OK, I'll start with a favorite of mine (see above post)
IBUs
Palmer explains this extremely well (see link below), but I've given an example in metric units.
http://www.howtobrew.com/section1/chapter5-5.html
The "International Bitterness Unit" is a measurement of concentration of alpha acids in the beer, expressed in milligrams per liter.
factors affecting the IBU are:
- the mass (weight) of the hops (in grams)
- the percentage of alpha acids in the hops (%AA)
- the specific gravity of the boil
- the final volume of the batch (in litres)
- the time the hops are in the boil (in minutes)
the IBU is calculated like this:
IBU = (mass of hops) x (% Utilization) x (%AA) / [(boil volume) x 10]
where the percentage values are whole numbers (ie if you have %AA = 6.5%, use 6.5 for the calculation, not 0.065)
add the IBU contributions up if you have more than one hop addition.
The utilization is a function of boil time and boil SG, and represents the fraction of alpha acids in the hops that make their way into the wort.
this can be looked up on a table like the one in Palmer:
http://www.howtobrew.com/section1/chapter5-5.html
(you'll have to scroll down a bit, and multiply these figures by 100 to get the percentage values)
Utilisation values are highly variable depending on whose data you use. Palmer uses the Tinseth values, which is usually the default setting for most brewing software (I think).
However, if you are using another brewing manual, such as Papazian's "Complete Joy of Homebrewing," then you may use a different scale. It is therefore best to use just one scale and state which one you are using in your recipes.
(Papazian uses the Rager scale, which is much higher than Tinseth. for example, for a 45 min boil with SG = 1.070, the Rager value is 24%, and the Tinseth value is 17.7%)
formulae for Utilization are complicated, so tables are good unless you are doing a spreadsheet:
Tinseth: %utilisation = 165 x 0.000125^(Gb - 1)x[1 - e^(-0.04 x T)] / 4.15
where Gb is the boil SG, and T is the boil time
(see the link to Palmer)
for Rager:
% utilisation = (18.11+13.86*TANH((T-31.32)/18.27))
for a boil SG of 1.050 or less
if the boil SG is greater than 1.050, divide the above value by 1+(Gb-1.05)/0.2)
(source:
http://www.realbeer.com/hops/FAQ.html#units)
now THAT'S why the tables are useful
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so for an example (using the Tinseth scale):
if we have these hop additions:
40g of hop xyz AA = 7% boiled for 45mins
25g of hop zxy AA = 4% boiled for 15 mins
boil SG = 1.050
final volume = 20L
for the first contribution: Utilization = 21.2%
IBU1 = 40 x 21.2 x 7 / (20x10) = 29.7
for the second contribution: Utilization = 11.4%
IBU2 = 25 x 11.4 x 4 / (20x10) = 5.7
TOTAL IBU = IBU1 + IBU2 = 29.7 + 5.7 = 35.4
(Rager comes out at about 42)
