Metallic odour caused by vinyl ketones formed in the oxidation of butterfat. the identification of octa‐1,cis‐5‐dien‐3‐one

Oct‐1‐en‐3‐one and the more potent octa‐1,cis‐5‐dien‐3‐one have been identified as being responsible for the metallic odour which developed when a model system containing butterfat, α‐tocopherol and cupric palmitate was allowed to oxidise at 35°C for 35 h. These vinyl ketones were synthesised, their mass and n.m.r. spectra determined, and odour descriptions and odour thresholds assessed. Octa‐1, cis‐5‐dien‐3‐one in aqueous solution could be detected at a dilution of 1 in 1012. Its metallic odour was also described as like geranium leaves. Oct‐1‐en‐3‐one (detectable at 1 in 1010 dilution) was described as mushroom like at low concentrations and only exhibited a metallic odour at higher concentrations. Both compounds had an odour threshold concentration about five hundred times greater in groundnut oil than in water. The significance of these compounds is discussed with reference to the published literature and their formation from (n‐3) and (n ‐ 6) polyenoic fatty acids.