The present study investigated the Kinetic parameter determination of edible argan oil (cold-pressed from roasted argan kernels) and cosmetic argan oil (cold-pressed from unroasted argan kernels) under the Rancimat test conditions. The physicochemical parameters of edible and cosmetic argan oil immediately after preparation and after accelerated oxidation test Rancimat at different temperatures 90 °C, 100 °C, 110 °C, 120 °C, 130 °C and 140 °C were determined and compared. The natural logarithms of the kinetic rate constant (kvalue) varied linearly with respect to temperature. An increasing rate of oxidation could be observed as temperature increased. On the basis of the Arrhenius equation and the activated complex theory, frequency factors A, activation energies Ea, Q10 numbers, activation enthalpies ΔH, and activation entropies ΔS for oxidative stability of the vegetable oils were calculated. The accelerated oxidation and Kinetic parameters have shown that edible argan oil can be stored much better than cosmetic oil.
Argan oil, a typical Moroccan product, is prepared from the fruits of argan trees (
For years, argan oil has been prepared exclusively by Berber women through a traditional multistep process (Charrouf
Unfortunately, this method is time consuming for producing oil batches with different organoleptic properties due to the non-reproducible roasting (Charrouf
Recently, in the women's cooperatives, argan oil has been extracted using mechanical presses producing a high quality of oil on a large scale (Charrouf
Lipid oxidation in vegetable oils is one of the most important reactions that can cause the deterioration of their quality. The oxidation process of edible oil results in the development of undesirable flavors which affect the sensory quality and considerably reduce the pharmaceutical and food use of these oils (Jacobsen and Nielsen
A number of accelerated methods have been developed to test the resistance of edible fats and oils to oxidation. All these accelerated methods involve the use of elevated temperatures because it is known that the rate of the reaction is exponentially related to temperature (Reynhout,
This test allows for the determination of the induction period (IP) or oil/oxidative stability index (ISO-
A number of kinetic parameters can be determined under the Rancimat test conditions. Kinetic data can be used to characterize the differences or similarities in the oils. These data are very useful for predicting the oxidative stability of vegetable oils under various heat processing, storage, and distribution conditions (Tan
The aim of the present study was to determine the relative oxidative stabilities of argan oil obtained from roasted and unroasted kernels and the kinetic parameters of their oxidation under the Rancimat test conditions.
Argan fruits (600 Kg) were collected in Ait Baha, (Province of Chtouka Ait baha, Morocco). The fruit was dried and peeled using the argan-cooperative traditional technique (Charrouf and Guillaume,
All chemicals and solvents used were of analytical reagent grade (Merck, Darmstadt, Germany).
The determination of physicochemical parameters (acidity, peroxide value, UV-light absorption (K232 and K270), was carried out according to the analytical methods described by Regulation EEC/ 2568/91 and EEC/ 1429/92 of the European Union Commission (
For the determination of fatty acid composition, the methyl esters were prepared by cold transmethylation in a basic medium (IOOC,
The tocopherol content was determined on the basis of the AOCS Official method Ce8-89 (American Oil Chemists' Society,
A Metrohm Rancimat model 743 (Herisau, Switzerland) was used. In order to establish air-saturated conditions in the oil samples, the tests were carried out with 3 g of the oil samples at temperatures of 90 °C, 100 °C, 110 °C, 120 °C, 130 °C and 140 °C at an airflow rate of 20 L/h. Samples for all determinations were randomized on their position in the heating block.
The glassware was rigorously cleaned between runs to avoid any contamination that would catalyze peroxidation. Measuring vessels, electrodes, and connecting tubes were cleaned several times with alcohol and distilled water, and were blown out with nitrogen before the experiment (Farhoosh
All Rancimat experiments and measurements were carried out in triplicate, and the data were subjected to analysis of variance (ANOVA). ANOVA and regression analyses were performed according to the SPSS software. Significant differences between means were determined by Duncan's multiple range Tests; p values less than 0.05 were considered statistically significant.
The initial compositions of the argan oils studied are summarized in
Initial quality characteristics and fatty acid composition of the argan oils
RAO | UAO | |
---|---|---|
Acid Value (%) | 0.18±0.01 | 0.38±0.01 |
Peroxide value (meq O2·kg−1) | 0.54±0.06 | 0.98±0.02 |
K232 | 1.38±0.026 | 1.25±0.01 |
K270 | 0.25±0.023 | 0.22±0.01 |
β-carotene (ppm) | 19,96±0.13 | 8.62±0.14 |
Phosphorus (ppm) | 42.81±0.14 | 2.06±0.03 |
Phospholipid (mg·100 mg−1) | 0.3 | 0.006 |
SFA (mg·100 mg−1) | 19±0.7 | 19.45±0.17 |
MUFA (mg·100 mg−1) | 46.35±0.72 | 46. 25±0.37 |
PUFA (mg·100 mg−1) | 34.65±0.72 | 34.30±0.37 |
Tocopherols (ppm) | 791.91 | 667.04 |
SFA: Saturated fatty acid; MUFA: Mono unsaturated fatty acid; PUFA: Polyunsaturated fatty acid.
We also found that there was no statistically significant difference among the percentages of saturated fatty acids SFA and unsaturated fatty acids (MUFA+PUFA) of the roasted and unroasted argan oil.
The initial content of tocopherol, in the oil prepared from roasted seeds was 791.2 ppm; it differs significantly from the corresponding values in oils from unroasted seeds of 667.04 ppm. The beta-carotene and the phosphorus/phospholipid contents were significantly lower in the beauty oil than in the edible oil. Beta-Carotene is a very lipophilic molecule that is known to present antioxidant properties in synergy with α-tocopherol (Palozza and Krinsky,
Semi-logarithmic relationship between k and T values for lipid oxidation of the argan oils.
Semi-logarithmic relationship between k and (1/T) values for lipid oxidation of the argan oils.
Semi-logarithmic relationship between (k/T) and (1/T) values for lipid oxidation of the argan oils.
The reaction rate constants (k) of the argan oils at different temperatures
K±SD (×103)[h−1] | ||||||
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Oil | 90 | 100 | 110 | 120 | 130 | 140 |
Roasted Argan Oil | 7.09±0.15 | 16.69±0.69 | 32.26±0.25 | 63.13±0.89 | 131.58±0.98 | 294.12±0.85 |
Unrosted Argan Oil | 16.89±0.72 | 36.66±0.34 | 83.33±0.48 | 158.73±0.57 | 335.57±0.84 | 781.25±0.75 |
SD, Standard deviation.
Regression parameters for Arrhenius relationships between the reaction rate constant and the temperature for the argan oils
ln (k) = ln(A)-(Ea/R)*(1/T) | ||||||
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a | b | R2 | Ea | A | Q10 | |
•RAO | −10.91 | 32 | 0.997 | 90.71 | 7.89 1013 | 2.11 |
♦UAO | −11.32 | 33.97 | 0.997 | 94.12 | 5.66 1014 | 2.16 |
The results obtained showed that a high content of β-Carotene, Phospholipid and Phosphorus would improve resistance to lipid oxidation (increase the Ea value). These would result in delaying the onset of the initial oxidation process where bond scission takes place to form primary oxidation products. The B Carotene, Phospholipid and Phosphorus contents of the argan oils (
The magnitude of the temperature effect on the oxidation rate of the argan oils was evidenced by the Q10 numbers. In general, a higher Q10 number implies that a smaller temperature change is needed to induce a certain change in the rate of lipid oxidation. As can be seen in
The ΔH and ΔS values estimated based on the activated complex theory and the corresponding regression parameters are summarized in
Activation enthalpies ΔH and entropies ΔS for lipid oxidation of the argan oils
ln (k/T) = ln (kB/h)+(ΔS/R) – (ΔH/R)×(1/T) | |||||
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a | b | R2 | ΔH | ΔS | |
•RAO | −10.52 | 18.13 | 0.996 | 87.47 | −46.81 |
♦UAO | −10.93 | 20.11 | 0.997 | 90.87 | −30.35 |
The ΔH and ΔS values for the argan oils studied ranged from 87.47 kJ·mol−1 and −46.81 J·mol−1·K−1 for roasted argan oil to 90.87 kJ·mol−1 and −30.35 J·mol−1·K−1 for unroasted Argan oil, respectively. In their study on the determination of the oxidative stability of rapeseed, sunflower and soybean oils by the Rancimat test, Kowalski
The experimental results allowed us to draw the following conclusions: An increasing rate of oxidation could be observed as temperature increases and, edible argan oil can be stored much better than cosmetic oil.