Three reasons. First, if you use the blowoff method, either the hops will clog the blowoff tube or the kraeusen will pump hops out of the fermentor and into the blowoff vessel, which will give your blowoff a nice hop nose but which won’t do your beer any good. Second, when carbon dioxide actively bubbles out of the green beer it takes with it dissolved gases (like excess oxygen) and other aromatics (like hop aromatics). (For this same reason, if you’re adding fruit to your beer, I recommend that it not be added until the really ferocious fermentation is over.) Third, it’s safer to wait until fermentation is almost finished to minimize sanitation worries.
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