Hops are the female flower of the hop plant, humulus lupulus. Used in beer for flavour, aroma, bittering, and as an antimicrobial agent (a preservative against bacteria and other spoilers). Originating in China, the first documented cultivation was in 736 in the Hallertau region in Germany, though not documented in brewing until 1079. It was the 1400s before hops were becoming popular across continental Europe and the 1500s before they made their way to the Kent region of England.
The hop plant grows throughout the area along the 48th parallel and are mostly in grown Europe, China, and North America in the north and New Zealand in the south. The hop plant grows from spring until late summer/early fall when it is harvested. The plant is typically cut down to almost ground level and the cones are removed. Large farms will harvest and remove the cones with machinery. The cones are usually dried for longer storage and often pelletized. Occasionally hops are used fresh (within a day or so of harvest) for a fresh “wet hopped” ale. For longer storage, hops are vacuum packages, kept cold and out of direct light—even then, most hops have a year or so of shelf life.
One exception to hop storage would be aged hops. These are hops that have been aged in less than ideal conditions for 3 or more years. These hops have lost most of their bittering, flavour, and aromatic properties. They’re used in Belgian-style sour ales where bitterness and hop flavour are not desired, but rather, the balancing flavour components come from the use of bacterial cultures to impart a sour/tartness in the beer.
The hop cone is mostly the green, leafy part, but at the centre of the cone, when the cone is pulled apart, there is a yellow, powdery substance known as lupulin. The lupulin is where the good stuff is. Hops contain various essential oils that contribute to the favour and aroma.The more of these oils, the more flavour and aroma But they are volatile and are easily boiled away. That’s is why flavour additions are done with 15 or less minutes left in the boil and the aroma additions are done with 5 or less minutes. Early additions will impart some flavour and aroma, but to a much less extent.
The main contributor to bitterness, and is often the only stat given on the hops, is Alpha Acid (AA%). This is given as a percentage of weight, with higher levels meaning more bittering potential. Alpha acid is not readily soluble in water, however, and to get the acid into the beer it has to be boiled for an extended time; typically 60 to 90 minutes will max out the hops and less than that producing predictably less bittering. When boiled, the acid is isomerized into the wort. The amount of bittering in a beer is given in International Bitterness Units (IBUs).