Effects of esterification on the formation and decomposition of steryl hydroperoxides

Abstract

The aim of the study was to investigate the effects of esterification on the hydroperoxide formation and decomposition of steryl ester to be able to evaluate early phases of autoxidation. In order to study hydroperoxides of intact steryl esters, a novel HPLC–ELSD method was developed. Steryl ester hydroperoxides were separated using a silica column with heptane-MBTE gradient. The method was applied to study the formation and decomposition of steryl moiety hydroperoxides in cholesteryl stearate during heating at 100 °C. To investigate the effect of esterification, also the formation of hydroperoxides in free cholesterol was measured. To confirm the results obtained by HPLC and to follow the decomposition of the hydroperoxides, peroxide values and secondary oxidation products were determined. The new HPLC showed that 7α- and 7β-OOH were the predominant hydroperoxides. In purified cholesteryl stearate, their amounts were highest (47–62 mg/g) after heating for 2–3 days, and after 4 days only small amounts were found. The formation of secondary oxidation products was marked when the amount of hydroperoxides was high. Presence of impurities in the commercial steryl ester preparation increased significantly the oxidation rate. Free cholesterol oxidised more slowly than the ester. The amount of 7-OOH epimers was only 2.6 mg/g for free cholesterol after 3 days. In conclusion, the new method was shown to be a useful tool for the determination of individual intact steryl ester hydroperoxides. The esterification of sterol affected both the rate of oxidation and the amount of formed oxidation products.