Bitterness, flavor, and aroma are all derived from the hops added to beer (beer is called wort before it is beer). Which of these three is derived from the hops depends on when the hops are added to the wort. Sometimes only bitterness is obtained from a hop infusion and sometimes your beer will get all three from a single infusion.
Bitterness in beer comes from the alpha acids prevalent in hops. Flavor and aroma in beer is derived from the oils.
Hops intended to bitter beer are added at the beginning of the wort boil while hops intended to flavor beer or give it aroma are added near the end of the boil because the oils dissipitate quickly (15 minutes or so) in the boil and if infused into the wort too early they will all disappear before the end of the boil.
Thus, most homebrewing recipes call for the infusion of "bittering" hops at the start of the boil time (usually 60 minutes) while flavoring and aroma hops are added near the end of the boil (usually with only 15 minutes of boil time remaining).
The term "dry hopping" is used to describe the process of adding hops after the wort has been cooled. Dry hopping can be done in the primary fermenter or you can wait until the beer is transferred to the secondary fermenter or hops can be added to the beer as it is being kegged. I have often used pellet hops in the secondary fermentation stage by putting the pellets in a small muslin bag and then tying a string around the top of the bag. I stuff the bag through the neck of the carboy and suspend the bag of pellets in the beer during the duration of the secondary fermentation. Of course, the bag swells up and cannot be retrieved from the carboy! Simply siphon your beer into your bottling bucket for bottling, then pull the bag up to the neck of the carboy, snip the bag with a scissors and shake out the hop residue. Throw the bag and string away and rinse the carboy.
Tomorrow: Why Dry Hop?
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