"The oxidative stability of different phytosterol. compounds during pan-frying was evaluated. Rapeseed oil, rapeseed oil-based liquid margarine and butter oil were used as lipid matrices. Gas chromatographic-mass spectrometric analyses indicated that pan-frying may induce phytosterol oxidation but has no marked effect on phytostanol oxidation; up to 5.1% of original sitosterol and 0.1% of original sitostanol were found as oxides. High-performance size-exclusion chromatographic determinations also revealed a moderate degree of polymerization of lipid matrices. The extents of these deteriorative reactions were determined by frying temperature and time, but also by unsaturation degree and water content of the lipid matrix as well as by structure of the phytosterol compound added. The highest phytosterol oxide content was measured in saturated butter oil enriched with free phytosterols, whereas the highest matrix dimer and polymer contents were found in free phytosterol-enriched liquid margarine, which contained 20% of water. (c) 2006 Elsevier Ltd. All rights reserved."