Acetoin is used as a food flavoring (in baked goods) and a fragrance. It can be found in apples, butter, yogurt, asparagus, black currants, blackberry, wheat, broccoli, brussels sprouts, cantaloupe.
Acetoin is a neutral, four-carbon molecule used as an external energy store by a number of fermentive bacteria. It is produced by the decarboxylation of alpha-acetolactate, a common precursor in the biosynthesis of branched-chain amino acids. The conversion of acetoin into acetyl-CoA is catalysed by the acetoin dehydrogenase complex, following a mechanism largely analogous to the pyruvate dehydrogenase complex; however, as acetoin is not a 2-oxoacid, it does not undergo decarboxylation by the E1 enzyme; instead, a molecule of acetaldehyde is released. Acetoin is used as a food flavoring (in baked goods) and a fragrance. It can be found in apples, butter, yogurt, asparagus, black currants, blackberry, wheat, broccoli, brussels sprouts, cantaloupe. The conversion of acetoin into acetyl-CoA is catalysed by the acetoin dehydrogenase complex, following a mechanism largely analogous to the pyruvate dehydrogenase complex; however, as acetoin is not a 2-oxoacid, it does not undergo decarboxylation by the E1 enzyme; instead, a molecule of acetaldehyde is released. 4 Acetoin is naturally present in buttery foods such as beer, butter and cheese. 5 Determination of diacetyl and acetoin to evaluate the workplace exposures at a popcorn processing facility has been reported. Acetoin is suitable reagent used in 2,3 butanediol and acetoin production using Paenibacillus polymyxa (safe microorganism). My research suggests that acetyl propionyl and acetoin are two of the most common substances these companies are using, and as a result, any e-liquid that has a buttery flavor but does not contain diacetyl may contain one of these chemicals. It seems that it is — or was — common for e-liquid manufacturers to use diacetyl in constructing their flavors, and the phrasing of this entry in the Halo FAQ led me to believe that it might be a dangerous substance, so I did a bit of research. Acetoin is a volatile compound widely employed as flavour and fragrance compound in foods, cigarettes, cosmetics, detergents, chemical synthesis, plant growth promoters and biological pest controls. 2 Acetoin is the main extracellular product of Bacilus subtilis grown on glucose and other fermentable carbon sources. Acetoin is suitable reagent used in 2,3 butanediol and acetoin production using Paenibacillus polymyxa (safe microorganism). Acetoin, also known as 3-hydroxybutanone or acetyl methyl carbinol, with the molecular formula is C 4 H 8 O 2, is a colorless or pale yellow to green yellow liquid with a pleasant, buttery odor. Acetoin is a chiral molecule. The form produced by bacteria is (R)-acetoin. The conversion of acetoin into acetyl-CoA is catalysed by the acetoin dehydrogenase complex, following a mechanism largely analogous to the pyruvate dehydrogenase complex; however, as acetoin is not a 2-oxoacid, it does not undergo decarboxylation by the E1 enzyme; instead, a molecule of acetaldehyde is released. Diacetyl (IUPAC systematic name: butanedione or butane-2,3-dione) is an organic compound with the chemical formula (CH 3 CO) 2. It is a yellow/green liquid with an intensely buttery flavor. It is a vicinal diketone (two C=O groups, side-by-side) with the molecular formula C 4 H 6 O 2.  The excretion of acetoin, which can be diagnosed by the Voges Proskauer test and serves as a microbial classification marker, has its vital physiological meanings to these microbes mainly including avoiding acification, participating in the regulation of NAD/NADH ratio, and storaging carbon.  Medical Definition of ACETOIN. : a colorless liquid hydroxy ketone C4H8O2 formed from various carbohydrates by fermentation—called also acetylmethylcarbinol. What made you want to look up acetoin? 
https://en.wikipedia.org/wiki/Acetoin https://en.wikipedia.org/wiki/Acetoin http://www.sigmaaldrich.com/catalog/product/aldrich/w200808?lang=en&region=US http://edripping.com/about-ecigarettes/what-are-diacetyl-acetyl-propionyl-and-acetoin/ http://www.sigmaaldrich.com/catalog/product/aldrich/w200808?lang=en&region=US https://en.wikipedia.org/wiki/Acetoin https://en.wikipedia.org/wiki/Diacetyl http://www.ncbi.nlm.nih.gov/pubmed/17558661 http://www.merriam-webster.com/medical/acetoin
what is acetoin