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For bittering purposes hops need to boil for at least an hour
(and this should be a good boil not a simmer). This boil causes the
alpha acids & oils to be extracted from the hop tissues, and
then to be isomerized into a soluble form which means they will
stay in your finished beer. If you have boiled your hops for 45 to
60 minutes you will have extracted most of their bittering power,
and really consider 30 minutes the minimum amount of boiling time
for bittering. Boils under 30 minute risk partial converted alpha
acids which may have unpleasant qualities (see George Fix, Analysis
of Brewing Techniques)
Bittering is a matter of degree, and every beer style has its
own range of desired bitterness. Recipes will usually indicate the
target bitterness with reference to HBU (home brew units) which is
also expressed as AAU (Alpha Acid Units). These (HBU and AAU) work
on the same idea. Take the alpha acid number of the hop you plan on
using, and multiply by the ounces of hop. If a 23 litre recipe
calls for 10 HBU of Fuggle and your Fuggles have 4.0% AA, then you
know you will have to use 2.5 ounces of hops (2.5 X 4.0 = 10). Some
popular book use this type of reference (Papazian Miller, Line)
A more accurate system of relating bitterness levels (and one
becoming very popular) is the IBU (International Bitterness Unit).
This system indicates the actual amount of isomerized alpha acids
which are in the beer providing bitterness (units are mg/L) You
could use the on-line Hop
Bitterness Calculator, or anygood brewing software. Fancy
programs will take into account the gravity of the wort boil which
affects the rate at which alpha acids are isomerized for
bitterness, e.g., the higher the gravity of your boil, the lower
the rate of isomerization of alpha acids.
There are many factors which affect the actual bittering
attained. I have mentioned both wort gravity and boiling time
above. These are two very important factors, but one which may be
missed is the use of a hop bag. These bags may make it easy to
separate the hops from the wort after the boil, but they do reduce
bittering potential by about 10% (so if you use these bags, you'll
have to take that into account and adjust the hopping rate).
Remember as well that if you employ a "blow-off" hose to remove
the sludge from the top of the krauesen, you may lose another 15%
of the bitterness which you calculated.
Lastly, Garetz also suggests yeasts affect bitterness too.
Highly flocculent yeasts such as Wyeast 1007, 1278, 1728, 1968 and
others, may increase apparent bitterness by 5%. Low flocculent
yeast, such as Wyeast 1388, 2042, 2247 and others may decrease
apparent bitterness by 5%.
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