Aroma hops are put into the boil, usually, around 15 minutes before the boil ends. They restore some of the flavour and aroma that are lost during the long boil to extract bitterness from the hops. If dry hopping is not being utilised, then these should be of the best quality hops available, and sometimes this can only be obtained by growing your own ! The Beta acid, and essential oil content are considered to be of prime importance at this stage.