Acetoin

Acetoin, also known as 3-hydroxybutanone, is an organic compound with the formula C4H8O2. It is a chiral molecule that exists as two enantiomers, (R)-acetoin and (S)-acetoin. In the laboratory it is a colorless liquid with a mild odor and is soluble in many organic solvents and to some extent in water.

Acetoin is produced by many microorganisms during fermentation and plays a role as an intermediate in the butanediol fermentation pathway. In this pathway, pyruvate is converted to α-acetolactate, which is decarboxylated to acetoin. Acetoin can be further reduced to 2,3-butanediol or oxidized to diacetyl (2,3-butanedione). Diacetyl is more strongly associated with buttery flavors, while acetoin contributes a subtler note.

In food and beverage contexts, acetoin may be present as a flavor-active metabolite and can contribute to aroma alongside diacetyl. It is also used as a flavor precursor and aroma compound in some formulations. In microbiology, acetoin production is one of the parameters assessed by the Voges-Proskauer test, a standard biochemical test for certain bacteria; a positive result indicates acetoin formation.

Overall, acetoin is a small, naturally occurring metabolic intermediate with relevance in fermentation chemistry, flavor science, and microbial identification. The compound exists as two enantiomers and participates in redox interconversions that link to other butanediol pathway products.