Not something I've noticed or heard anyone else talk about, but we all taste things our own way.
There are two main sources of tannins (polyphenols) in beer, one is malt husks. The polyphenols in husks are mostly extracted late in the wort separation. To provide an analogy, ever made "Billy Tea" add tea, sugar and cold water to a billy, heat to boiling - you get strong sweet tea with out much in the way of tannins. Do the same thing without the sugar and it could strip paint. The sugar blocks the tannins from coming into solution.
In mashing and lautering when there is plenty of sugar in the wort it to blocks tannins, when we sparge the falling sugar concentration and often increasing temperature will start to cause the tannins to come into solution.
Higher pH, high temperature (>80oC) and larger sparges volumes will increase tannin extraction.
The other source of tannins is hops, generally the more hops the more tannins they will contribute. Worth noting that the tannins in hops are softer tasting.
In the boil polyphenols combine with protein to form hot break. There are squillions of different proteins and more than a few different polyphenols, as a general rule larger protein and larger polyphenols will combine more readily in the boil.
If you suspect you might have a bit of an excess of either, boiling for a while (say 30 minutes) before adding hops will tend to reduce the more offensive tannins and the larger proteins (more inclined to cause staling and chill haze).
Acidifying sparge water, not over extracting, good temperature control and longer boils will all help reduce tannins.
Squeezing the bag will tend to increase the amount of high molecular weight protein, and I suspect the last runnings would be the highest in tannins - better to let it gravity drain and collect very slightly less better quality wort.
Mark
