Effect of microwave heating on phytosterol oxidation

Nowadays, there is an increasing trend to use fast cooking/reheating techniques, such as microwave (MW). Vegetable oils used for product formulation and/or cooking contain phytosterols (PS), which are structurally similar to cholesterol and thus can generate phytosterol oxidation products (POPs), known for their adverse biological effects. Therefore, it is important to understand how MW can impact POPs formation in model systems and in lipid matrices, especially if the growing tendency of the phytosterol-enriched product market is considered. In this study, two different phytosterol model systems (alone (PF) and in combination with triolein (PS+OOO)) were used to assess the thermoxidative behavior of PS during MW heating (1000 W) for 1.5, 3, 6, 12, 20 and 30 min. Different oxidative stability was observed depending on the model system. In particular, PS degradation fit a first order kinetics, being faster in PS microwaving alone than in PS+OOO. In both systems, a marked PS degradation (~30%) was noted in the first 1.5 min of microwaving, followed by a fast drop rate after 6 min and 12 min for PF and PS+OOO, respectively. At 12 min of heating, a higher POPs amount was found in PF (90.96 µg/mg phytosterols) with respect to PS+OOO (22.66 µg/mg phytosterols). At the end of the MW treatment (30 min), the POPs content were highest in PS+OOO (159 µg/mg phytosterols). POPs formation in the PF system was directly correlated with the temperature reached during microwaving, while no correlation was found in the PS+OOO system. The latter further confirms the important “protective/dilution” role of triolein towards PS oxidation, despite the prooxidizing effect of temperature actually detected in PF.