To the OP: FWIW: I like to skim the ‘stuff’ from the BK. Not sure it matters. Some say it doesn’t. ...but I like my results so I continue to do it. (I skim soups and stews also.)
I've gotten compact gray debris (slime-like) in my mash tun before too...I figure it's just insoluble proteins from the grains.
Possibly what you’re seeing is teig (which is an indicator of HSA according to the MBAA).
Bamforth states,"oxygen reacts with gel proteins to cross-link them and that causes this sticky gel formation called the 'teig'".
Teig can cause lauter problems because it forms an impenetrable layer so the wort flows around the grain bed if at all (stuck lauter). You can cut the layer with a knife to help the liquid flow thru the grain vs around it.
The thought is reduce oxygen in the mash to reduce teig. I believe proper pH helps as well but I can’t find a reference so that’s just me saying that.
One way that I found helps is to underlet the mash with brewing liquor. I started underletting to reduce the hazard of lifting 5.5 gal of ~160*F strike liquor to pour it into a mash tun. I add the grain to the mash tun and pump the water thru the drain valve. It’s a lot safer to pump than lift.
A welcome byproduct of underletting is a lack of dough balls I’d get when I added grain to water.
Another nice side effect is no more teig. I understand that pumping water from below the grain bed helps reduce O2 in the mash.
Also to limit O2 pick up 1) I do not open the MLT during the mash. I tried a mash cap but it was a PITA and my MLT is nearly full anyway so I figure the MLT lid is the mash cap. 2) I recirculate during the entire mash and ensure the recirculation return is below the liquid level so I don’t pick up O2 that could be caused by splashing.
I nearly always had a teig layer but using these techniques has eliminated it altogether.
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