This is an ancient thread but maybe this could still help someone. Different finings act upon different components to improve clarity of the beer. Opposites attract, like magnets.
Negatively charged finings (e.g., Irish Moss) attract positively charge proteins in the wort.
See Charlie Papazian's The Complete Joy of Homebrewing, 4th edition, p. 113.
Good book, but he often plugs his other book, The Homebrewer's Companion, Second Edition throughout, and that gets a little annoying.
[and as the thread above mentions, some of these proteins may also be ones that contribute to head formation and stability]
(CP recommends 1/4 - 1/2 tsp. in the final 10 minutes of boil rather than 1 tsp for 30 minutes)
On page 114, CP continues, regarding flocculation:
Negatively charged yeast can be removed from suspension by the use of positively charged finings (e.g., gelatin). He gives instructions and agrees that boiling is bad.
But I know I read recently about the use of IM being unnecessary or not recommended for extract beer...
In John Palmer's "How to Brew" (3rd edition), he cautions that too much IM can lead to poor head retention and potentially insufficient levels of Free-Amino Nitrogen (needed for yeast health). p. 280
The summary table on p. 281 includes the statement that IM is "not recommended for ...extract-based worts."
THERE it is!
By the way, Palmer recommends IM in the last 20 minutes of the boil. Ask 5 homebrewers, get 6 opinions.
Reading between the lines, and from using extracts, my guess is that the hot-break proteins that IM would be targeting have already settled out or have been dealt with prior to the evaporation of the wort to make L/DME. Maybe the extract makers have already utilized IM or something similar by that point and adding the "normal" dose may therefore be an overdose in this case, resulting in low FAN / compromised head retention.
