Butter flavoring.
Diacetyl is one of the many chemicals that give butter its characteristic flavor. Low levels are present in butter (including unsalted butter, to which extra diacetyl is added to prolong its shelf life). Much higher levels have been used in butter-flavored popcorn, margarine, and butter-flavored cooking oils and sprays. The low levels are safe, but workers in factories that produce microwave popcorn learned the hard way that long-term exposure to diacetyl causes obstructive lung disease, which is potentially fatal. Widespread publicity around 2005 to 2007 and several lawsuits persuaded most major American food manufacturers to protect their workers (and restaurant cooks) by switching to supposedly safer ingredients. But more recent studies indicate that one substitute, 2,3-pentanedione, chemically similar to diacetyl (also called 2,3-butanedione), may be just as damaging to the respiratory tract.

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