Form the Anglo-Saxon word "hoppan," hops are a perennial herb whose flowers grow into cone-shaped structures on a vine. Hops are used in beer primarily for their bittering and aromatic qualities but also have a preservative effect and aid in clarification and head retention. The leaves of the female hop plant carry the lupulin glands which produce the resins and oils that provide the plant's distinctive aroma, flavor and bitterness. It is these characteristics of the hops that are used to balance the sweetness of the malt.

The bitterness of hops comes from the alpha acids that exist in the resin of the lupulin glands. Alpha acid percentage (AA%) is a measure of the percentage of the hop flower's weight that is composed of the alpha acid resin. The higher the alpha acid percentage, the more bitter the hop. Alpha acids only become water soluble and release their bitterness when boiled, which is why hops used for bittering are added at the beginning of the wort boil. The flavor and aroma of the hops come from hop oils, not the resins, Unfortunately, those oils are destroyed if boiled too long, which is why hops added for flavor or aroma are added at the end of the wort boil. The lupulin glands also contain beta acids, but they have little to no effect on beer.

Hops suitable for homebrewing come in four different forms: flowers, plugs, pellets and extracts. All hops (except, of course, hop extracts) are dried before packaging. However, it is advisable to get the freshest hops possible as they degrade over time, especially if exposed to heat and oxygen. Hops should be packaged in air-tight oxygen-proof or nitrogen-purged bags to maintain their freshness.

Whole hops flowers are generally considered the freshest if purchased within one year of harvesting, but they will retain their bittering, flavoring and aromatic qualities if refrigerated. However, if they are not packaged in oxygen-proof bags, hop flowers are more subject to heat damage and oxidation than plugs or pellets. Furthermore, hop flowers require a longer boil (approximately 90 minutes) to attain their full bittering potential. Flowers should be green or goldish-green when used. Brown hops are stale and should not be used.

Hop plugs are whole hop flowers compressed into a half-ounce round disc. When added to the boil, the plugs rehydrate and begin to resemble whole flowers. Although some of the aroma may be lost in the compression process, they are a good middle ground between the freshness of whole flowers and the convenience of pellets.

Hop pellets are hops that are ground into powder then compressed into pellets. Pellets have a much longer shelf-life than flowers or plugs and increase the utilization of bittering acids by crushing the lupulin glands. Almost all hop varieties are available in pellet form.

Hop extracts are created by distilling whole hop flowers and are used primarily for adding hop aroma to beer just prior to bottling.

Certain hop varieties are more suited for bittering, others more suited for finishing (flavor and aroma), while others are good all-around hops and can be used for bittering or finishing. Accordingly, the brewer selects the hops for his recipe depending on the hop character he would like to impart in his beer. The hops are then added to the wort at various times of the boil to achieve their optimum effects. For example, bittering hops should be added to the beer at the beginning of the boil to allow the alpha acids to break down and impart their bittering potential. Flavoring hops should be added approximately 15 minutes prior to the end of the boil so as not to destroy the delicate hop flavors by over-boiling. Likewise, aromatic hops should be added no more than 5 minutes before the end of the boil, again so as not to destroy the delicate hop aroma by over-boiling. Hops used for aroma can also be dry-hopped, or placed directly in a secondary fermenter prior to bottling or kegging.

The two primary ways of achieving proper bittering rates for homebrewing is by measuring HBUs (Homebrew Bittering Units) or IBUs (International Bittering Units). Only an approximate measure of the total alpha acids contributed to the beer, HBUs are calculated by multiplying alpha acid percentage by the total ounces of hops boiled for more than 15 minutes. For example, if you're brewing an American Pale Ale and hopping it with 1 ounce of Perle at 8.0% AA for 60- minutes, 1 ounce of Cascade at 5.0% AA for 15 minutes and another 1 ounce of Cascade at 5.0% AA dry-hopped in the secondary fermenter, the beer would have 8 HBUs. The Cascade hops would not be considered when calculating HBUs because they are not boiled for more than 15 minutes. This measurement only approximates actual bitterness, as hops boiled for 16 minutes certainly contribute bitterness to the beer, but much less bitterness than hops boiled for 60 minutes.

On the other hand, the accepted worldwide standard for measuring bitterness in beer is by calculating IBUs. One IBU equals 1 milligram of isomerized alpha acid in 1 liter of wort or beer. IBUs may be calculated using the following formula:

Because the calculations for IBUs take another sort of calculation - percent utilization of the hops - into consideration, this concept requires a brief explanation. As indicated above, the longer hops are boiled, the more bittering character they impart. Pellets usually achieve maximum hop utilization with a 60 minute boil, whereas flowers and plugs require a 90 minute boil to bring out their full bittering potential. In addition, the gravity of the wort affects hop utilization - the higher the gravity, the lower the hop utilization. The following chart (lifted directly from Charlie Papazian's "The Complete Joy of Homebrewing, 3rd Edition") will help explain this concept. The hop utilization percentage for hop pellets is in parentheses, while the hop utilization percentage for whole hops and plugs is not.

Using the same hops as in the HBU example and the Hop Utilization Chart above, the calculation for IBUs would go as follows (assuming we were using all pellet hops and had approximately 2 pounds of extract for each gallon of water):

Therefore, the total IBUs for the Perle hops would be approximately 36. Below are the calculations for the flavoring Cascade hop addition:

The IBUs of the Cascade hops at 15 minutes to the end of the boil adds approximately 6 IBUS. Recall that dry-hopped hops are not boiled, so they impart absolutely no bittering to the beer. Therefore, taking the Perle IBUs and the Cascade IBUs together, this hypothetical beer has a total of 42 IBUs.

As stated above, different hops are better suited for different purposes: bittering, flavoring or aroma. Below is a list of the most commonly used hops, including a brief description, the purpose for which they are most commonly used and the alpha acid percentage.

1. Brewers Gold - 6-10% - Bittering hop traditionally used in German lagers and English ales.

2. Bullion - 5-10% - Bittering hop with strong, spicy flavors. A good substitute for Brewers Gold. Used in dark ales and stouts.

3. Cascade - 4-7% - Fuggles cross with a citrusy flavor. Primarily a flavoring or aroma hop but can be used for bittering. Used in American ales and lagers and is a favorite of homebrewers.

4. Centennial - 9-11% - Good for bittering and aromatic purposes, often described as a stronger Cascade. Used primarily in medium to dark American ales.

5. Challenger - 7-10% - Primarily a bittering hop, but has a nice aroma. A favorite of British homebrewers. Used in British and Belgian ales.

6. Chinook - 11-14% - A bittering hop, similar to Bullion, but stronger. Also used for its strong aroma. Suitable for American ales and lagers.

7. Cluster - 5-7% - A mild bittering hop with a nice floral aroma. Used in light and dark American lagers.

8. Columbus - 12-16% - A strong Centennial substitute, used for bittering and its very pungent aroma. Used in American ales, porters and stouts.

9. East Kent Goldings - 4-6% - The classic aromatic ale finishing hop. Used in pale ales, bitters, porters and stouts.

10. Fuggles - 4-6% - Another classic ale finishing hop. Suitable for English ales, especially pale ales, porters and stouts.

11. Galena - 11-14% - A bittering hop that is said to blend well with finishing hops. Used in American ales and lagers.

12. Hallertau - 2-7% - Grown in Bavaria in the largest hop-producing region in Europe. A good all-around bittering and finishing hop with a mellow, spicy aroma. Used in brewing altbiers, wheats, American and continental lagers, pilsners and Belgian ales.

13. Liberty - 4-5% - A softer version of the Mt. Hood hop, used primarily for finishing. Suitable for American and German ales and lagers.

14. Mt. Hood - 4-6% - A finishing hop with a light, delicate aroma. Derivative of the Hallertau Mittelfruh. Used primarily in brewing American and German ales and lagers.

15. Northdown - 8-11% - A versatile hop used for bittering, flavoring and aroma. Derivative of the Northern Brewer. Used in pale ales, porters and stouts.

16. Northern Brewer - 7-11% - Primarily for bittering, but also suitable for flavoring or aroma in small quantities. Used in California common beers, dark English ales and German lagers.

17. Nugget - 11-16% - A Brewer's Gold derivative, extremely bitter bittering hop with a distinct herbal and spicy aroma. Used in medium to dark American ales and lagers.

18. Perle - 6-11% - A hybrid of Hallertau and Northern Brewer, used primarily for bittering. Can also be used sparingly as an aroma hop. Used primarily in lagers, wheats and pilsners.

19. Pride of Ringwood - 8-10% - An excellent bittering hop grown primarily in Tasmania. Used in British ales, Australian ales and lagers.

20. Saaz - 3-6% - Aromatic, spicy and fragrant, a Czech hop used for bittering and aroma. Considered the quintessential Czech pilsner hop but also used continental lagers and wheats.

21. Spalter - 4-5% - An aromatic and flavorful hop considered an alternative to Saaz hops. Ideal for German lagers.

22. Styrian Goldings - 3-7% - No relation to East Kent Goldings, this hop is basically a Fuggles variety grown in Austria. Used as a finishing hop for its strong aroma but also used for bittering. Suitable as a finishing hop for English-style ales and a bittering hop for Viennas, Oktoberfests, Belgian ales and pilsners.

23. Target - 9-13% - Britain's most popular hop. Used as a bittering hop in British ales and lagers.

24. Tettnanger - A mild, floral and very aromatic all-purpose hop. Interchangeable with Spalter hops. Used in wheats and German lagers.

25. Willamette - 4-6% - An American Fuggles derivative with a spicy, aromatic aroma used as a finishing hop. Suitable for American and British ales.

26. Yakima - 10-16% - An American bittering hop used in American strong ales.