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. The substances responsible for hop aromas are essential oils. They make up about 0.5% to 3% of the hop’s weight. Essential oils are composed of two fractions: hydrocarbons and an oxygenated fraction. Hydrocarbons, however, which make up 80% to 90% of the essential oil mass, are not necessarily responsible for hop aromas in finished beer, because they are extremely volatile and evaporate during the boil. Oxygenated fractions—made up of terpenes and sesquiterpenes—tend to be more aromatic and are less volatile. Late hop aroma in beer, therefore, may be largely derived from a few intensely aromatic compounds in oxygenated fractions. There is also the possibility of a synergistic effect between several compounds that are each present in amounts too small for the human nose to detect, but which, in combination, may achieve the sensory threshold. Some of these compounds have citrus and floral characteristics. The extent to which they survive the kettle boil, however, depends greatly on the method of late hopping and varies among breweries. In addition, there are additional aromas associated with late hopping that do not appear to be related to the characteristic scent that caused the brewer to select a particular hop variety. American craft brewers have been pioneers in the use of late hopping, not only adding aroma hops in the minutes before the end of the boil but also flavoring the wort with successful dosages of hops right to the very end of the boil. Some brewers wait until the boil is actually over, only adding the final hop pellets before the wort is sent into a whirlpool vessel. To some extent, this practice mirrors the purpose of the hop back, a vessel that strains the wort over a bed of whole hop flowers before it is sent to the heat exchanger and the fermentation vessel.
In many craft breweries, the majority of the hops used are part of the late addition. Conventional wisdom has held that hops need to be boiled to provide bitterness to wort, but this is not true. Even when added as late as the whirlpool, late hop additions add bitterness as well as aroma. Isomerization and extraction of hop bittering compounds are a matter of temperature, contact time, and surface area; actual boiling is not strictly required, especially if the wort remains in turbulent motion. The bitterness extraction is inefficient, because the hops are barely boiled, or not boiled at all, but large late hop additions can provide a majority of bitterness in styles such as American pale ale and India pale ale as well as contributing aroma and flavor. The aromatics given to beer by late hop additions are noticeably different than those given by dry hopping, with the latter giving a greener “hop sack” aroma. Late hopping and dry hopping are therefore often used in combination for the production of beers intended to have intense hop aromatics.