Hops are the flowers (also called seed cones or strobiles) of the hop plant Humulus lupulus.They are used primarily as a bittering, flavouring, and stability agent in beer, to which, in addition to bitterness, they impart floral, fruity, or citrus flavours and aromas.Hops are also used for various purposes in other beverages and herbal medicine. The hop plant is a vigorous, climbing, herbaceous perennial, usually trained to grow up strings in a field called a hopfield, hop garden (nomenclature in the South of England), or hop yard (in the West Country and US) when grown commercially. Many different varieties of hops are grown by farmers around the world, with different types used for particular styles of beer.
Hops are usually dried in an oast house before they are used in the brewing process. Undried or “wet” hops are sometimes (since ca.1990) used.
The wort (sugar-rich liquid produced from malt) is boiled with hops before it is cooled down and yeast is added, to start fermentation.
The effect of hops on the finished beer varies by type and use, though there are two main hop types: bittering and aroma.Bittering hops have higher concentrations of alpha acids, and are responsible for the large majority of the bitter flavour of a beer. European (so-called “noble”) hops typically average 5–9% alpha acids by weight (AABW), and the newer American cultivars typically range from 8–19% AABW. Aroma hops usually have a lower concentration of alpha acids (~5%) and are the primary contributors of hop aroma and (nonbitter) flavour. Bittering hops are boiled for a longer period of time, typically 60–90 minutes, to maximize the isomerization of the alpha acids. They often have inferior aromatic properties, as the aromatic compounds evaporate during the boil.