2. Physicochemical properties
2.1 Prickly pear oil
Opuntia ficus indica oil obtained by mechanical extraction has a lower acid index (4.37 ± 0.10 mg KOH/g oil) compared to the chemical extraction (5.85 ± 0.03 mg KOH/g oil) and the extraction by maceration (5.66 ± 0.07 mg KOH/g oil). This significant difference (P < 0.05) is probably due to the solubility of the acids in the solvents (Table 1). This high acidity value could indicate a strong enzymatic hydrolysis of the seeds during harvesting, handling or processing of the oil or could also be due to the wrong storage. Compared to our results, the study carried by Kandji (2001) and R'bia et al (2017), showed a low acidity index of OFI oil produced in Tunisia, which was 1.28 ± 0.007%.
The results of saponification index obtained from Opuntia ficus indica show a significant difference (P < 0.05) between the three types of extractions (mechanical, chemical and maceration) which varies from (181.12 ± 0.18, 183.77 ± 1.23 and 179.08 ± 3.45) respectively (Table 1). This index was in accordance with the CODEX STAN 210–1999 standard and these results are comparable to that of the castor oil (185.83 mg KOH / g oil and 181.55 mg KOH / g oil) (Akpan et al. 2006). Our results are also in agreement with El Mannoubi et al (2009) which shows a saponification index of 173.3 mg KOH / g oil. The high saponification index could due to the geographical origin of the seeds and indicates that the oils have a high amount of triglyceride content and are therefore very useful in cosmetology.
The peroxide value (PV) of Opuntia ficus indica oil showed a significant difference (P < 0.05) for the three extraction types (mechanical, chemical and maceration) with 5.75 ± 0.08, 6 ± 0.06 and 5.97 ± 0.04 meq/kg respectively. Compared to our findings, the research performed by Brahmi et al (2020) shown a higher PV value (12.0 ± 0.4 meq O2/Kg, P ≤ 0.05) of OFI samples.
This difference may be due to the extraction time and the solvent used (Table 1). The PV may also be affected by the oxidation of the oil studied under the extraction and conservation conditions. This can lead to the oxidation of unsaturated fatty acids, and thus their reduction.
The density of Opuntia ficus indica oil for the three types of extraction (Mechanical, Chemical and Maceration) are (0.926 ± 0.003, 0.925 ± 0.001, 0.919 ± 0.005) respectively, which is in accordance with CODEX STAN 210–1999. Therefore, the density of this oil is comparable to both olive oil (0.910) and almond oil (0.917) (Ollé 2002).
The refractive index depends, like density, on oil chemical composition and temperature. The refractive index increases with the level of unsaturation and availability of minor functions in the fatty acid chain. (Boukeloua et al. 2012).
The refractive index shows that there is a significant difference (P < 0.05) between the values of the three types of extraction (mechanical, chemical, maceration) with (1.475 ± 0.001, 1.476 ± 0.003 and 1.476 ± 0.001) respectively, which is in accordance with CODEX STAN 210–1999.
Table 1
Physicochemical properties of the oil extracted from Opuntia ficus indica by three methods of extraction.
Extractions
Parameters studied
|
Opuntia ficus indica
|
Mechanical
|
Chemical
|
Maceration
|
Acid index
(mg KOH / g oil)
|
4.376 ± 0.10a
|
5.854 ± 0.03b
|
5.667 ± 0.07c
|
Saponification index
(mg KOH / g oil)
|
181.12 ± 0.18b
|
183.77 ± 1.23c
|
179.08 ± 3.45a
|
Peroxide index
20milieq/Kg
|
5,75 ± 0.08a
|
6 ± 0,06b
|
5,97 ± 0,04b
|
Density
(20°C)
|
0,916 ± 0.003b
|
0.915 ± 0.001a
|
0.919 ± 0.005c
|
Refractive index
(n20d °C)
|
1.475 ± 0.001a
|
1.476 ± 0.003b
|
1.476 ± 0.001b
|
3. Gas chromatography analysis
The fatty acid composition of vegetable oils depends on several factors: plant origin, genetic factors, fruit ripening and specific climatic conditions (Tlili et al. 2011).
The prickly pear oil is rich in unsaturated fatty acid (UFA), the predominant one is the linoleic acid in the three types of extraction (chemical (75.80%±0,10), maceration (74.07%±0,14) and mechanical (71.58%±0,14)). In the second place, we find the saturated fatty acids (SFA), the palmitic acid with (17.32%±0,02 for chemical extraction, 22.41%±0,06 maceration and 26.58%±0,00 for mechanical extraction), followed by the stearic acid (6.87%±0,12 for chemical extraction) and the arachidic acid which represents (3.50%±0,02 for maceration and 1.83%±0,01 for mechanical extraction).
The chromatographic analysis of prickly pear oil for the three types of extraction clearly shows that the fatty acids are stable whatever the extraction and the temperature used. This stability is remarked for the three major fatty acids. The proportion of linoleic acid always remains around (70%) despite the different temperature extraction (maximum 120°C), it is also the case of the saturated fatty acids where we observe no degradation of those acids (palmitic, stearic and arachidic).
The SFA content varies slightly between 24.19% (chemical extraction), 25.92% (extraction maceration) and 28.41% (mechanical extraction), while there is a clear predominance of UFA with slightly variable proportions and the dominance of linoleic acid (Table 2). This is in agreement with the results of El Hachimi et al (2015) and that of Tlili et al (2011) which show that prickly pear oil is also rich in polyunsaturated fatty acid with the dominance of linoleic acid. Similar results were obtained by Özcan and Al Juhaimi (2011) which showed that linoleic acid was the most important fatty acid (61.01%), following by oleic acid (25.52%) in OFI oil seeds harvested in the Mersin province of Turkey. These values are in the range of the main fatty acids of OFI oil from another Moroccan regions that are linoleic acid (60.2–64.6%) and oleic acid (18.2–22.3%) (Taoufik et al. 2015), and those of OFI oil of South Africa: linoleic acid (56.86 ± 0.07 to 67.32 ± 0.37%) and oleic acid (15.20 ± 0.14–22.51 ± 0.52%) (De Wit et al. 2017). The results of R'bia et al. 2017 shown that the same fatty acids have been identified in the OFI from the area of Nabeul, Tunisia: linoleic acid (61.42%) and oleic acid (20.55%), and from Palermo, Sicily, Italy: linoleic acid (58.5 ± 1.1%) and oleic acid (15.8 ± 1.0-18.1 ± 0.9) (De Wit et al. 2017).
Moreover, according to Ennouri et al (2005; 2006), the rate of linoleic acid overtakes 70% and oleic acid 12% in OFI seed oil of Sfax (Tunisia).
Other parameters related to nutritional aspects are the unsaturated/saturated ratio, in fact, OFI oil has an average value of 3.13 due to its high linoleic acid content. In addition, linoleic acids (omega-6) can be transformed by organisms into a series of long-chain fatty acids (Letawe et al. 1998), a precursor of the biosynthesis of arachidonic acid, a substrate for the synthesis of eicosanoids (Ghazi et al. 2013) which play an important role in vascular level and blood coagulation. Also, this oil has useful properties for the skin that’s why it is used in the cosmetics industry (Ramadan and Mörsel 2003).
However, the work of Chaaben et al. 2015 shows that the average content of SFA in prickly pear oil is only 16.64%. This difference is probably due to the genetic, climatic and soil type of the cultivated plant which is in agreement with the work of Ramadan and Mörsel (2003), which shows that the fatty acid composition of the oil of Opuntia ficus indica, is very much influenced by climatic factors, soil type and genetic factors in which they are cultivated.
Table 2 Gas chromatography of the fatty acids of Opuntia ficus indica and Argania spinosa Oils extracted by three methods of extraction.
|
Opunitia ficus indica
|
|
|
Fatty acids
|
Retention Time
|
% Air
|
IM
|
Extraction
|
Chemical
|
Maceration
|
Mechanical
|
Palmitic acid, methyl ester C16 :0
|
23.29
|
17.32%±0.02
|
22.419%±0.06
|
26.58%±0.00
|
MS
|
Linoleic acid, methyl ester C18 :2
|
24.99
|
75.80%±0.10
|
74.07%±0.14
|
71.59%±0.14
|
MS
|
Oleic acid, methyl ester C18 :1
|
25.03
|
***
|
***
|
***
|
***
|
Stearic acid, methylester C18:0
|
24.62
|
6.88%±0.12
|
_
|
_
|
MS
|
Arachadic acid, methyl ester C20:0
|
25.24
|
_
|
3.51%±0.02
|
1.83%±0.01
|
MS
|
Fatty acids
|
SFAa
|
24.198
|
25.928
|
28.413
|
-
|
UFAb
|
75.802
|
74.072
|
71.587
|
-
|
UFA / SFAc
|
3.13
|
2.85
|
2.51
|
-
|
IM: Identification Method, MS: Mass Spectrometry, SFA: Saturated Fatty Acid, UFA: Unsaturated fatty acid
4. Tocopherols
The major tocopherol founded in Opuntia ficus indica oil is β-tocopherol, it represents 502,04 ± 2,7mg/kg for the mechanical extraction followed by the chemical and the maceration extractions 499,6 ± 1,56 mg/kg and 315,47 ± 1,11mg/kg respectively (Fig. 3). Our results also shown that γ-tocopherol was detected in the chemical and the maceration extractions (257,86 ± 2,01 mg/kg and 260,61 ± 2,9 mg/kg) respectively, but missing in the mechanical extraction. The extraction by maceration presents a high value of α-Tocopherols with 237,12 ± 0,33, followed by the chemical and the mechanical extractions (98,71 ± 1,06 and 93,49 ± 0,89) respectively. The γ-tocopherol was missing in mechanical extraction and that may be due to temperature of extraction or the absence of the solvent.
Our results are in accordance with those of Matthäus and Özcan (2011) who find that β-tocopherol contents of Opuntia seed oil varied between 3.9% (Eskioba) and 50.0% (Adana). But less than Gharby et al (2015) shown that prime level of tocopherols in cactus seed cold press oil 946 mg/kg decided, it’s much above that of the Tunisian (447 mg/kg ) and therefore the Germany cactus seed oil (403 mg/kg). Total tocopherols content of cactus oil is almost that of sunflower-seed oil (490 mg/kg), however, much above that of vegetable oil (220 mg/kg), and less than that of the soya bean (650 mg/kg)