Irradiation-Induced Changes in Milk Fat

Summary Gamma irradiation of milk fat under reduced atmospheric pressure produced free monocarbonyls at a rate proportional to irradiation dose. The 2-thiobarbituric acid number and total carbonyls increased, but not at a linear rate. Small quantities of hydroperoxides were produced; radiation dosages higher than 1.5Mrad decreased the concentration. The flavor of the irradiated fat appeared to have three components: hydrolytic rancidity, oxidized, and candle-like. The monocarbonyls of irradiated fat were isolated and analyzed by gas chromatography and as 2,4-dinitrophenylhydrazine derivatives. Alkanals predominated and their order of decreasing concentration in the irradiated fat was: C 16 , C 15 , C 14 , C 12 , C 10 , C 6 , C 8 , C 7 , C 4 , iso-C 4 , C 9 , C 5 , C 3 , C 2 , C 1 . Methyl ketones also were produced by irradiation and the following compounds were found: C 15 , C 11 , C 9 , C 7 , C 5 , C 4 . Indirect evidence is presented indicating that the long-chain aldehydes are responsible for the candle-like defect. It is suggested that the long-chain aldehydes and methyl ketones are produced via hydrolytic rather than oxidative mechanisms.