Trub

Trub is the layer of solid material that settles out during brewing and fermentation, made up of coagulated proteins, hop matter, and later dead or dormant yeast cells. It first appears at the bottom of the kettle after the boil, then again as a fermenter sits and yeast finishes its work and drops out of suspension.

Leaving trub behind matters because it can carry off-flavors and unwanted texture into the finished beer if it gets transferred along with clear beer at bottling or kegging time. This is a major reason brewers rack, or siphon, beer off the trub layer into a clean vessel rather than pouring straight off the sediment, and why conical fermenters with a bottom dump valve are popular for making that separation easy.

A common mistake is disturbing the trub layer during racking by placing the siphon tip too close to the bottom, pulling sediment up along with the beer. Leaving a small amount of beer behind on top of the trub, rather than trying to get every last drop, is normal practice and worth the trade-off for a cleaner pour into bottles or a keg.