this isn’t really a bitter beer at all, it’s relatively balanced. Conversely, our Conifer Conspiracy IPA has a BU/GU Ratio of 1.19! That’s a bitter beer. Seeing the relationship between all of the variables can really help you get a better understanding of the bigger picture.
Remember, especially at a homebrew scale, sensory evaluation is everything. Take your numbers, write them down and use them as a guide when you plan, but your perceptions will be your best tool.John LaPolla