Effect of Fatty Acid Unsaturation on Phytosteryl Ester Degradation

This study examined the thermo-oxidative degradation of stigmasterol fatty acids esters. Stigmasterol stearate, oleate, linoleate and linolenate were synthesized by chemical esterification and their purity evaluated by (1)H-NMR and GC–MS. The degradation of stigmasterol esters was examined after heating them at 60 and 180 °C for 1, 2, 4, 8 and 12 h. It was established that stigmasterol esters were prone to thermo-oxidative degradation, with time and temperature affecting the degree of degradation. The unsaturation of fatty acids affected the rate of stigmasteryl ester degradation. The kinetics of StS and StO degradation were similar and the additional double bonds in StL and StLn resulted in their faster decomposition. The esters degraded faster at 180 than at 60 °C. The sterol and fatty acid molecules degraded at different rates, such that the fatty acid moiety deteriorated faster than the sterol at both temperatures, independent of the time of heating and the level of unsaturation.