Sitosterol thermo-oxidative degradation leads to the formation of dimers, trimers and oligomers: a study using combined size exclusion chromatography/mass spectrometry.

Phytosterols are recognized as functional food components with cholesterol reducing properties in humans. The formation of phytosterol oligomers as a result of the thermo-oxidative degradation of sitosterol is shown to occur. The existence of oligomers is demonstrated by size exclusion chromatography (SEC) and confirmed by combined SEC-atmospheric pressure chemical ionization mass spectrometry (SEC/APCI-MS). A speculative structure for the sitosterol dimer with 3,7' linkage is proposed consistent with data from tandem mass spectrometry and exact mass measurements. Higher molecular weight species arising from the formation of trimers or higher oligomers are seen in the mass spectra. Fragments of sitosterol formed by thermo-oxidative processes are also shown to oligomerize and their common structural characteristics are demonstrated by tandem mass spectrometry. The results presented provide evidence for the possible formation of oligomeric species involving sterols in addition to those known for acylglycerides in vegetable oils subjected to extreme oxidative stress such as in frying.