Characterisation of non-polar dimers formed during thermo-oxidative degradation of β-sitosterol.

Thermo-oxidative degradation of sterols at temperature typical for frying leads to the formation of oxidised derivatives, fragmented sterols and oligomers. Recent research on sterol oxidation focuses mainly on the oxysterol derivatives formation to the exclusion of compounds with high molecular mass. The aim of this work was to decipher the chemical structure of non-polar dimers formed during β-sitosterol oxidation at 180°C in the presence of oxygen. The dimer fraction was separated by size-exclusion chromatography (SEC) after pre-fractionation on silica gel. The chemical structure of the dimers was assessed by 1D and 2D NMR, IR, Raman and MS spectroscopies. NMR data confirmed that the predominant non-polar dimer formed during β-sitosterol oxidative degradation has a configuration of 3β,3β'-disitosteryl ether. Data from IR and Raman spectroscopies further proved it chemical structure. Applied analytical techniques also confirmed presence of dimers with different configuration than disteryl ethers.