Determination of cholesterol oxides in dairy products. Effect of storage conditions

A methodology to quantify cholesterol oxidation products (COPS) in dairy products has been established by gas-liquid chromatography (GLC). Lipids were extracted according to the Folch method and saponified at room temperature using sodium hydroxide in methanol/benzene (3:2, v/v). Cholesterol oxides were derivatized to trimethylsilyl ethers and resolved by GLC, confirming the structures of cholesterol oxides detected in selected samples by GLC/MS. This methodology allows quick, simple, and reliable quantification of free and esterified cholesterol oxidized derivatives. This method was applied to milk powders, butter oil, and isolated milk protein powders. Effects of adverse storage conditions were studied in whole and skim milk powders. Storage temperature severely affected levels of COPS even under mild storage conditions. Packaging under nitrogen retarded cholesterol oxidation. Slight differences in COPS profiles were observed between different storage conditions. 7-Ketocholesterol was the major cholesterol oxide formed. A good correlation between thiobarbituric acid reactive substances and COPS concentration was observed.