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Vaccenic acid is an omega-7 fatty acid. It is a naturally occurring trans-fatty acid found in the fat of ruminants and in dairy products such as milk, butter, and yogurt. It is also the predominant fatty acid comprising trans fat in human milk.
Its IUPAC name is (E)-11-octadecenoic acid, and its lipid shorthand name is 18:1 trans-11. The name was derived from the Latin vacca (cow).
Vaccenic acid was discovered in 1928 in animal fats and butter. It is the main trans fatty acid isomer present in milk fat. Mammals convert it into rumenic acid, a conjugated linoleic acid, where it shows anticarcinogenic properties.
Its stereoisomer, cis-vaccenic acid, also an omega-7 fatty acid, is found in Sea Buckthorn (Hippophae rhamnoides) oil. Its IUPAC name is (Z)-11-octadecenoic acid, and its lipid shorthand name is 18:1 cis-11.

(Z)-11-Octadecenoic Acid, is a natural 18:1 ω-7 fatty acid. It is readily incorporated into triglycerides in perfused liver.

ChEBI: Cis-vaccenic acid is the cis isomer of vaccenic acid. It is a conjugate acid of a cis-vaccenate(1-).

Purify the acid by fractional distillation under high vacuum or crystallisation form its melt in an inert atmosphere away from light. [Beilstein 2 I 198, 2 III 1384, 2 IV 1639.]