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How do I calculate hop bitterness?

 

Answer: Ever since Reinheitsgebot, the German Beer Purity Law of 1516, hops have been the primary spice additive in beer. Hops were originally added as a preservative, but brewers quickly realized they also impart unique flavor characteristics.Among the various flavor aspects is bitterness, which provides a balance to the sweet flavor of malt. Getting the proper balance of maltiness to bitterness is one of the keys to great beer brewing.

Hop bitterness comes from a family of chemical compounds called humulones. Humulone dissolves very slowly in boiling wort, so the amount you get in your beer depends both on the amount of hops in the beer, and the length of boiling. In addition, the amount of Humulone varies from one batch of hops to another. The standard measure of Humulone is called alpha acid, (AA, or AAU) and is quoted as a percentage. Whenever you buy hops, make sure you note the alpha acid percentage.

There are two common scales of bitterness in beer. The official scale is called an International Bitterness Unit, or IBU. Technically, this is the concentration of alpha acid in the beer, expressed in parts per million. A simpler method to use is the Homebrew Unit, or HBU. For a five-gallon batch, one HBU is the amount of bitterness you get from one ounce of hops at 1% alpha acid when boiled for one hour. As an example, if you want 11 HBU for your recipe and you have Cascade hops that have an alpha acid percentage of 5.5%, you need 2 ounces.

Your flavoring and finishing hops will contribute bitterness too, but not as much. I use the following table to figure out how much is extracted, based on the boiling time:

 

Boiling Time, Minutes

Extraction % for Cones or Plugs

Extraction % for Pellets

60

100%

110%

40

80%

90%

30

66.6%

75%

20

50%

60%

15

40%

50%

10

29%

35%

5

15%

20%

0

0%

5%

 

Note that pellets will give you more bitterness than cones. This is because the pellets break up rapidly to yield a larger surface area for extraction.

To calculate your total bitterness, you need to add the contribution of each hop addition. Suppose you're using the following recipe for 5 gallons of Pale Ale:

 

60 minutes boiling Hops -- Nugget
11% AA, 0.55 ounces of cones

20 minute flavor hops -- Cascade
5.5% AA, 1.0 ounce of pellets

5 minute finishing hops -- Columbus
13% AA 1.0 ounce of plugs.

 

Multiply the AA by the amount and extraction efficiency for each addition, like so:

 

Nugget:11% x 0.55 x 100% = 6.05 HBU

Cascade: 5.5% x 1.0 x 60% = 3.3 HBU

Columbus 13% x 1.0 x 20% = 2.6 HBU

Total:                                       11.95 HBU

 

The last piece of information you need is how bitter your beer ought to be. Different styles have different requirements. Here are some typical values:

 

Beer Style

Bitterness, HBU

Wheat Beer

6.5

Scottish ale

10

Pale Ale

12

India Pale Ale

15

 

If you're using someone else's recipe, add up the HBU contributions based on their hop addition rates and use that overall. Then rework the number based on the hops you find at the brewshop. I usually use the amount of flavor and finishing called for in the recipe, and then make an adjustment in the amount of boiling hops. One final note: If you cool your wort slowly you'll get a bit more extraction. This is especially true if you're using pellets.