Below, a primer.
Hops are native to temperate climates and were found in North America, Europe, and Asia. The first hops were cultivated domestically in Germany in the 8th century near Hallertau. In the 9th-12th centuries, hop cultivation started in Bohemia, Slovenia, and Bavaria.
Europeans pretty much instantly discovered hops when they arrived in America. Records show that the Dutch were growing hops and brewing on Manhattan Island in 1612. It's not clear where the hops came from, but those settlers describe good, wild hops growing on the island. In colonial times, hops were grown throughout the Northeast, but particularly in Massachussets; by the mid-19th century, major production had shifted to Wisconsin, where beer-brewing German residents settled. Unfortunately for the Badger state, aphids and powdery mildew wiped out the Wisconsin fields, and the West Coast picked up the slack. Now, all commercial hops come from the Northwest, 70% from the Yakima Valley.
Hops balance malt, adding bitterness, aroma, and flavor to beer. Generally two elements of the hop flower are referenced in this regard--alpha acids, which provide bitterness, and beta acids, which contribute aroma. That's true, but it's incomplete. In fact, hops have all kinds of elements which add subtle flavors and aromas:
- Alpha acids. These are one of two major compounds of the soft resin that turn a beer bitter when "isomerized" (eg, boiled in beer).
- Beta acids. Another compound of the soft resin that contribute aroma, beta acids are added for aroma. Their aromatic properties are destroyed by boiling, so aroma hops are added at the end of the boil.
- Co-humulone. Alpha acids exist in three "analogous forms," humulone, ad-humulone, and co-humulone. Co-humulone accounts for 16-50% of the alpha acids in a hop plant, and they are suspected of contributing a harsh bitterness. This hasn't been proven.
- Lupulin. This is the yellow pollen-like powder on hops. High-alpha hops have more lupulin, but it's not clear what they contribute to beer.
- Oils. These are more subtle contributors to aroma and bitterness. The oil content of 100 grams of hops ranges from .5 mls to 3 mls. The four main components in the oils are Caryophyllene, Farnesene, Humulene, Myrcene, though the percentage of each within a hop variety varies dramatically. They are volatile hydrocarbons and driven off during the boil.
Hop varieties are expanding every year, and the number is now a bit overwhelming. In advance of the weekend, I'll assemble a post with a description of the hops used in this year's crop of fresh hop beers--you can use it as a cheat sheet, though of course the wet-hopped variants are substantially different. If you're dying to know about a particular variety, click around on the "sources" links below--those are commercial hop growers, and they have some information about the major varieties. (New ones like Teamaker--apparently 0% alpha [!]--and Mt. Ranier are harder to locate. I'll try.)
Sources: Hopsteiner, Hop Union, Oregon State University, Yakima Chief.