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The purpose of this study was to investigate the effects of temperature, oxidation time, presence of water, pH, type of buffer and form of substrate used on cholesterol oxidation. Microcrystalline cholesterol films, both solid and melted, and aqueous suspensions of film fragments were used as substrates. Use of dispersing agents was avoided. Quantitative analysis of the unaltered substrate and the products of its autoxidation was carried out by gas chromatography over the course of oxidation. Solid cholesterol films were found to be resistant to autoxidation in the dry state. However, when heated at 125 degrees C, a sudden increase in oxidation rate occurred at a point coinciding with the visible melting followed by a plateau of the oxidation rate. All of the autoxidation products formed underwent further decomposition. Film fragments of cholesterol oxidized at a faster rate in aqueous suspensions than when oxidized in the dry state. In aqueous suspensions, the differences in the resistance of cholesterol to oxidation were not significant within the pH range 6.0-7.4, except for the early stages of oxidation. The 7-ketocholesterol/7-hydroxycholesterol ratio dropped significantly with increasing pH. However, at all pH levels tested, this ratio remained relatively constant during the 6 h of heating. While the 7 beta-hydroxycholesterol/7 alpha-hydroxycholesterol ratio was not affected by pH in the range of 6.0-7.4, at pH 7.4 a high preference was observed for the cholesterol beta-epoxide over its alpha-isomer.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of oxidative interactions between cholesterol and milk fat globule membrane (MFGM) components, i.e., nonlipid fraction, total lipid, nonpolar lipid and polar lipid, on cholesterol oxidation were studied in the presence and absence of water. In the dry state, cholesterol natively present in MFGM appeared to be protected at 135 degrees C. The nonpolar lipid and nonlipid fraction contributed to the protective effect of MFGM. Added cholesterol accelerated the oxidation of membrane lipid fractions. At 75 degrees C, pure cholesterol and membrane lipid fractions did not show significant interaction. However, cholesterol and other lipids in MFGM were less stable than when these were heated separately. When cholesterol and membrane lipids were mixed in an aqueous medium at 75 degrees C, each accelerated the oxidation of the other. The MFGM exhibited a high protective effect on cholesterol oxidation in an aqueous environment. The nonlipid fraction protected cholesterol against oxidation, whereas the lipid fraction was destructive. In the absence of water, the net balance between these two opposing factors was destructive. The presence of water reversed the balance in favor of protection.

The two major high-boiling volatile compounds produced during refrigerated storage of haddock fillets were found by gas chromatography and mass spectroscopy to be phenethyl alcohol and phenol.

The monohydroperoxides of methyl palmitate oxidized at 150C were concentrated, purified, and reduced to yield a mixture of isomers of monohydroxy palmitate. No evidence of unsaturation in these molecules could be obtained, and no direct method for the resolution or identification of the individual isomers was found. Nuclear magnetic resonance (NMR) spectroscopy of the reduced esters suggested that the hydroxy groups were not located at either end of the chain of the fatty ester.To establish the location of the initial oxidative attack on saturated esters, the reduced esters of oxidized methyl palmitate were chemically cleaved at the location of the hydroxy group by means of chromium trioxide oxidation and by the Beckmann transformation. The resulting mono- and dibasic fatty acids were methylated and analyzed by gas chromatography. This analysis indicated that the oxidation of methyl palmitate did not occur selectively at one location along the ester. Although all carbon atoms can apparently be oxidized, preferential oxidation was observed toward the center of the molecule.

A highly purified methyl palmitate free of all detectable impurities was oxidized by aeration at 150C. Monohydroperoxide was shown by thin-layer chromatography (TLC), spot and spray test, and polarography to be the initial autoxidation product.