A creamy, buttery aroma compound — 3-hydroxy-2-butanone, also called acetylmethylcarbinol — generated by lactic acid bacteria and yeasts during fermentation. It is the milder cousin of diacetyl, and the balance between the two defines the flavour of cultured butter, buttermilk, sour cream and some wines: more diacetyl for punch, more acetoin for soft creaminess. Analysts measure acetoin alongside diacetyl to judge fermentation character and maturity, and flavour houses use it to build dairy notes without the harsh edge of pure diacetyl. Enzymes that shunt precursors toward acetoin are prized in brewing for taming buttery off-flavours.