Formation and decomposition of stigmasterol hydroperoxides and secondary oxidation products during thermo-oxidation

The oxidation mechanisms of stigmasterol at 100 and 180 degrees C were investigated by using the HPLC-UV-FL method. An overall picture of the oxidation status was achieved with a single HPLC analysis, enabling us to monitor the formation and decomposition of both primary and secondary oxidation products. The oxidation behavior of stigmasterol was different at the two temperatures. At 180 degrees C, the amounts of hydroperoxides increased sharply during the first 10 min and then began to decrease. At 100 degrees C, the amounts of hydroperoxides increased over the entire experimental period. At 180 degrees C, all major secondary oxidation products, except 7-ketostigmasterol, reached a plateau after 40 min of oxidation, while at 100 degrees C their amounts increased constantly. The same oxidation products were formed at both temperatures, but their distribution differed. At 180 degrees C, the formation of free radicals at position 7 was more favorable than formation of radicals at position 25. The situation was the opposite at 100 degrees C; radicals formed more easily at the tertiary position 25. At 180 degrees C, 7-ketostigmasterol was dominant after 40 min of oxidation, whereas at 100 degrees C it was the main oxidation product over the entire experiment.