In the process of brewing beer, trub is the term used for the material, along with hop debris, left in the whirlpool or hopback after the wort has been boiled then transferred and cooled. Brewers generally prefer that the bulk of the trub be left in the whirlpool rather than stay in contact with the fermenting wort. Although it contains yeast nutrients, its presence can impart off-flavors in the finished beer.
It is composed mainly of heavy fats, coagulated proteins, and (when in fermenter) inactive yeast.
The term has its origins in the German word trübe (also trüb), which means cloudy, via the brewing and winemaking terms Trubstoff (cloudy + material) and Weintrub (wine + cloudy). 
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