The Hop Back
A hop back is nothing more than a strainer that was used in days of old to separate the whole hops in the kettle from the sweet wort running into the fermenter. At some point brewers discovered that adding fresh whole hops to the hop back first and then straining boiling wort through them accomplished two things. First, it acted as a filter bed, straining out the hops and trub, and second, it imparted a fresh hop aroma.

In the homebrewery this method is effective if you are straining boiling wort into a fermenter filled with cold water or using a counterflow chiller to cool your wort. In an open straining system the hot wort flowing over the hops carries oils into the wort, though some of the volatiles will be lost to the open air.

The system is most effective when used in conjunction with a counterflow chiller and constructed so that the hop back is sealed, allowing hot wort from the kettle to flow through it on the way to the chiller without allowing any of the volatiles to escape. It has little practical use in an immersion chiller system.

In general a hop back will be a bit more time consuming, costly, and messy, but it can impart a very close-to-fresh hop aroma.

Dry Hopping
Dry hopping is perhaps the most widely used and most talked about method of adding fresh hop aroma to beer. In principle the method involves adding a quantity of whole or pelletized fresh hops in a straining bag to the secondary fermentation of the brew.

Because the secondary fermentation will be taking place at room or cellar temperature, the chemistry of the hops is not altered or affected by heat. Therefore, the aromatic oils can be slowly extracted from the hops, and the delicate volatiles will not escape. Again, a liberal amount of hops should be used to achieve a full aroma (about two ounces in a five-gallon batch).

For those who employ kegging systems, add the hops at kegging time and allow them to steep for a few weeks before the beer is served. This is the most traditional way of dry hopping and will produce the best results.

If you bottle your beer, you should employ a secondary fermentation for at least a week (two is better) to fully extract the hop aroma before bottling.

Dry hopping during the primary fermentation is possible, but it should only be done when the primary fermentation has subsided (after three or four days) so that the volatiles will not be carried off in the escaping carbon dioxide of fermentation. In any case, secure the hops in a straining bag and weight the bag so that they are fully submerged in the beer.

Proper dry-hop aroma will take some time to develop, and the aroma should continue to improve for months in the keg. Dry hopping, as seen by the success of the first well-traveled and generously dry-hopped India pale ales, also has a great preservative effect on beer. And because fermented beer is an environment hostile to bacteria, don’t worry about contamination from unsterilized hops; it is highly unlikely to occur.

While many beers are late-kettle or whirlpool hopped and not dry hopped, most dry-hopped beers have also seen late kettle and/or whirlpool hopping. The fresh hop aroma of dry-hopped beer seems to be complemented by the flavor quality of late hopping.

Hop Oils and Teas
Isolated hop oils are also available to the homebrewer. Using a carbon dioxide extraction process, manufacturers are able to provide pure, liquid hop oils that can be added directly to the secondary fermenter or keg to produce a calculated amount of hop aroma. Extracted hop oils need to be diluted into extremely small amounts and therefore are difficult for homebrewers to use.

There are also products on the market called late hop essences. Manufacturers of these products have isolated the individual compounds of the oils that give the distinctive late kettle hopping or whirlpool flavor. These are easier for homebrewers to use than the aroma oils.

Hop teas also provide some of the flavor associated with late kettle and whirlpool hopping. Bring a quart of water to a boil in a saucepan, add a generous amount of aroma hops, stir well, turn off the heat, and allowthe hops to steep for about 20 minutes. You can strain this hop tea directly into the secondary fermentation, keg, or simply make the tea when you are preparing your priming sugar and add it directly to the bottling bucket.

Capturing the perfect balance of hop aroma and flavor in your beer will depend on the style and gravity of the beer you are brewing, the brewing method you use, and of course the type and variety of hop. Certain aroma hops, Cascade for example, have become the defining aroma hops in American pale ales.

There are many hop varieties to choose from, and they all have subtly different, wonderful properties. Do some controlled experimentation. You will be rewarded upon opening your next bottle of homebrew with the same tingly sensation you get when smelling freshly cut grass for the first time in the spring.
 

(from:byo.com)