My understanding is that acetoin, on its own, is not thought to be harmful. If I remember right, acetoin occurs frequently in combination with diacetyl... possibly as a by-product, or vice versa? I bought Belgian Waffle several months on the understanding that acetoin by itself was considered OK.
Here is a quotation from
the thread Huck referenced above:
"In the
interview with Russ Wishtart, starting at about 99 minutes in, Dr. Farsalinos responds to a caller concerned about acetoin in flavorings and discusses the relationship of acetoin to diacetyl:
'Acetoin is one of the chemicals that produce a buttery flavor. It is one of the three main chemicals that produce this flavor. The other ones are of course diacetyl and acetyl propionyl.
Acetoin by itself is not a reason for concern...however, since...
during the production of acetoin diacetyl is also produced, it is possible that the [diacetyl] contamination issue we saw [in our study]--in some liquids that contained diacetyl, but not at very high levels, so [our conclusion was that in those cases] it is not intentionally added [and we theorize that] the contamination levels that we found may have come from acetoin.
Therefore, you may get an acetoin flavoring which is not very pure acetoin but contains traces, or somewhat higher levels of diacetyl... So from this perspective, it is a problem.
But acetoin itself, as a substance, is not a problem when inhaled.Unless--and this is something we are going to test--unless acetoin is transformed into diacetyl during heating. ... It's very unlikely that [diacetyl] will come out during the steeping process or [over time sitting in a bottle] but, by heating,
there is a possibility that we have to check, whether when you heat acetoin [to the temperatures used in e-cigarettes] that there is a production of diacetyl. We have to check that to make sure, otherwise, acetoin is not a reason for concern by itself.'"
The above interview is from many months ago and I don't know if the research on heating acetoin has been done yet. e-cigarette-research.com is a good place to check on that. If you do, please post what you find in this thread for the sake of keeping the info together:
http://vapingunderground.com/threads/diacetyl-acetyl-propionyl-and-acetoin-in-flavorings.3351/