Ketones are responsible for many natural flavors and odors. Ketones exist in their precursor form in fresh butter as alkanoic acids. As acids, they may be only marginally important in contributing to the flavor of fresh butter. However, when heated, these acid precursors are converted to different ketones and their total concentration rises above their flavor threshold value. Thus, they are very important in providing flavors associated with heated or cooked foods containing butter. Aliphatic ketones are important flavor components, especially in blue cheeses; Penicillium roqueforti, for instance, produces 2-pentanone, 2-heptanone and 2-nonaone (F-1).a
4-Hexen-3-one (FEMA# 3352, CAS# 2497-21-4) (F-2), which is this month’s aroma ingredient, is a colorless to pale yellow liquid, which has an ethereal, whiskeylike and metallic flavor that is also fruity with tropical nuances,b and is applied in vegetable and meaty formulations, as well as rum, butterscotch and horseradish flavors.
4-Hexen-3-one can be prepared from propylene and propionyl chloride by Friedel-Krafts type reaction, firstly to afford 5-chlorohexan-3-one, which loses an HCl molecule by elimination, to give both trans and cis isomers in ca. 9:1 ratio (F-3).