I find that during sparging, the very last of the runoff (maybe the last 1-2 quarts) get very cloudy and full of grain particulate. I think this is because the liquid level is now well below the level of the grainbed and therefore is not being well filtered.
At this point, I stop the flow into the kettle, prop one side of the tun up with a large book, and drain the last of the runoff through a fine mesh hop bag into a pitcher. The hop bag catches all of the little grain particules but there is some white fluffy stuff that it does not filter out. This stuff is very fine and looks a lot like cold break. It does eventually settle out to the bottom of the pitcher, but it takes a while.
My questions are (1) what is this stuff (protein?) and (2) is it a bad idea to add it to the boil kettle? It's nice to be able to drain pretty much all of the wort out of the tun and add it to the boil but if this stuff is detrimental to the beer I'll start discarding it. My guess is that it's harmless and probably settles out with the break material during chilling, but I want to know what you all think.