Evolution of Oxidative Values during Kinetic Studies on Olive Oil Oxidation in the Rancimat Test

A comparative study was carried out in order to evaluate the kinetics of the formation of a number of primary and secondary oxidation products during oxidation of olive oil in the Rancimat test at 100–130 °C. There were good correlations between the Rancimat index (OSI) and stability indices (IP) measured in the Rancimat test with no significant differences in kinetic parameters calculated from them. Mean values of the temperature coefficient, Q₁₀ number, activation energy (E_a), frequency factor (A), and free energy of activation (ΔG⁺⁺) for olive oil oxidation were calculated to be ?3.44 × 10^?2°C^?1, 2.21, 98.91 kJ/mol, 12.17 × 10¹² h^?1, and 128.25 kJ/mol, respectively. Each unit change in E_a was accompanied by an average 1.43 × 10¹² change in A, indicating a higher contribution for factor A than for E_a to the olive oil stability. The E_a and A correlated well with the values of enthalpy and entropy, respectively. The values of OSI or IP could be described well by the ΔG⁺⁺ values. Kinetic data indicated that olive oil stability is more affected by the indigenous antioxidants than by the fatty acid composition.     

Oxidative stability of virgin olive oil as affected by the bene unsaponifiable matters and tertiary-butylhydroquinone

During 16 h heating at 180 °C, the oxidative stability (OS) of virgin olive oil (VOO) as affected by the same concentrations (200 ppm) of tertiary-butylhydroquinone (TBHQ) and unsaponifiable matters of bene kernel (UKO) and hull (UHO) oils in terms of the inhibitory effect on the formation of conjugated diene hydroperoxides (OS(CDV)) and off-flavor carbonyl compounds (OS(CV)) was investigated. TBHQ was not able to considerably increase the OS(CDV) (7.51) of the VOO (7.2) and showed no synergistic effect with indigenous antioxidative compounds of the VOO (IOV) in this respect. However, it could significantly improve the OS(CV) (from 2.49 to 4.52), which was mainly due to its synergism with the IOV. The UKO increased considerably the OS(CDV) (to 11.8), and its OS(CV) (4.22) was nearly the same as that of TBHQ. The IOV still had marked contributions to the prevention of VOO oxidation but the majority of stabilizing effect was related to the UKO and its synergism with the IOV. The OS(CDV) in presence of the UHO was less than that of the VOO (5.96), although it significantly increased the OS(CV) (to 5.2), mainly due to the stabilizing effect of UHO and its synergism with the IOV.     

Shelf-life prediction of olive oils using empirical models developed at low and high temperatures

Induction period of the formation of hydroperoxides and conjugated dienes at 50 °C and oxidative stability index at 100–130 °C as the oxidative stability measures of different types of olive oils with a wide range of chemical compositions were determined. Regression models (R² ? 0.95) developed at low and high temperatures showed different contributions of compositional variables (the ratio between monounsaturated and polyunsaturated fatty acids, the content of total tocopherols and phenolics, peroxide value, acid value, and total polar compounds content) to the oxidative stability measures. To estimate the shelf-life of olive oils at low temperature, three empirical models with errors of ±1.5%, <±10%, and ±21.2% were developed.