No abstract

When plant sterols are oxidized at moderate temperatures ( 100 7C), products mainly derive from hydroperoxides, but at temperatures close to 200 7C, thermal reactions such as dehydration and condensation become important. Although sterols are often subjected to frying conditions, very little is known of their thermal reactions. In this study, stigmasterol was thermo-oxidized at 180 7C, and the formation of dimers and polymers and the amounts of monomers were measured by high-performance size-exclusion chromatography. The products were further characterized by polarity using solid-phase extraction fractionation. During heating, the amounts of monomers decreased at a steady rate, and those of dimers and polymers increased. After 3 h of heating, 21% of the material existed in higher-molecular-weight products. The amount of polar monomers increased especially during the first hour, demonstrating the formation of oxides and their further reactions, while that of mid-polar monomers decreased constantly, indicating losses of stigmasterol. Polar dimers contributed to approximately 60% of the dimers, and polar polymers to approximately 78% of the polymers, which suggests that in most higher-molecular-weight products at least one of the sterol moieties was oxidized. This study showed that a significant proportion of thermooxidation products are not polar monomeric oxides which are commonly analyzed as oxidation products.

No abstract

The oxidation mechanisms of stigmasterol at 100 and 180 7C 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 7C, the amounts of hydroperoxides increased sharply during the first 10 min and then began to decrease. At 100 7C, the amounts of hydroperoxides increased over the entire experimental period. At 180 7C, all major secondary oxidation products, except 7-ketostigmasterol, reached a plateau after 40 min of oxidation, while at 100 7C their amounts increased constantly. The same oxidation products were formed at both temperatures, but their distribution differed. At 180 7C, 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 7C; radicals formed more easily at the tertiary position 25. At 180 7C, 7-ketostigmasterol was dominant after 40 min of oxidation, whereas at 100 7C it was the main oxidation product over the entire experiment.

No abstract