Trans 4,5 Epoxy (E) 2 decenal

trans-4,5-Epoxy-(E)-2-decenal is an oxygenated α,β-unsaturated aldehyde found in mammalian blood that gives blood its characteristic metallic odor. It is used by predators to locate blood or prey. Humans can smell it at a concentration of 1.5 pg/l in air, at 15 ng/L in water and 1.3μg/L in oil. It is permitted as a food flavouring in the EU.

It can be formed during baking fats that contain linoleic acid. 13-Hydroperoxy-9,11-octadecadienoic acid and 9-hydroperoxy-10,12-octadecadienoic acid are intermediates in the process. The aldehyde also forms in cooked beef when it sits in the refrigerator for too long contributing to a stale smell. It is also an important part of the smell of raw and cooked mutton.