In the process of brewing

Brewing

Brewing is the production of beer through steeping a starch source in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BCE, and archeological evidence suggests that this technique was used in ancient Egypt...

 beer, trub (from the German for lees

Lees (fermentation)

Lees refers to deposits of dead yeast or residual yeast and other particles that precipitate, or are carried by the action of "fining", to the bottom of a vat of wine after fermentation and ageing. The yeast deposits in beer brewing are known as trub...

) refers to the layer of sediment that appears at the bottom of the fermenter after yeast

Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 has completed the bulk of the fermentation. It is composed mainly of heavy fats, proteins and inactive yeast.

Trub also refers to the material, along with hop debris, left in the boil kettle or hopback after the wort

Wort

Wort may refer to:* Wort, the liquid created by the mashing of malted barley to use in brewing beer* Worting, Hampshire, a large district and suburb of the town of Basingstoke, in Hampshire, England....

has been transferred and cooled. Brewers generally prefer that the bulk 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.