Phytosterol oxidation in oil-in-water emulsion and bulk oil

Dietary plants sterols (phytosterols) have been shown to lower total and LDL cholesterol level in humans. Since phytosterols may protect against coronary heart diseases, they have been recently incorporated into a growing spectrum of functional food. However, phytosterols are prone to oxidation, as cholesterol, and their oxidation products (POP) may have toxic effect on human and animal cells similar to those of cholesterol (COP). The purpose of this study was to evaluate the formation of POP in both oil-in-water (o/w) emulsions and bulk vegetable oil. The samples were stored in the dark at 55oC for 73 days. The extent of lipid oxidation was monitored by measuring the lipid hydroperoxides and hexanal, whereas phytosterols oxidation was determined by gas chromatography. The 7-keto derivatives of phytosterols were chosen as reliable markers of their degree of oxidation. A higher POP level was found in the o/w emulsion with respect to the bulk oil; the latter actually displayed a lower POP content and formation rate. This may be due to the high surface activity of sterols, which would allow them to migrate to the oil-water interface where the oxidative stress is high. This hypothesis was actually confirmed by the interfacial tension measurements performed with the digital tensiometer, which showed that phytosterols had a high degree of surface activity which would allow them to concentrate at the interface of emulsion droplets. Surprisingly, the monounsaturated phytosterols were found to oxidize faster or at a similar rate as the omega-6 polyunsaturated fatty acids in the o/w emulsion and bulk oil, a factor that could also be related to their high surface activity.