DSC evaluation of microwave-heated olive oils

Lipid oxidation is one of the major degradation reactions in heated oils, which results in a significant loss of quality. Microwave heating is commonly applied in both domestic cooking and food catering. Differential scanning calorimetry (DSC) was recently utilized to assess thermoxidative stability of vegetable oils subjected to conventional heating; however, few reports have dealt with the application of this technique to the evaluation of thermal properties of microwave heated oils and, to the authors’ knowledge, this aspect has not been faced in olive oils yet. The aim of this work was to evaluate the influence of exposure time to microwave heating on DSC thermal properties of three commercial categories of olive oil for direct human consumption (extra virgin olive oil (EVOo), olive oil (Oo), pomace olive oil (POo)). Oil samples were placed in 250 mL open flasks, microwaved (at 2450 MHz, 0.72 kW) for 1.5, 3, 6, 9, 12, 15 min, and then cooled to 25°C in the darkness before analysis. Temperatures reached by the oils were determined immediately after microwaving with calibrated K-type thermocouples. Thermograms of the heated oils were obtained by means of DSC (DSC Q100, TA Instruments, New Castle, DE) by cooling from 30 to -80°C at 2°C/min, holding 3 min at -80°C and heating from -80 to 30°C at 2°C/min. Enthalpy (H, J/g), onset (Ton), and offset (Toff) temperatures of the transitions were obtained. Chemical oxidative parameters of the oils (free acidity, peroxide value (POV) and p-anisidine value (PAV)), were also measured. Thermo-oxidation affected the crystallization process of both EVOo and POo, significantly increasing the transition enthalpy and shifting Ton towards higher temperatures. Moreover, crystallization occurred over a larger temperature range, probably due to the interaction of lipid oxidation molecules and free fatty acids with triacylglycerol crystals. This may be related to the higher increase of PAV and free acidity in EVOo and Poo, as compared to Oo. The lineshape of the melting curves significantly changed in EVOo and POo; in fact, the smaller endothermic event peaked at higher temperature and slowly disappeared with increasing treatment- time. On the contrary, Oo exhibited few changes in its thermal properties, induced by the microwave process.