Effect of phytosterol structure on thermal polymerization of heated soybean oil

This study determined the effect of phytosterol structure, including the degree of unsaturation and the presence of an ethylidene group in the side chain, on the thermal polymerization of heated soybean oil. Indigenous tocopherols and phytosterols were removed from soybean oil by molecular distillation. Pure phytosterols were added back to the stripped soybean oil at concentrations of 0.5, 1.0, and 5 mg/g oil (0.05, 0.1, and 0.5 wt-%). These oils were heated at 180 7C over a period of 8 h, and triacylglycerol dimers and polymers, fatty acid composition, and residual phytosterol content were determined. None of the phytosterols prevented triacylglycerol dimer and polymer formation when used at 0.5 mg/g; however, phytosterols with two or more double bonds, regardless of the presence of an ethylidene group in the side chain, provided slight protection when added at 1 mg/g. Ergosterol addition at 5 mg/g reduced polymer formation by 16–20% compared to the control oil, but at this level none of the other phytosterols provided protection of any practical significance. Thus, under the conditions used for this heating study, the degree of phytosterol unsaturation was more important for its anti-polymerization activity than the presence of an ethylidene group.