http://bavarianbrewerytech.com/news/boilhops.htm
Boiling the hops longer than one hour will start generating sharp, undesirable and unpleasant flavors.
http://byo.com/european-pale-lager/item ... nd-trouble
But when doing 90-minute boils, add the kettle hops about 15 minutes into the boil to avoid unpleasant bitterness from over-boiling the hops.
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