Beer with no hops would be very sweet and fairly tasteless. It's the bitterness of the hops that balances the residual sweetness of the malt, and adds aroma and flavour.
Hops are the flower cone of Humulus lupulus, a vine related to cannabis. Only the female plant produces flower cones, which are light and the consistency of tissue paper.
Hops contain alpha acids, beta acids and essential oils, all of which contribute to the flavour or aroma of beer.
Alpha acids are the main bittering component in hops and have an antiseptic and preservative effect, which lengthens a beer's shelf life. However, alpha acids are not soluble in water and must be boiled in order to isomerise them and allow their bitternes to be released into the beer. The longer hops are boiled the more of their alpha acids are isomerised. A typical boil is between 60 and 90 minutes, although some are up to 120 minutes. The closer to the end of the boil hops are added the less bitterness they will impart, and hops added during fermentation will add no bitterneess, just aroma and flavour.
The amount of alpha acid (AA) in a hop varies between varieties, and even the location in which the hop is grown and the season can influence the amount of AA. The AA content of hop varieties can range from 3 per to 19 per cent. Hops are usually sold labelled with the season and percentage alpha acid (AA) by weight so that brewers can add the correct amount to ensure the beer has a given bitterness, which is expressed in International Bitterness Units, or IBUs. (Sometimes bitterness is expressed in European Bitterness Units, or EBUs, are used but they are in essence the same as IBUs.)
The alpha acid content of hops is important for commercial breweries because the more AA a hop contains the less needs to be used to achieve a given bitterness and therefore the lower the cost. Saving a small amount of money usually isn't such a consideration for homebrewers, who may choose to use lower-AA bittering — or kettle — hops. In addition, many brewers and beer connoissers believe that using high-AA hops for bittering can lead to a harsh bitterness in beer and so use low-AA alternatives.
Beta acids in hops are not isomerised during the boil and contribute little bitterness and taste to the beer but some aroma.
Hops also contain essential oils, which add flavour and aroma to beer. These oils evaporate during the boil, and so to impart these in a beer the hops must be added towards the end of the boil, after the boil has finished, or during fermentation or maturation (known as dry hopping).
Different hops impart different flavours and aromas to beer. These characteristics can be described variously as "grassy", "floral", "citrus", "spicy", "piney," "lemony" and "earthy". The term "new world hops" refers to newer hop varieties from countries such as the US, Australia and New Zealand.
A group of hops with low alpha and beta acids and high essential oil content that are used in classic European lagers such as Pilsner Urquell are known as "noble hops". Because these hops — Hallertau, Tettnanger, Spalt, and Saaz — usually have 3 per cent to 5 per cent AA a lot of them are needed to bitter the beers they are used in. This makes producing these beers expensive, but the results speak for themselves.
There are hundreds of hop varieties but only a few dozen of these are widely used by commercial brewers. Small breweries, microbreweries and homebrewers may use a wider variety of hops, including new varieties developed around the world.
Hops are available to brewers in three forms:
Hop cones or leaf hops (a misnomer because they are not actually leaves), are whole hop flowers that have been picked, usually dried and compressed into bales. Because the processing for whole hops is minimal, most of the flowers' delicate glands containing the aromatic oils and resins are intact. However, because the entire surface of the hop flower is exposed to air, they lose their bitterness, aroma and flavour relatively quickly. Therefore, freshness is essential when buying and using hop cones. If your recipe calls for hop pellets and you use cones, use about 25 per cent more cones.
Some hops are used immediately after picking in a process called "wet hopping". This is thought to impart more flavour and aroma than using hops that have been dried because no essential oils have been lost during processing and storage. Examples of wet-hopped beers include Sierra Nevada Harvest Ale, from California, and Tuatara Brewing's Conehead, from New Zealand.
Plug hops are whole hop flowers compressed into large pellets. The processing leaves the hop flower somewhat more intact than with hop pellets and helps preserve them better than hop cones because of the lower surface area.
Hop pellets are pulverised hop flowers compressed into small cylindrical pellets. Many feel these are the best form of hop because there is minimum exposure to the air and the oils and aromas remain trapped inside the pellets. Usually, the pellets are sealed in a vacuum-packed foil bag, which helps preserve them. If a recipe calls for hop flowers or cones and you use pellets, use about 20 per cent less pellets. Pellets are by far the most common form of hop that Australian homebrewers will come across.
When buying hops, make sure they have been stored in a refrigerator at the shop, away from light and in a vacuum-sealed bag. Heat, light and oxygen are the enemies of hops. As with all your homebrew ingredients, buy from a reputable supplier that turns over plenty of stock. If your homebrew shop does not declare the AA and year of the hops it sells, be wary of their quality. When you get hops home, store unopened packets in the fridge for short periods, or in the freezer for longer-term storage. Once the packets are opened seal them tightly in another bag and keep them in the freezer.
Some homebrew shops sell hop "tea bags" that are used to add hops at the end of the boil or for dry hopping during fermentation or maturation. Often these have not been stored well and it is an expensive way to buy hops. You are better off buying a small bag of pellets and using what you need then freezing the rest in a zip-lock bag. In any case, the small "tea bags" contain only a small amount of hops, and much less than most brewers would normally add as late or dry hops. Hop pellets won't do any harm if left in the beer while it ferments and will settle to the bottom. If you prefer, buy a "hop bag", which is a small mesh bag with a zip or tie, and put your hops in that before you throw it into the boil or fermenter.
It should be noted that old hops aren't necessarily bad hops. In fact, some Belgian brewers use aged hops, which lose most of their bitterness, flavour and armoma, yet retain their preserving power.
To find out more about hop varieties and their characteristics and uses, see our Hop varieties page.
Read more about hops at Wikipedia.