From Canberra Brewers Club Wiki
The term trub can refer to two things:
- The layer of sediment that appears at the bottom of the fermenter after yeast has completed the bulk of the fermentation. It is composed mainly of heavy fats, proteins and inactive yeast, plus some hop debris.
- The material left in the kettle after the wort has been boiled and drained out. It is mainly composed of hop debris, plus coagulated protein ("hot break") from the boil. Brewers generally prefer that most of the trub be left in the kettle rather than stay in contact with the fermenting wort. Although it contains yeast nutrients, its presence can impart off-flavors in the finished beer.