Hops

Hops are what gives beer that distinct flavor and bitterness.

Surprisingly, hops is the one part of brewing that is fairly uncomplicated. Yes, there are somewhere around 75 different popular varieties of hops, and they each have their own "flavor", but deciding on a particular kind for a recipe is often quite simple.

Alpha Acids

The resin of the hop flower (or cone) is made from the lupulin gland of the hop. If you look at a flower, you can see a nice pile of yellow pollen-looking deposits. This is the good stuff. The main acid that makes up the resin is called alpha acid. There is also a beta acid in a lesser amount, and these do not impart bitterness or a lot of taste to the brew. For good reason, as hops with high beta acids can really screw up a beer.

The general rule is the higher the alpha acid content (usually maxes out around 16-18%) and the longer the hops stay in the boil, the more bitter the beer will be. This is imparted by the isomerization of the alpha acids in the boil, which is basically how bitterness is measured commercially. The amount of bitterness of a brew is given in International Bittering Units (IBU). Really clever name. A beer that is low in bitterness, like a wheat will be around 10-15 IBU, while a heavy beer like an IPA may have upwards of 75-100 IBU (which is the theoretical threshold of bitterness that we can taste).

The nice thing is that although different varieties of hops have slightly different flavors, even as a bitter hop, the bitterness they contribute is fairly easy math. Very generally, it is:

IBU = Bigness Factor (like gravity) * Boil Time factor (like boil time) * mg/ml of alpha acid

Software is the best way to determine this, as it has to do with the gravity of your wort (or how much sugar is present), so you change one thing and they all change.

Flavor and Aroma

This is where it gets a little more complex, and a lot of it has to do with personal preference. Flavors of hops vary from being earthy, piney, citrusy, sharp, floral, grassy, woody, spicy, clean...you name it, there's a hop for it. Some new strains are now being popularized like Citra that has a distinct passion fruit flavor and aroma. The list is too long to go into here, but when I mention a certain hop for a recipe, I'll state why I used that one over another, and some substitutes that are very similar.

Hop Schedule

You basically have 3 different ways to use hops in the boil: bitterness, flavor and aroma. To get the full bitterness out of a given amount of hop you want to add it at the beginning of the boil. Therefore it makes sense to use a high alpha acid hop for the boil. Hops are not exactly cheap, and depending on the recipe and the variety, it can easily make up 20-30% of the cost of a brew. Flavor components typically come mid-boil, in the 15-30 minutes prior to flame-out range. Aromas come from adding the hops with a very short boil, around 0-5 minutes before flame-out. To get even more aroma, hops are often added directly to the fermenter after the main fermentation is complete, called dry hopping. You also have a long list of cool ways to up the hops aroma like using a hopback after flame-out or Randall (Dogfish Head) that infuses the beer with fresh hops on it's way out of the keg into your glass. Awesome.

Pellets, Whole Leaf, Fresh, or Plugs?

Pellets are the most common form of hops for the homebrewer, although many prefer whole leaf or even fresh. Pellets are compressed, shredded leaves that are relatively easy to deal with and store. Whole leaf hops are just that, dried and cold stored cones that you dump right in like it was off the vine. It doesn't take any other calculation differences, only the leaves soak up much more wort when separating them after the boil, so that has to be accounted for. Fresh hops are taken during the harvest in the fall, so they are only used around that time. These hops are not dried, and often give a very earthy, grassy tone to the brew. Plugs are pretty rare these days, but they are still around, and people still use them. Some think that the pelletizing process causes too much degradation to the lupulin glands and compromises the quality of the hop. Whatever, they make good beer.