Lately we've been fielding a lot of questions about using The Brew Bag as a hop bag (which we recommend) VS a hop spider. The hop spider name thing is mostly because of the way they are set up on the kettle. Most folks get a piece of PVC pipe, 4" or so, and figure a way to add horizontal pieces that rest on the kettle edges and thus the PVC is suspended above the wort, so from the top is looks like spider legs.
Then a bag of of some sort is banded to the bottom of the open PVC which creates a tube that dangles into the wort, to which hops are added. Mostly I've seen that the tube is the same size as the PVC opening. I don't know why that is, it really doesn't make much sense to me, since pro-brewer's dump hops directly into the wort, then whirlpool so the trub (proteins, fats, hops) settle in the center of the vessel and are eliminated from being picked up when the wort is pulled out of the kettle.They do this for a number of reasons, with all of them based on their knowledge of molecular interaction post boil/pre-fermentation. Clean wort = fewer variables = consistent beer. But, back to the point of the tube thing. I think home brewers don't realize that when hop pellets are rehydrated they swell to 4 times the dehydrated volume. That hop bag looks big enough for 10 packets of 1 oz hops, but that just ain't so!
There's a bit of science here, so bear with me. In order for the hop resins to be isomerized (hooked up molecularly with water and dissolved in the wort) they must reach a temperature of 185ºF, and they must be allowed to contact the water. If the hops are constricted into a tube and not free floating, the resins are restricted inside the layers of hops that are packed into the tube.
So why do home brewers think that adding hops via a constricting mush bag tube is better than free-floating full contact? I wish I could tell you. I think the answer is that no one has ever compared the results...and the beer still tastes good. No joke, I can't find any scientific evidence that hop bag beer is better or worse than free-floating hop beer.
Here's what I know - when used as a hop filter bag, The Brew Bag allows full contact with the water/wort and thus eliminates the question of whether constriction equals less utilization, which by the way is only 30% of the potential in the first place.
The excerpt below is from: http://www.howtobrew.com/section1/chapter5-4.html
The utilization is influenced by the vigor of the boil, the total gravity of the boil, the time of the boil and several other minor factors. The vigor of the boil can be considered a constant for each individual brewer, but between brewers there probably is some variation. The gravity of the boil is significant because the higher the malt sugar content of a wort, the less room there is for isomerized alpha acids. The strongest bittering factors are the total amount of alpha acids you added to the wort, and the amount of time in the boil for isomerization. Understandably then, most equations for IBUs work with these three variables (gravity, amount, and time) against a nominal utilization. As mentioned earlier, the utilization for alpha acids in homebrewing is generally accepted as topping out at about 30%.