What is it and where is it found?
Like diacetyl and pentanedione, acetoin is used to create or enhance a buttery flavour or scent.
It is naturally found in apples, butter, yogurt, asparagus, black currants, blackberry, wheat, broccoli, Brussels sprouts, and cantaloupe.
How harmful is it and what are the effects?
Though there is no evidence that acetoin itself is harmful it has been considered worth monitoring since it has often been found to be contaminated with diacetyl. During production it can catalyze somewhat into containing trace amounts of diacetyl though if it is not contaminated before blending it will not catalyze after.
Where can it occur in e-cigarettes?
A buttery flavour or scent might indicate the presence of acetoin.
What is a recommended threshold for this constituent?
There is little reason to be concerned about the presence of acetoin. In testing, if acetoin is detected along with trace elements of diacetyl it may in fact indicate that there is some contamination of the acetoin.