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International Bittering Units

Alpha Resins and Hop Oils
Hops flowers contain alpha and beta resins and essential oils within the lupulin glands of the flower. Alpha resins are not very soluble in water, therefore, need to be boiled to extract the proper bittering from the alpha acid(at least 60 Minutes).

Hop oils are very soluble in water and will boil off quickly with the steam of the boil. For flavor and aroma, you should be adding your hops in the last 15 minutes.

Dry Hopping
Dry hopping gives no bitterness but lots of aroma and should be done for at least a week to two weeks in the secondary.

International Bitterness Units - How to Determine IBUs
Here is a quick formula for determining International Bitterness Units (IBUs) from Fred Eckhardt:

For a 5 gallon batch of homebrew, you take Ounces x Alpha Acid x Percentage Utilization(boil time) divided by 7.25.

Example - we have a Pale Ale using Cascade hops with 6% alpha acid, it would look like this:

1 Ounce(Boiling Hops) x 6%(Alpha Acid) x 30%(Utilization) divided by 7.25 =24.8 IBUs
1.5 Ounces(Flavor Hops) x 6%(Alpha Acid) x 15.3%(Utilization) divided by 7.25 =15.8 IBUs
1 Ounce(Aroma Hops) x 6%(Alpha Acid) x 5%(Utilization) divided by 7.25 =4.1 IBUs

24.8 IBUs + 15.8
IBUs + 4.1 IBUs = 44.7 IBUs. This beer would have 44.7 IBUs

Percentage Hop Utilization for a 5 gallon batch of homebrew
31%      over 60 Minutes
30%      51 to 60 Minutes
28.1%   46 to 50 Minutes
26.9%   45 to 41 Minutes
22.8%   40 to 36 Minutes
18.8%   31 to 35 Minutes
15.3%   26 to 30 Minutes
12.1%   21 to 25 Minutes   
10.1%   16 to 20 Minutes
8%        11 to 15 Minutes
6%          6 to 10 Minutes
5%         Up to 5 Minutes

Tyler Barber
Adventures in Homebrewing