Late Hopping is the addition of hops during the latter part of the kettle boil. During the boil, bittering acids are isomerized from hops into the wort, but the volatile oils responsible for hop flavor and aroma are largely boiled off and therefore lost.
Late hopping is employed to keep more aromatic hop components in the wort so that they can later become part of the finished beer. There is no set point during the boil that brewers perform late hopping, but hops added any time within 30 min of end of the boil are generally considered “late hops.”
Today many beer styles incorporate late hop additions during the kettle boil, and these additions are often blends of different hop varieties. Such blends can become part of the signature aroma of a particular beer. Although late hopping may be a well-accepted technique for adding hop flavor and aroma to beer, the actual mechanism and hop compounds that create these are imperfectly understood.
The variety of late hops chosen by a brewer is often based on the aromatic qualities of a particular hop variety. Careful hop breeding has resulted in cultivars with exceptional aromatics, which is why they are often called aroma hops.