Investigation of thermal oxidation kinetics of refined hazelnut oil

In this study, the changes in fatty acid composition, polymer triglyceride content, tocopherol content, peroxide value, p-anisidine value, TOTOX value, induction period and the color of hazelnut oil during heating from 80 °C to 180 °C at 20 °C intervals were investigated. Kinetic calculations were carried out between 80°C - 180°C except peroxide value, induction period and color values. It was determined that the changes in α-tocopherol content, induction period, peroxide value, p-anisidine value, polymer triglyceride content, a* and b* color values and absorbance value at 460 nm were in accordance with zero-order reaction kinetics. The rate constants determined for p-anisidine value and polymer triglyceride content of refined hazelnut oil increased between 80°C - 160°C while the rate constants for peroxide value decreased between 80°C - 140°C. The activation energy values for p-anisidine value and polymer triglyceride formation were 47.49 and 52.65 kj/mol while the activation energy for peroxide value between 80°C - 140°C was 29.95 kj/mol. Values Q10 for peroxide, p-anisidine and polymer triglyceride content between 80°C - 100°C were 0.91, 2.72 and 2.37, respectively. These values were higher than those determined at other temperatures. Increasing the resting time of hazelnut oil at a given temperature resulted in minimal change in the C18:1/C16:0 ratios whereas the C18:2/C16:0 ratios decreased. TOTOX value which is indicative of the sum of primary and secondary oxidation products increased directly proportional to the increase in oxidation time (p<0.05). The increase in b* value depending on the oxidation time and temperature showed that the heating process intensified the yellow color of the oil. Heating the hazelnut oil at different temperatures led to the degradation of α- tocopherol. Further degradation of γ-tocopherol took place when α-tocopherol decreased to a certain level. Rate constants for the degradation of α-tocopherol were between 0.5099 and 1.7222 mg.kg-1.hour-1. The induction period of hazelnut oil decreased as the heat exposure time increased. The induction period showed a steady decline as the temperature increased between 80°C - 120°C. Strong correlations were determined between p-anisidine value and polymer triglyceride content and olso p-anisidine and TOTOX values. Additionally, negative correlations were found between oxidation parameters and α-tocopherol content, and oxidation parameters and induction period values. The correlation between b* values and oxidation products were above 0.9 except at 120°C.