First Wort Hopping
First Wort Hopping is a bittering technique where you provide a flavor/aroma hop addition into the warm wort as it is
received from your mash tun before boiling. As the boil kettle fills, the hops immediately begin releasing
their oils into the wort. Because they are added below boiling temperature, where the coumpunds that provide
aroma to your beer normally boil off, the oils are converted to more soluble compounds in the liquid, and do not
boil off as readily during the boil. You should use some of the hops that you would normally use in the last 20
minutes of your boil.
Common wisdom says that the amount of bittering imparted to your beer is roughly equivalent to the same hops added for 20 minutes. However, there is still some debate over this fact on the online message boards.
The technique is quite straight-forward. Add a portion of your lower-acid aroma or flavor hops into your boil kettle by themselves right before you are ready to drain your mash tun. As you drain your mash tun the warm wort will start to go to work on the hops. The easiest way to calculate the final IBU contribution is to use a software program like BeerSmith. I usually set my first wort hop addition to 90 minutes because I am a batch sparger, and it takes me about a half hour to get all the wort into the kettle.
First Wort Hopping seems to result in a bit more refined hop aroma. The bitterness is less harsh and more uniform. It is something I would encourage you to try for yourself to see the differences.
The only real disadvantage I can see in this technique is the difficulty you may have in calculating exacly the amount of IBU imparted. Sparges never really take exactly the same length of time, and other variables always seem to come into play in even the best brew day. But, hey, the worst that can happen is you have a bit hoppier beer ... not a bad thing in my personal estimation.
Appropriate Beer Styles
I would not use this technique in beer styles that do not showcase the hop flavor and aroma as one of the major constituents of the beer. Try this in APA, IPA, ESB, and even hop-forward Pilsner style beers.