Early bitterness vs late bitterness

Hey guys, so I've been brewing for a couple of years now, I'm doing all grain and I'm curious as how I can control early flavor of bitterness vs a late bitterness on the end of a drink? I understand that the length of the boil will affect your bitterness from hops, but how do you control where that bitterness shows up in the finished beer? Is it the cohumulone level, myrecene? I hope I'm clear on what I'm asking?
Thanks
Jp

There are two things that could be going on. Hop bitterness will generally be tasted on the back of the tongue. Bitterness up front is more often called astringency. Overcrushing your grain, oversparging or sparging with boiling-hot or alkaline water, and bacterial infection can cause astringency. Excessive oxidation also produces a rather unpleasant astringency.

Here's an article on Late Hop additions if your interested, I found it pretty interesting

http://www.homebrewersassociation.org/attachments/0000/1249/MJzym06_LateHops.pdf




Bob

I'll take another look at that thread to see if that refers to it. I'm not talking about a harshness... Basically it seems that there are some ipa's that have a bitterness that transitions into the flavor of the hops and then you have some that carries a flavor first and transitions into the bitterness.