Effect of Phytosterol Structure on Thermal Polymerization of Heated Soybean Oil

Authors: Moser, Jill; Warner, Kathleen

Submitted to: European Journal of Lipid Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2008
Publication Date: 8/9/2008
Citation: Moser, J.K., Warner, K.A. 2008. Effect of Phytosterol Structure on Thermal Polymerization of Heated Soybean Oil. European Journal of Lipid Science and Technology. 110:1068-1077.

Interpretive Summary: There is a lot of interest in finding natural alternatives to synthetic antioxidants to help increase the stability of vegetable oils during frying and other high-heat applications. Phytosterols are naturally abundant in vegetable oils, but little is known about their impact on vegetable oil stability. In this study we added phytosterols with varying degrees of unsaturation or structures to soybean oil, and heated the oil to frying temperature for 8 hours. Then we monitored the accumulation of polymers in the oil, which increase as the oil degrades. It was found that a phytosterol with three double bonds was effective at reducing the amount of polymers that formed in the heated oil by 16-20%. Other phytosterols with fewer double bonds had little to no impact on polymer formation. This research indicates that the type of phytosterols present in an oil can impact its stability to high-temperature applications such as frying. This research provides information to vegetable oil processors by demonstrating a natural additive that may improve stability to frying.

Technical Abstract: 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-1 oil (0.05, 0.1, and 0.5 wt-%). These oils were heated at 180 deg C over a period of eight hr, 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-1, 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-1. Ergosterol addition at 5 mg g-1 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.