Autoxidation of Milk Lipids. I. Identification of volatile Monocarbonyl Compounds from Autoxidized Milk Fat

Abstract

Milk fat, free of detectable phospholipids, was oxidized to a peroxide value of 14.3 and a TBA (2-thiobarbituric acid) number of 15.0 by exposure to air at 20 to 25[degree] C in diffuse light. The volatile compounds were isolated by warming the fat to 40[degree] C at 2 to 3 mm Hg pressure and stripping with steam generated at the same pressure. The carbonyl compounds were converted to 2,4-dinitro-phenylhydrazones. The monocarbonyl derivatives were extracted from the reaction mixture with hexane and the resulting mixture was separated by column chromatography. Identification of the 2,4-dinitro-phenylhydrazones was based on chromatographic behavior light absorption spectra and melting point data. The C1 through C10 n-alkanals, the C5 through C11 alk-2-enals and acetone were conclusively identified. Evidence for the presence of but-2-enal and the odd numbered C5 through C15 n-alkan-2-ones was obtained. Quantitative data revealed that 63% of the monocarbonyls were n-alkanals and 34% were alk-2-enals. Identical compounds were found in less severely oxidized milk fat.