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Even if you've only brewed a couple batches of beer so far, you've probably realized (if you didn't know already) that hops are a major factor in determining how your beer tastes. The primary contribution hops make to your beer is adding bitterness, but they add other dimensions as well, namely flavor and aroma. There are a wide variety of hops out there to choose from, and selecting the right ones can make the difference between a good beer and a terrific beer. In this node, we'll talk a little more technically about the properties of hops, as well as the characteristics and best uses for the more common varieties.

The hop plant produces tiny oil-bearing sacs at the base of its flower petals. These sacs are called lupulin glands, and the oils and resins they contain have two major characteristics: a pungent, tangy aroma and a strong, bitter flavor. In nature, the former helps attract bees and other pollinating insects to the flowers, while the latter discourages small mammals and herbivorous insects from trying to eat them. In our beer, they serve similar purposes. The fragrant oils lend aroma and flavor to the brew, while the resins add bitterness.

The resins in the lupulin glands contain two important types of acids, which are referred to as alpha and beta acids. Of these, the alpha acids are both more bitter and more numerous, and so a specific hop's bittering capacity is measured by its alpha acid content. This content is represented as a percentage - a number which tells you what percent of the dry weight of the hops is attributed to the alpha acid resins. Most hops have an alpha acid content of between 3% and 14%.

These acidic resins are not water soluble, and as such they need some coaxing to properly mix with your wort. This coaxing comes in the form of prolonged boiling, which will cause a chemical reaction in the resins called isomerization. Basically, this means their molecular structure changes to more closely resemble that of the medium around them (the wort). They can then dissolve and become part of the brew. As a result, to get the full effect of a particular hop's bitterness in your final beer, it must be boiled for the duration of your wort (at least 60 minutes).

Unfortunately, the aromatic and flavorful oils mentioned previously are more soluble in water and also more delicate in nature. As a result, this prolonged boiling will cause them to evaporate from the wort entirely. This means that if you want to add these characteristics to your beer, you will need to add more hops towards the end of the boil. Flavoring hops should be added in the last ten to fifteen minutes of your boil, and aromatic (or finishing) hops should be added at the very end of the boil. Remember that because the resins in these hops won't have the chance to isomerize and merge with your wort, they will add little or no bitterness, only flavor and aroma.

For people who keep records, you may want to have some way to calculate the exact bitterness of your final product. There are two commonly accepted ways to do this: one for normal humans, and one for mathematicians. The layman's bitterness measurement is the HBU, or Homebrewer's Bitterness Unit. To calculate your beer's HBUs, simply multiply the % of alpha acids in your hops by the number of ounces you use. So, if you brewed an ale with 3 ounces of 5% AA Cascade hops, your beer would have 15 HBUs. This gives only a vague representation of how bitter your beer actually is, but it will allow changes to your recipe more easily. For example, since hops' AA content can vary from season to season, you may go to your homebrew shop and discover that the Cascade they're selling now is 6% alpha. By dividing your target HBUs by that 6% content, you would see that you only need 2.5 ounces of hops to achieve the same amount of bitterness you did before (15 / 6 = 2.5). Remember to only use your bittering (60-minute) hops for this calculation.

Some more dedicated and technically-minded homebrewers may wish to perform more precise calculations which will allow them to find out precisely how bitter their finished beer will be. For this, you will need to calculate your beer's IBUs (International Bitterness Units). The real problem here is that the exact amount of alpha acid resins isomerized will vary not only by the hops you use, but also by the specific gravity of your wort and the length of your boil. The calculations can become quite involved and, quite frankly, they are well over my head. If you are interested in pursuing this further, I suggest you begin by visiting http://realbeer.com/hops/research.html, where you will find more in-depth information, as well as a JavaScript calculator that will do most of the dirty work for you.

It is important to note that no matter how precisely you calculate the amount of bittering compounds present in your beer, this will still not be a completely accurate representation of how bitter you will perceive your beer to be when you drink it. In general, the bitterness in lighter beers is perceived more sharply than that of darker beers. For this reason, a 50 IBU porter will not seem as bitter as a 50 IBU pale ale, even though they technically contain the same amount of bitterness. Keep this in mind.

Now, with all that out of the way, let's move on to a breakdown of the more common varieties of hops and their uses:

Brewers' Gold: A strong bittering hop, most often used in pale ales or heavier German-style lagers. Its aromatic and flavorful characteristics are very mild.
Alpha acid content: 8-9%

Bullion: One of the oldest high-alpha hops around, Bullion is a strong bittering hop, but also possesses an intense aroma often described as being similar to black currants. Most commonly used in stouts, ESBs and IPAs.
Alpha acid content: 8-9%

Cascade: Probably the most popular variety of hop used by homebrewers, due mostly to its well-balanced nature. Cascade has a pleasant but not terribly strong bitterness, as well as a flowery, citrusy aroma and flavor. This makes it a great all-purpose ale hop, and very well-suited to dry-hopping. It is also commonly used in barleywine.
Alpha acid content: 4.5-7%

Centennial: Often referred to as "Super Cascade", Centennial is significantly more bitter while being slightly less flavorful and aromatic, although along the same lines as Cascade. It is another good all-around ale hop, and many brewers will substitute an ounce of Centennial for an ounce of Cascade as a bittering hop if they want to give their beer more bite without having a dramatic impact on the flavor.
Alpha acid content: 9-12%

Challenger: A good dual-purpose ale hop, combining bitterness with an assertive, spicy aroma. Grown in the UK, Challenger is a staple among British pale ales, bitters and porters. I've made many a good Bass-alike with Challenger.
Alpha acid content: 7-9%

Chinook: A strong bittering hop with some unique aromatics. Popular when brewing pale ales, IPAs, stouts and porters, and great if you want a lager with some bite. Its aroma is piney with strong grapefruit elements, and it makes a great dry hop for IPAs and lagers.
Alpha acid content: 11-13%

Cluster: Spurned by many homebrewers as being too "commercial", Cluster mixes moderate bitterness with a moderately spicy aroma and is very popular among large breweries. Generally a good bittering hop for lagers, and a good aromatic hop for ales.
Alpha acid content: 6-8%

Fuggles: The quintessential British hop, Fuggles imparts a mild, dry bitterness and an earthy aroma. Put to good use in any British variety of beer, especially lighter ales and lagers. Its low alpha content prevents its use as a bittering hop in pale ales and ESBs, but it is still commonly used as an aromatic in these styles.
Alpha acid content: 4.5-5.5%

Galena: Hang on to your hat, because Galena will kick your ass. A wicked bittering hop, its pleasant citrusy aroma is usually overlooked. Makes a good bittering hop in any strong IPA, ESB, porter or stout.
Alpha acid content: 12-14%

Hallertauer: A superior aromatic and flavoring hop, Hallertauer has both German and American varieties. The American variety is slightly more bitter and less flavorful, making the German variety the choice for most homebrewers despite its lower availability. Hallertauer is good for lagers and bocks, and excellent in pilseners and weiss (wheat) beers.
Alpha acid content: 3.5-5% (German), 4.5-5.5% (American)

Kent Goldings: Another classic British hop, Kent Goldings' gentle, hoppy fragrance makes it a very popular choice for dry hopping. Also used widely in British ales and bitters, sometimes alongside something with more bittering potential.
Alpha acid content: 4-6%

Mt. Hood: Derived from Hallertauer, Mt. Hood shares many of the same characteristics, but with a stronger AA content. Some feel that Mt. Hood's aroma is more pungent and resinous than Hallertauer, and its flavor a bit more tannic.
Alpha acid content: 4-7%

Northern Brewer: A strong bittering hop combined with an equally strong aromatic component, most often used in pale ales, ESBs and porters.
Alpha acid content: 8-10%

Nugget: Another dual-purpose bittering/aromatic hop, Nugget's aroma is very strong and often described as "herbal". It is best suited to stouts, but finds some use in stronger ales from time to time as well.
Alpha acid content: 11-13%

Saaz: The classic (okay, only) Czechoslovakian hop, Saaz is used almost exclusively in pilseners, to which it lends its characteristic mild, herbal aroma. Occasionally used in weiss beers as well.
Alpha acid content: 3-4.5%

Tettnanger: A traditional "noble" aromatic hop, German Tettnanger has a mild, herbal aroma which is slightly spicy. Like Hallertauer, it also has an American counterpart, which actually has more characteristics in common with Fuggles than with Tettnanger. Make sure you're getting the German variety if you are using this classic lager and pilsener hop. It also enjoys some popularity in weiss beers.
Alpha acid content: 4-5%

Willamette: A variant of Fuggles grown in the USA, Willamette is enjoying some popularity as an aromatic hop, as its mild but complex fragrance blends spicy, fruity and floral elements. Very pleasant, and commonly used in weiss beers, pale ales, bitters, porters and stouts.
Alpha acid content: 4-6%.

That about wraps up the most common varieties of hops. Dozens of others do exist, of course, but these are the ones you are most likely to encounter early in your homebrewing career, and the ones which will be most available to you. In practice, it's best to follow recipes at first, and later begin experimenting by substituting one variety of hops for another to see what effect it has. And if you get a recipe that uses hops only for bittering, try adding half an ounce each of flavor and aromatic hops to round out the beer.

One excellent way to enhance the flavor and aroma of beer is by dry-hopping: that is, adding hops to your beer after it's already been put into your carboy. This technique will be covered in more detail in the next lesson, Homebrewing 203: Secondary Fermentation. You may also wish to go back to the homebrewing node to browse through earlier lessons.

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