Diacetyl is an organic compound that gives buttermilk its distinctly creamy flavor and aroma. It's an essential flavoring chemical that improves the natural taste and scent of food. It's also a natural by-product of the conversion of glucose to ethanol by yeast during fermentation, which is why it's often found in fermented products, such as butter, beer and wine. Diacetyl is also naturally found in low concentrations in coffee, dairy, fruits, honey and vinegar.
Aside from butter-flavored microwave popcorn, edible products like candies, pastries and frozen foods as well as beverages prepared by baristas on-site like shakes and frappes may also contain flavorings that have diacetyl. However, these flavorings were likely kept at a very minimal amount per serving and that would render them practically harmless to consumers.
When there's too much diacetyl produced during the fermentation process, it produces a butterscotch flavor. At low levels, diacetyl contributes to the slippery texture of certain beverages.
Diacetyl and its twin, acetyl propionyl, are generally safe for humans and animals to ingest, and they're also safe to inhale as long as e-cig users practice moderation in vaping and carefully choose the e-juice flavors they want to vape.
However, these flavoring chemicals may not be completely safe for e-juice makers to handle without sufficient protection and in a workplace that doesn't have a ventilation system. Manufacturing plants for food products and e-liquids are proverbial "hot spots" for high levels of diacetyl because they usually keep large quantities of these flavoring chemicals in stock and process them in bulk for mass production.