DSC evaluation of thermoxidative decomposition of extra virgin olive oil: effect of microwave and conventional heating

The aim of this work was to evaluate DSC thermal properties of extra virgin olive oil subjected to microwave and thermal heating. Fresh extra virgin olive oil was stored in dark bottles under nitrogen at room temperature before analysis. Sample were placed in a 250 ml open flask and either heated in an electric oven (with air convection) at 180°C for 30, 90, 120, 180, 360, 900 and 1440 min, or microwaved (at 2450 MHz, 0.72 kW) for 3, 6, 9, 12, 15 and 20 min. All samples were cooled at room temperature in the dark to 30°C and then analyzed. Thermograms were obtained by means of DSC, 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. Lipid crystallization Ton of fresh extra virgin olive oil was at -11°C and developed over a 35–40 degrees range. Two well defined events (minor, peaking at -16 and major peaking at -40°C) were distinguishable. Heating thermograms of fresh sample showed a minor exothermic peak and successively a major endothermic event occurring over the -18°C/12°C temperature range, with the presence of a major endothermic peak (-5°C) and a small shoulder at 8°C. Enthalpy of both crystallization and melting transition significantly decreased in all samples with increasing length of heating treatment; this effect was more pronounced for the microwaved sample. Crystallization peak line-shape dramatically changed with increasing heating time. The major crystallization peak decreased in height, and the transition spanned over a larger temperature range, because of a significant shift of Toff towards lower temperatures. Crystallization Ton remain constant in conventionally heated samples, whereas it significantly shifted towards higher temperature in samples microwaved for at least 12 min. Melting line-shapes also dramatically changed; in fact, the major endothermic peak flattened, decreased in height, and spanned over a larger temperature range, while the minor endothermic peak disappeared in the longest treated samples. An additional endothermic peak also appeared at lower temperature after 20 and 900 min of microwave and conventional treatment, respectively. Ton of the heating transition significantly shifted towards lower temperatures at increasing treatment time. In conclusion, changes in all thermal properties were already evident in microwaved samples after 9 minutes of treatment, whereas similar values were reached after 360 min of air convection heating.