castoreum is a fragrant food additive harvested from castor sacs at the base of a beaver’s tail. Beavers discharge castoreum, a yellowish, viscous fluid, from their cloacal opening to mark their territories. In high concentrations, the substance can be quite pungent; diluted, it exudes a musky, leathery smell.
The unique scent is derived from the beaver’s diet. Scientists believe that antifeedants (predator-repelling chemicals) found in many trees are distilled by the beaver’s body into castoreum’s rich blend of phenolic and neutral compounds.