After the wort is boiling, the first hops for bittering should go in.
These are the hops that will be in contact with the wort for the longest time.
Therefore, they will be responsible for the majority of the beers bitterness.
As the boil continues the acids extracted from the hops increases.
After about 45 min's, the rule “cost of diminishing returns” takes hold as less and
less of the oils are being isomerized into bitterness.
There is a finite amount of bitterness that can be imparted to a beer. Bitterness
is measured in IBUs ( International Bitterness Units ). An IBU is one part per million
of isohumulone dissolved in the beer.
The scale for reference to styles:
15-25 Brown Ale
15-30 Blonde Ales
18-25 Marzen and Oktoberfest
40 + IPA ( India Pale Ale )
After the initial hop additions, one can add more hops throughout the boil. Usually
there are late hop additions very near the end of the boil or at flameout. This would
be done to give the wort a big hop presence without really extracting further bitterness.
The idea being to have say a hop nose ( floral or otherwise ) or up front hoppiness.
The flowery notes of the hop are quickly burned off unless added near the end. Some
even prefer to add a late hop addition minutes after the boil during cool down. It’s
all what you want, so experiment.
In addition to all that, there is dry hopping. After fermentation in a secondary
fermenter or when kegged, hops are added usually in a hop sock or bag so as not to
muddy up the beer. They then sit in the beer for a few days and add a huge bouquet
to it. Mostly used for things like IPAs.