Several oxidation products of cholesterol have been reported to have biological activity in animals. Cholesterol oxidizes readily in solution, in aqueous dispersion and in foods when it is exposed to air, elevated temperatures, free radical initiators, light, or a combination of these. The main outline of the pathway for cholesterol oxidation is fairly well understood and involves initial formation of allylic hydroperoxides. HPLC and GC techniques are available for measuring the concentration of cholesterol oxides in animal-derived lipids. Biologically active cholesterol oxides have been reported to be present in egg products, dairy products, frying oils and other foods. Their concentration may be subject to process control.

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Cholesterol in purified triolein was subjected to saponification by two procedures employing hot alkali and was found to give rise to oxidation products despite precautions. The isomeric 5,6-epoxycholestanols and 7-hydroxycholesterols were stable under saponification conditions, but 7-ketocholesterol and 6-ketocholestanol were destroyed by hot alkali. The oxidation of cholesterol during saponification was not inhibited by the addition of BHT. The determination of a number of standard cholesterol oxides by direct on-column GC was demonstrated with good resolution. Their order of elution was different from that of the TMS-derivatized products.

Cholesterol in aqueous dispersion with sodium stearate or Triton surfactants was aerated at various pH values at 50 and 80 C. Analysis of the reaction mixtures by TLC during the oxidation produced qualitatively similar patterns regardless of pH or temperature. Major oxidation products observed were 7‐ketocholesterol, the isomeric 7‐hydroperoxy‐ and 7‐hydroxycholesterols, the isomeric 5,6‐epoxycholestanols and 3β,5α,6β‐trihydroxycholestanol. The concentrations of 3β,5α,6β‐trihydroxycholestanol and an unknown compound increased greatly at the lower pH values. Recovery of the 5,6‐epoxide isomers by preparative TLC followed by capillary GC allowed theα‐ andβ‐epoxide isomers to be quantitated. Oxidations at pH 8 and 12 produced increasing amounts of the epoxides with time, without significant changes in theα/β‐epoxide ratio. However, oxidations at the acidic pH values of 5.5 and 3 showed large changes in theα/β‐epoxide ratio during the oxidation. Measurement of the hydrolysis rates of the 5,6‐epoxides at pH 5.5 showed that theβ‐epoxide isomer is more labile than theα‐isomer by a factor of 2.5. The rate constant for the hydrolysis of theα‐epoxide isomer was 5.3 × 10−7 sec−1 and that of theβ‐isomer 13 × 10−7 sec−1.