Neuroglottis

The Neuroglottis, also called the Devourer, is the 15th of the 19 genetically-engineered gene-seed organs implanted into a Space Marine Neophyte to create a new Astartes. The organ is implanted within the Astartes' upper nasal passages and after it is functioning, chewing, tasting or smelling a substance allows a Space Marine to biochemically test it for toxicity and nutritional content, essentially determining if the substance is edible or poisonous. The organ also allows the Space Marine to identify extremely subtle odours with the same fidelity as the average canine bred for tracking, allowing him to even track his quarry by taste alone.

Sources

Codex: Black Templars (4th Edition), pg. 10
Codex: Space Marines (5th Edition), "The Making of a Space Marine," pg. 11
Index Astartes I, "Rites of Initiation - The Creation of a Space Marine" by Rick Priestley & Gav Thorpe, pg. 5
Warhammer 40,000: Compendium (2nd Edition), "The Origins of the Legiones Astartes" by Rick Priestley, pg. 8
White Dwarf 98 (UK) (1988) "Chapter Approved: The Origins of the Legiones Astartes" by Rick Priestley, pg. 14