Browsing by Author "Cherif, Marwa"
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- Kinetic study of the microwave-induced thermal degradation of cv. Arbequina olive oils flavored with lemon verbena essential oilPublication . Cherif, Marwa; Rodrigues, Nuno; Veloso, Ana C.A.; Pereira, J.A.; Peres, António M.The effect of typical domestic microwave heating (0–15 min, at 800 W) on the thermal degradation of unflavored and flavored olive oils' minor bioactive compounds and related antioxidant activity was studied. Olive oils from cv. Arbequina were flavored with lemon verbena essential oil (0%, 0.2% and 0.4%, w/w) leading to a linear increase of total phenols (112–160 mg gallic acid kg−1 oil, R-Pearson = +0.9870), total carotenoids (2.19–2.56 mg lutein kg−1 oil, R-Pearson = +0.9611), and, to a less extent, of chlorophyll (2.32–3.19 mg pheophytin kg−1 oil, R-Pearson = +0.8238). However, no such linear trend was observed for the oxidative stability (6.5–7.8 h) or the radical scavenging activity (inhibition rates: 40%–43%). The contents of total phenols, total carotenoids, and chlorophyll decreased with the rise of the microwave heating time, following their thermal degradation, a second-order kinetic model (0.8784 ≤ R-Pearson ≤ 0.9926). The essential oil addition did not influence the estimated second-order rate reaction constants of total phenols (0.00070–0.00072 kg oil min−1 mg−1 gallic acid)and total carotenoids (0.14–0.17 kg oil min−1 mg−1 lutein), with a broader variation observed for chlorophyll (0.014–0.022 kg oil min−1 mg−1 pheophytin). Globally, total carotenoids degraded faster than total phenols and chlorophyll (half-life of 2.3–3.4, 8.8–12.8, and 14.5–30.8 min, respectively). Moreover, except for chlorophyll, the half-life of total phenols and carotenoids linearly decreased with the essential oil addition (R-Pearson: −0.9999 and −0.9421, respectively), showing that flavoring did not have a protective effect against degradation when subjected to a microwave heating.
- Kinetic-thermodynamic evaluation of olive oils fortified with phenolics compoundsPublication . Cherif, Marwa; Peres, António M.; Rodrigues, Nuno; Zaghdoudi, KhalilFlavoring olive oils represents a traditional practice in Mediterranean area aiming to enhance the quality and the shelf life of the olive oils, namely of lower quality oils. Flavor is one of the main factors that influences the consumer’s perception of quality. So, the present research intends to evaluate the phenolic enrichment of EVOO, from cv. Arbequina since it is expected to have a low total phenols content, with known concentrations (from 0.1 to 0.4%) of an essential oil of Lemon Verbena plant (Aloysia citrodora) as well as the effect on oils’ quality parameters, oxidative stability and antioxidant activity. The study also focused on a kineticthermodynamic approach, in order to deeper understand the lipid oxidation that olive oils undergo under Rancimat test conditions (110, 120, 130, 140 and 150°C). The results pointed out that the addition of the essential oil led to a slight decrease in the oils’ acidity, while the remaining quality parameters (peroxide value and extinction coefficients) were negatively influenced. On the other hand, the antioxidant capacity was positively influenced by the essential oil addition. The total reducing capacity increased by 17% with the increase of the essential oil addition (from 0.0 to 0.4%). Also, the same trend was found in DPPH values and it increased by 22%. Unexpectedly, the fortification of olive oils didn’t improve the oils’ oxidative stability. The kinetic-thermodynamic data, determined by applying the activated complex theory, showed that olive oils without any essential oil addition had a significantly more negative temperature coefficient (-0.0268°C-1), higher temperature acceleration factor (1.852), greater activation energy (82.7 kJ mol-1) and frequency factor (10.9×109 h-1), higher positive enthalpy of activation (79.4 kJ mol-1), lower negative entropy of activation (-131.8J mol-1 K-1), and greater positive Gibbs free energy of activation. These results allowed to conclude that the lipid oxidation process in the studied olive oil (without and with essential oil flavoring), was a non-spontaneous, endothermic, and endergonic process with activated formed complexes structurally more ordered than the reactants.
- Kinetic-thermodynamic study of the oxidative stability of Arbequina olive oils flavored with lemon verbena essential oilPublication . Cherif, Marwa; Rodrigues, Nuno; Veloso, Ana C.A.; Zaghdoudi, Khalil; Pereira, J.A.; Peres, António M.Arbequina extra-virgin olive oils were flavored with lemon verbena (Aloysia citrodora) essential oil (0.1–0.4%, w/w), being evaluated quality parameters (free acidity, peroxide value, UV-extinction coefficients), oxidative stability, antioxidant and total reducing capacity. The kinetic-thermodynamic nature of the lipid oxidation was evaluated by Rancimat (110–150 °C). The essential oil addition promoted the antioxidant and total reducing capacities but, unfortunately, increased primary and secondary related quality parameters. Moreover, flavoring decreased the oils' oxidative stability. The kinetic-thermodynamic data showed that unflavored oils had significantly lower oxidation reaction rates (0.055–0.06492 h−1), more negative temperature coefficient (−0.0268°C−1), higher temperature acceleration factor (1.852), greater activation energy (82.7 kJ mol−1) and frequency factor (10.9 × 109 h−1), higher positive enthalpy of activation (79.4 kJmol-1), lower negative entropy of activation (−131.8 J mol−1K−1) and greater positive Gibbs free energy of activation (129.95–135.23 kJ mol−1), showing that oils’ oxidation was negatively influenced by the essential oil incorporation. Overall, oxidation had a non-spontaneous, endothermic and endergonic nature. Finally, olive oils could be satisfactorily classified (principal component and linear discriminant analysis) according to the flavoring level, using quality-antioxidant-stability or kinetic-thermodynamic datasets. The latter showed a less predictive performance, although ensuring the full discrimination of unflavored from flavored oils.