Additive 25 / 75 – Diacetyl
In its concentrated form, it totally stinks. Considering that it is the primary element in artificial butter flavor (think movie popcorn), this would be counterintuitive. But when it is freshly manufactured it must be handled especially carefully due to its intensely disgusting odor. You might recognize it as the smell of overfermented beer, spoiled fruit juice, or rancid butter.
See full post here: Dwight Eschliman Photography2015-09-04.