Why Are Hops Used?
Brewers covet the magical properties of the hop and in particular their mix of acids and oils. The powdery lupulin consists largely of complex molecule chains called “humulones,” collectively named “alpha acids.” In addition there are aromatic essential oils.
Beer's bitter bite is derived from these acids, but there's a challenge. Alpha acids won't dissolve in water. With the application of heat the molecules isomerize (rotate) into isohumulone. Basically, a chunk of the molecule flips around, allowing it to bind with water. Wort's roiling boil dissolves the acids by inefficiently extracting a maximum of 30 percent of the available humulones even after 120 minutes of churning.
Why does beer get that skunky aroma?
A reaction between ultraviolet light and isohumulone is responsible. Light causes the isohumulone molecule to decompose into 3-methylbut-2-ene-1-thiol, primary component in skunk's infamous “spray.” The effect is more noticeable in beers bottled in clear or glass bottles that block less UV. One major American brewer uses stabilized isohumulone extracts, preventing skunking in their clear bottle beers.
Malt and sugar's strong, sweet flavors explain the need for a bite. Even after an efficient fermentation, there are residual sugars left. Great for richness and body, the resulting beer tastes toothachingly, sickeningly sweet. The bitterness cuts the remaining sweetness, increasing a beer's drinkability.
Experiment by mixing extract and water, boiling briefly, and transfering to a jug to ferment. After a week's time, take the beer, measure the final gravity, and taste. Notice how the beer lacks zippiness or punch?
In addition to the bitterness, hop oils contribute aroma: pine, woody resin, flowers, grassiness, spices, citrus, cattiness, and more. The variety and amount of oils diverge widely, based on hop type and location grown, which is important to remember when shopping for hops. Using Argentinean Cascades in place of American Cascades yields disappointing results, as the Argentinean crop is herbal and spicy, lacking the citrus character of American Cascade.
Before there was microbiology, brewers knew that beer soured quickly. The beers with hops lasted longer and stayed pure. Hops have a secret property, a limited antibacterial effect that is particularly effective against lactobacillus, beer's biggest natural predator. This is also why you don't have to worry about sanitizing hops when dry hopping. Hopped beer pours clearer because hop tannins bind with haze-causing proteins.