Factors contributing to the oxidative stability of phytosterols were studied. Unsaturated stigmasterol and saturated sitostanol were used as model compounds and were heated at different temperatures in different lipid matrices for various periods of time. Accumulations of the major secondary oxidation products were used as a marker of the stability of heated compounds, and the products were analyzed by gas chromatography-mass spectrometry. The results showed that both temperature and heating time, as well as sterol structure and lipid matrix composition, affected phytosterol oxidation. In particular, the interactions between different lipid matrices and temperatures had drastic effects on the total contents of the phytosterol oxides formed and also on the reaction pathways of oxidation. During heating at high temperatures for prolonged periods, >20% of stigmasterol was oxidized. At moderate temperatures the oxidation of stigmasterol was rather slow. Sitostanol oxide contents were low under all heating conditions studied.

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The effect of esterification with rapeseed oil fatty acids on the oxidation reactions of sitosterol, campesterol and sitostanol was investigated, as well as the interactions between phytosterol/stanol compounds and the saturated lipid matrix at 100 7C and 180 7C. Free and esterified phytosterols differed in their reactivity in terms of the formation and profile of secondary oxidation products. Phytosteryl esters were more reactive than free phytosterols during prolonged heating at 100 7C. In contrast, free phytosterols were slightly more reactive than phytosteryl esters at 180 7C. The oxidation reactions of phytostanol compounds were low under all conditions studied. Changes in the phytosterol compounds during heating were also studied via the losses in the original phytosterol contents. This study revealed that the formation of secondary oxides did not account for all the phytosterol losses; this indicates the presence of other oxidation products, especially at 180 7C, and during the heating of free sitosterol. Thus, in order to understand the overall deterioration of phytosterol and phytostanol compounds, both the secondary oxide formation and the sterol loss need to be studied. The deterioration of the saturated lipid matrix used in this study was rather low and was mainly associated with the heating temperature and time.

A new GC-MS method for characterization and quantification of phytosterol oxidation products was developed.Applicability of this method was tested by characterizing sitostanol oxides formed in bulk and then quantifying selected oxides in purified rapeseed oil and tripalmitin matrices in which the complex matrix made oxide analysis difficult. In bulk, nine different sitostanol oxides were characterized, including epimers of 7-and 15-hydroxysitostanol and 6-and 7-ketositostanol. In both lipid matrices, the amounts of sitostanol oxides generated in thermo-oxidation were very low. According to statistical analyses, depending on the oxide, the GC-MS results were the same or slightly higher than those quantified by the more common GC-FID method. Thus, GC-MS provides a powerful alternative for characterization and quantification of phytostanol oxides found in low amounts in complex matrices and is a promising method for future phytosterol oxide studies.