Humulus lupulus, more commonly called hops, have been used almost exclusively in the brewing of beer for centuries. Hops provide bitterness, aroma, foam stability and act as a preservative in beer.
The soft resins found in the lupulin glands of the female cone contain the hop acids which are used to impart bitterness to beer. The essential oil in the lupulin glands provides the aroma or spicy flavor to beer.
Hops are harvested in late August or early September. During harvesting the hop cones are stripped from the vine and dried to about 3–5% moisture. The dried cones are then baled or milled or pelletized. Though most breweries use pelletized hops in the brewing of beer, increasingly many breweries are enjoying the advantages of brewing with hop extracts. Hop pellets are routinely extracted with liquid CO2 at about 2,400–3,400 psi and 40–55°C. The CO2 extract contains α- and β-acids and essential oils.