Microwave heating of different commercial categories of olive oil: Part II. Effect on thermal properties

The effect of microwave heating of commercial categories of olive oil for human consumption (extra virgin olive oil [EVOo], olive-pomace oil [Po] and olive oil [Oo]) on DSC thermal properties was evaluated at different times of microwave treatment. Marked changes of DSC cooling profiles were found for EVOo and Po subjected to microwaving, with the major exotherm that shifted towards lower temperature and decreased height with increasing treatment time. Thermal properties (during DSC cooling analysis) changed in all samples: crystallization enthalpy significantly decreased and the phase transition developed over a larger temperature range, due to more heterogeneous chemical composition of all oils that resulted from triacylglycerol lysis and formation of lipid oxidation products. Heating profiles of EVOo and Po were also modified by microwave treatment, as the minor endotherm progressively disappeared significantly shifting offset temperature of transition towards lower temperature. Oo did not exhibit such changes of thermal properties and phase transition profiles as described for EVOo and Po. This could be mainly related to its lower water content although the simultaneous presence of small amounts of antioxidant molecules (polyphenols) may have contributed to partially prevent thermal degradation of this oil in comparison with the others. These preliminary results suggest that DSC can be useful not only for monitoring modifications of chemical composition with increasing microwave treatment time, but also to discriminate among olive oils according to their response to microwave exposure.