Artificial butter flavoring

Artificial butter flavoring may contain diacetyl, acetylpropionyl, or acetoin, three natural compounds in butter that contribute to its characteristic flavor. Because of this, manufacturers of margarines or similar oil-based products typically add diacetyl, acetylpropionyl and acetoin (along with beta carotene for the yellow color) to make the final product butter-flavored, because it would otherwise be relatively tasteless.
Chronic industrial exposure to diacetyl fumes, such as in the microwave popcorn production industry, has been associated with bronchiolitis obliterans, a rare and life-threatening form of non-reversible obstructive lung disease in which the bronchioles (small airway branches) are compressed and narrowed by fibrosis (scar tissue) and/or inflammation.