Thermal oxidative process in extra-virgin olive oils studied by FTIR, rheology and time-resolved luminescence

With the aim to characterise the antioxidant properties of different extra-virgin olive oils and to understand in more detail the mechanisms of oil degradation, we have made an experimental study on thermal induced oxidative processes of extra-virgin olive oils by using different techniques: Fourier Transform Infrared (FTIR) spectroscopy, rheology and time-resolved luminescence. The oxidation process was followed at three different heating temperatures (30, 60 and 90 °C) as a function of time up to 35 days. Thermal treatment induced changes in the FTIR spectra in the wavenumbers region 3100–3600 cm−1: in particular, the absorption profiles show an initial formation of hydroperoxides and a subsequent increase of alcohols and secondary oxidation products. In agreement with the FTIR data, rheology measurements show that after thermal treatment all examined samples exhibit an increase in the viscosity values with respect to pristine ones, indicating that the heat treatment induces the formation of polar molecules with propensity to form hydrogen bonds, which has as a consequence a viscosity increase. Finally, a lifetime increase of luminescence of chlorophyll is observed in agreement with the viscosity rise. Indeed, the viscosity increase reduces the frequency of collisions between the chromophore and its environment, consequently lowering the non-radiative contribution to the luminescence decay.