I have some debates on here about flavor imparted by hops when those hops have been boiled at a rolling boil for over 60 minutes.

I could not understand how some people insisted that there was a notable "flavor" difference for different bittering hops even when long boiled.

All the beta acids etc (read flavoring compounds) boil out of the beer after 60+ minutes.

The one thing I forgot was cohumulone levels. A hop with a considerable % of cohumulone will impart a more astringent or "harsh" bitterness. Usually these levels are higher in high alpha hops but not always. (cohumulone is an alpha acid after all).

For example Columbus is a high alpha acid hop that has very low cohumulone levels and imparts a fairly clean bitterness, in comparison to it's rather pungent flavoring/aroma compounds.

High cohumulone have a tendency to seem harsher with more delicate light srm beers and less noticeable with heavy dark beers.

So in fact there can be a difference in bittering hops but this "flavor" is not the same as the flavoring compounds that are in your beer when the hops are added at the end of a boil.

Se here for some basic information:
http://www.brewingwiki.com/w/index.php?title=Cohumulone#Cohumulone