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Hops are vines whose flowers, when boiled, release bitter resins. These balance the sweetness of the malt. By adding hops we can control the amount of bitterness in our beer (I wish we could do this with people).

Hops are sold in 3 forms: leaf (which is a misnomer since we use the flowers, not the leaves), plug and pellet. The latter is the most convenient and keeps fresh the longest, so I highly recommend it (of course, I also highly recommend staying single, but nobody listens to me).

How much hops should you use? It depends on a lot of things, not the least of which is alpha acid percentage, which should be written on the packages you buy (alpha acid is the main bittering component in hops). Different species of hops have different concentrations of bitterness, e.g. Fuggles might be 4%, while Nugget might be 15% (Nugget is the Roseanne Arnold of hops). A standard bitterness measurement is the "alpha acid unit", abbreviated AAU (another term is "home bittering unit", or HBU). One AAU = one ounce of hops with 1% of alpha acid, so, for example, 2 ounces of 4% Fuggles has 8 AAUs. Most homebrews have anywhere from 6-30 AAUs in a 5 gallon batch.

There's also a measurement of bitterness per unit of wort, called "international bittering unit", or IBU. But the calculations are so ridiculously complicated that you might as well try to figure out pi. Seriously, IBU is beyond the scope of this article.

As hops are boiled, the alpha acid is isomerized into a chemical that tastes bitter. A minimum of 30 minutes is necessary for a significant amount of the alpha acid to start isomerizing, and maximum utilization is reached at around 90-120 minutes. The higher the wort gravity, the longer isomerization takes. For this reason I usually boil my hops in water and add this hop tea to the sweet wort later.

Then there's aromatics. Hops, if boiled only briefly, give aroma but no bitterness, as opposed to a long boil in which bitterness is extracted but the aromatics are lost into the air. Aromatics, found in the hop oils rather than the resins, are not measured in AAUs.

Much of the aromatics are lost in the carbon dioxide that goes out the airlock during fermentation. To prevent this you can do the following: briefly boil the aroma hops at bottling time. Remove the hops, dissolve the priming sugar in this hop tea, and add to your bottling bucket. Or you can try what's called "dry hopping". This involves throwing the aroma hops into the secondary fermenter without even boiling them. The aromatics will dissolve in the next week or so. Dry hopping might introduce some bacteria into the wort, as the hops are not boiled, but since there is some alcohol in the wort at this point, most of the bacteria will die. It is a good idea to put hops in a fine straining bag, whether boiling them or dry hopping with them, otherwise removing them from the wort will be a pain in the a--.

Hops can also add flavor. A 15 to 20 minute boil will drive off the aromatic oils, and virtually no bitterness will be extracted, and what will be left is flavor. Longer boils tend to kill the flavor, according to more informed sources, which is why a special hop addition is needed for hop flavor.

Different hop species give different aromas and flavors, so which species you use will determine much of what the brew will taste and smell like.

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