Crystallization and melting properties of extra virgin olive oil by synchrotron XRD and DSC

Extra virgin olive oil (EVOO) is the primary source of dietary lipids in Mediterranean countries; but today it is becoming a widespread product internationally appreciated due to the evidence of its beneficial effects on health. As the majority of vegetable oils, EVOO is liquid at ambient temperature. However, the storage at chilled and frozen conditions causes the occurrence of crystallization phenomena. Consequently, EVOO in chilled and frozen foods is frequently present as a mixture of a liquid phase entrapped in a solid made of triglyceride crystals. It is well known that DSC results do not allow the direct identification of the crystal species formed in the system. However, coupling DSC results with synchrotron XRD analysis that allow structures generated by small amounts of crystallites dispersed in the amorphous matrix to be easily detected, a set of detailed information of food lipid phase transition can be obtained. The aim of this work was to apply DSC and synchrotron XRD to study the thermal and structural behavior of EVOO during cooling and subsequent heating at 2 °C/min from 60 to -60 °C and vice versa. To our knowledge, this represents the first attempt to systematically investigate the EVOO crystal polymorphism. Results showed that during cooling, EVOO formed a triple-chain length (3L) having a c parameter of about 58.38 Å and a quadruple chain length structure (4L) with a c parameter of about 89.99 Å, respectively. Both structures evidenced a cell packing arrangement ascribable to a ?? form. During heating, part of the metastable ?? crystals rearranged into the more thermodynamically stable ? form. Beside this interpretation of XRD data, a model considering a cell with a c parameter of about 170 Å and a hexagonal crystal was proposed. Even if more research is needed to validate this approach, it allowed all XRD events recorded during the experiments to be well described.