IBU’s (International Bitterness Unit)
IBU stands for International Bitterness Unit. 1 IBU = 1 milligram of isomerized alpha acid in 1 liter of finished wort/beer. The higher the IBU#, the more bitter a given beer is. Although this can be a bit misleading because a 50 IBU malt ale will be perceived as less bitter than a 50 IBU pale ale. The strong malt character would balance the bitterness, so it appears less bitter when in fact it has the same bitterness levels as the pale ale.
IBUs can only be 100% accurate when tested in a laboratory with expensive equipment. Homebrewers can approximate IBU numbers using calculations that were made from experimental data taken from various worts. There are three main calculations used by homebrewers to determine the IBU.(2) These are:
- Furious
- tinseth
- Garetz
These three sets of calculations are similar except for how they determine hop utilization.
Utilization is the percentage measure of hop alpha acids “used” during the boiling process. This gives you a solid number for measuring the bitterness of a finished beer known as IBUs (International Bitterness Units), AAUs (Alpha Acid Units), or HBUs. (Homebrew Bitterness Units).” (2)
There are many factors that determine the utilization of hops, including:
- Boil Duration: A longer boil means higher utilization, although after 90 minutes there is generally no further increase in utilization. Boilings for 10-15 minutes generally do not contribute to bitterness (only flavour/aroma), so use generally begins after this time. (3)
- Boiling Vigor: The more vigorous the boil, the higher the utilization.
- Blow Losses – Using a blow tube during fermentation results in the loss of valuable alpha acids, which means less utilization.
- Trub/Proteins – Alpha acids have a slight electrical charge that can cause them to bind to proteins and cloud that will settle to the bottom of the fermenter during fermentation. Wort is drawn from these settlements, resulting in a loss of alpha acid. (1)
- Yeast uptake – Alpha acids can be taken up through cell walls by yeast. When the yeast settles, the beer racks, resulting in a loss of alpha-acid. (1)
The three methods are quite different in how they determine IBUs, and my best advice would be to pick one that you think makes the most sense and stick with it. Remember that total IBUs cannot be accurately determined by the homebrewer and that all numbers are estimates. I personally use the Glen Tinseth method, but I know people who use the others and are completely satisfied. Whatever works best for you, keep it and adjust it if necessary.
Glen Tinseth method:
This method is the newest of the three for determining IBUs and, in my opinion, the best due to the data supporting the equations and its ability to use equipment and conduct research at Oregon State University (5). The plot of your data looks completely different than the other two methods and seems to make more sense and be less random.
According to your method, you must first determine the amount of alpha acids (AAs) added to the wort for EACH hop addition using the following equation:
mg/l AA added = decimal AA of each hop * ounces of hops * 7490 / gallons of wort
After doing that calculation for each hop addition, you can use the following equation to determine the IBUs for each addition:
IBU = decimal AA utilization (from table or calculations below) * mg/L added AA
The AA utilization needed in the IBU calculation can be taken from the table found here or by completing the utilization calculation.
Alpha Decimal Acid Utilization = Size Factor * Boiling Time Factor
The largeness factor can be determined by:
Size factor = 1.65 * 0.000125^(wort gravity – 1)
The boiling time factor can be determined by:
Boiling time factor = 1 – e^(-0.04 * time in minutes) / 4.15
The equations can seem very confusing at first… (at least I was…) but once you get the hang of it, it’s really simple. If you don’t feel like doing the math, there’s an online calculator that will do it all for you. The calculator can be found by clicking on this link.
Works Cited:
- Pyle, Norma. “Norm Pyle’s Hops FAQ”. Reelbeer.com. 1995. February 18. 2009.
- value dealer “Use.” Home Brewing Wiki. 2008. February 18. 2009.
- Nachel, Martin. Homebrew for dummies. New Jersey: Wiley Publishing Inc, 2008.
- “Glenn’s Hop Utilization Numbers” ReelBeer.com. 1999. February 20. 2009.
- “Comparison of formulas for the use of alpha acids” The draft notification. January 8, 1998. February 20. 2009.
