Introduction

Prickly pear (Opuntia ficus-indica L.) of the Cactaceae family is a plant that mainly grows in arid and semi-arid zones. Endemic in South America, it has been introduced in almost all (sub)tropical areas where it is cultivated or can freely develop. Because it is amazingly drought tolerant and presents very low agronomic requirements, prickly pear presents an undoubted interest in drylands where its cultivation could provide some incomes to poor rural population in the tropical world. Around most of the mediterranean basin as well as in Morocco, prickly pear can be frequently encountered and its fruit is eaten fresh. Although prickly pear fruit and stem are traditionally utilized for medicinal and cosmetic purposes, as forage, building material, and as a source for natural colors (1), its use is still mainly limited to the preparation of an alcoholic beverage and fresh fruit consumption (2-4). Prickly pear fruit nutritional importance mainly results from its content in antioxidants, fibres and free amino acids (1, 5).

Opuntia ficus-indica seeds, which are are frequently discarded as waste, constitute up to 15% of the fruit mass (6). Opuntia ficus-indica seed oil has been reported to be 10-15% of whole seed weight (6). This oil has already been characterized as presenting a high content in unsaturated fatty acids, linoleic acid being the major fatty acid (6-8). It also contains important amount of tocopherols and sterols (9). However, composition of Opuntia ficus-indica seed oil has also recently been reported to depend on the plant geographical origin (10). Therefore, the aim of this work was to study the physicochemical parameters, fatty acid composition, tricylglycerol, tocopherols, phytosterols as well as oxidative stability of Opuntia ficus-indica seed oil cultivated in Morocco and obtained by press-extraction. Because solvent-extracted oils can not be used for human nutrition purposes, we decided to exclusively examine press-extracted oil. Composition of Opuntia ficus-indica seed oil from Morocco prepared by press extraction was compared to that of Opuntia ficus-indica seed oil, obtained by solvent extraction and from other origin, and to other seed oils frequently found in Morocco to determine if Opuntia ficus-indica seed oil could be used as an alternative oil for nutritional or industrial uses.

Experimental

Seeds

Opuntia ficus-indica seeds were purchased from a local market in Sidi ifni region located in the south of Morocco in June 2012. The seeds were rapidly washed to remove impurities, and eventually air-dried.

Oil extraction

Oil extraction was carried out using cold-presses (IBG Monforts Oekotec GmbH, Mönchengladbach, Germany).

Oil analytical determination

Fatty acid composition

Fatty acids were converted to fatty acid methyl esters before analysis by shaking a solution of 60 mg oil and 3 mL of hexane with 0.3 mL of 2 N methanolic potassium hydroxide. Fatty acids were analyzed by gas chromatograph (Varian CP-3800, Varian Inc.) equipped with a FID. The column used was a CP- Wax 52CB column (30 m×0.25 mm i.d.; Varian Inc., Middelburg, The Netherlands). The carrier gas was helium, and the total gas flow rate was 1 ml/min. The initial column temperature was 170 °C, the final temperature 230 °C, and the temperature was increased by steps of 4 °C/min. The injector and detector temperature was 230 °C.

Triacylglycerol composition

The triacylglycerol (TAG) profile was obtained by reverse-phase high-performance liquid chromatography (RP-HPLC) using an Agilent liquid chromatograph equipped with an auto-injector and a refractive index detector. TAG were separated using a hypersil ODS column (125 × 4 mm) with a particle size of 5 m and was eluted from the column with a mixture of acetonitrile–acetone (65/35, v/v) at a flow rate of 0.5 mL/min; 10 L of sample solution prepared in chloroform–acetone (1:1, v/v) was injected into the HPLC system. The total run time was 45 min. TAG peaks were identified based on their equivalent carbon number (ECN). Peak areas produced by the data integrator were used to quantify the components based on relative percentages.

Sterol composition

Sterol composition was determined after trimethylsilylation of the crude sterol fraction using a Varian 3800 instrument equipped with a VF-1 ms column (30 m × 0.25 mm i.d.) and using helium (flow rate 1.6 mL/mn) as carrier gas. Column temperature was isothermal at 270 °C, injector anddetector temperature was 300 °C. Injected quantity was 1L for each analysis.

Tocopherol analysis

For the analysis of the tocopherol content, high performance liquid chromatography (HPLC) was used, using a solution of 250 mg of oil in 25 ml of n-heptane. Tocopherols were analyzed by HPLC using Shimadzu CR8A instruments (Champ sur Marne, France) equipped with a C18-Varian column (25 cm×4 mm; Varian Inc., Middelburg, The Netherlands). Detection was performed using a fluorescence detector (excitation wavelength 290 nm, detection wavelength 330 nm). Eluent used was a 99:1 isooctane/isopropanol (V/V) mixture, flow rate of 1.2 ml/min.

Physical and chemical parameters

Acidity, peroxide value (PV), extinction coefficients (K₂₃₂ and K₂₇₀), iodine and saponified values, refractive index, unsaponifiable matter and density were determined according to AOCS recommended practices Cd 1c-85, Cd 3a-94, Cc7-25, Ca 6a-40 and Ca 5a-40, respectively (11). Acidity was expressed as the amount of oleic acid as %. PV was expressed as milliequivalents of active oxygen per kilogram of oil (Meq O₂/kg oil), and extinction coefficient (K₂₃₂ and K₂₇₀) was expressed as the specific extinctions of a 1% (w/v) solution of oil in cyclohexane in 1 cm cellpath length,  using a CARY 100 Varian UV spectrometer. Total phosphorus content was determined using the NF T60-227 recommendation (12).

Oxidative Stability of Seed Oils

The oxidative stability of each sample was determined as the induction period (IP, h) recorded by a 743 Rancimat (Metrohm, Switzerland) apparatus using 3 g of oil sample. Samples placed into Rancimat standard tubes were subjected to the normal operation of the test by heating at 110°C with an air flow of 20 L/h.

Statistical Analysis.

Values reported are the mean ± SE of two to three replications.

Results and Discussion

Physical and chemical parameters

Oil yield and fatty acid composition of press-extracted cactus seed oil are presented Table 1. The oil yield ranged between 6 to 7%. Although such yield is much lower than  that reported for argan oil, another press-extracted Moroccan oil, or other well-known vegetable oils (Table-1), it is almost similar to extraction yields reported for cactus oil in different studies that used hexane as extraction solvent. Press-extracted cactus seed oil showed a high iodine value (130.5) likely due to its high content of unsaturated fatty acids (UFAs) (Table 1). This value is higher than that of argan oil (102), olive oil (90.2) but similar to that of soybean seed oil (134.5) and sunflower oil (130). The peroxide value of cactus seed oil was determined as 3.5 MeqO₂/Kg and extinction coefficients (K₂₃₂ and K₂₇₀) were found to be 1.72 and 0.31, respectively. These results suggest that cactus seed oil stability to oxidation is relatively low and its UFA high content is again likely responsible for this limited stability. The saponification value of cactus oil, determined as 186.63 mg KOH/g, is slightly lower than that of argan, olive, soybean and sunflower oil (Table-1) for which the average range of saponification value is 189–194 mg of KOH/g of oil. High saponification value indicated that oils have a high triglyceride content and hence are very useful in cosmetology. Density, insaponifiable matter, moisture and volatiles, and refractive index were similar to that of other vegetable oils (Table-1).

Cactus oil contained a significantly higher level of phospholipid/phosphorus than soybean and sunflower oil. Phospholipids can act as antioxidants or prooxidants (13, 14) depending on their concentration and the presence of metal ions (13) or tocopherols (14-16). Therefore, their influence in cactus oil deserve to be studied.

Table 1: Physicochemical parameters of cactus seed oil and other selected vegetable oils.

Fatty acid composition

Fatty acid composition determination is an important characteristic for vegetable oils. Unsaturated fatty acids are the major component of cactus oil. Values are listed in Table-2. Together oleic and linoleic acids constitute 84% of the fatty acids. -Linolenic acid was detected at a low level in cactus seed oil (<0.3%). The major unsaturated fatty acid detected was linoleic acid followed by oleic acid (Table-2 and Figure-1). Cactus seed oil constitutes an important source of polyunsaturated fatty acid (PUFA) (61%), while olive oil presents a high content in monounsaturated fatty acids (MUFA) (over (61-80%). Cactus seed, soybean and sunflower oil present a similar level of linoleic acid. Among the saturated long-chain fatty acids, palmitic acid was predominant (11.9%). Second major saturated fatty acid was stearic acid (3.4%). The dietetic value of the oil is high as the total unsaturated fatty acids/total saturated fatty acids ratio is approximately 5.4, which is similar to that of argan oil. No difference, in terms of fatty acid composition was observed between cactus seed oil from Morocco or other countries (9, 10, 19-21) (Table 3). This shows that the fatty acid composition only weakly depends on the geographical origine and press-extraction does not modify the fatty acid composition.

Table 2: Fatty acid [%] composition of cactus seed oil and other vegetable oils (17,18).

Table 3: Fatty acid [%] composition of cactus seed oils from different geographical origin (9, 10, 19, 20).

Fig. 1:    Chromatogram of fatty acid composition in cactus oil

Sterol and tocopherol composition

Sterols constitute a sizeable proportion of the unsaponifiable matter in vegetable oil. The total content of sterols in the unsaponifiable fraction of cactus oil is about 20%. Investigated sterol profile of seed oil of cactus oil is given in Table-4 and Figure-2. -Sitosterol is significantly the sterol present in high amount, it represents 75.3% of the total sterol fraction. This sterol is also abundantly found in soybean, sunflower and olive oil (Table 4).

Table 4: Percentage of sterols in total sterols in cactus seed oil and other vegetable oils (17-18).

Fig. 2:    Chromatogram of sterol composition in cactus oil.

Tocopherols are natural antioxidants. Tocopherol content of cactus seed oil is presented Table-5. Total tocopherol content of cactus seed oil is 946 mg/kg is much higher than that of olive (220 mg/kg), soybean (650mg/kg) and sunflower (490 mg/kg), but close to argan oil (850 mg/kg). The major tocopherol found in cactus oil is γ-tocopherol. It represents 90.5% of total tocopherols, whereas δ-tocopherol represents only 7.5% and α-tocopherol 2.2%. β-Tocopherol was not detected (Table-5). Cactus, soybean, argan are the richest source of γ-tocopherol a major difference with olive and sunflower oils where α -tocopherol is the major tocopherol. Tocopherol compounds are used in food, cosmetics and pharmaceutical industries (21). α-Tocopherol is recommended for human and animal consumption because it has a higher biological activity than other tocopherols, but γ-tocopherol shows a higher antioxidant capacity as compared to α-tocopherol (22). The main biochemical function of γ-tocopherol is its protection of polyunsaturated fatty acids against peroxidation (23). They also avoid the oxidation of vitamin A, β -carotene and essential fatty acids (24).

Table 5: Tocopherol contents of cactus seed oil and other vegetable oils (17,18).

Triacylglycerol composition

Nine triacylglycerols were identified in cactus seed oil. The three major compounds are LLL, LLO, LLP and OOL with 24.94%, 21.31%, 15, 90 and 13.76%, respectively. Therefore, cactus seed oil presents a triacyglycerol specificity that could be endowed with specific properties. A similar triacylglycerol composition has been reported for solvent extracted cactus seed oil from Tunisia (9).

Table 6:Triacyglycerols [%] content of cactus seed oil and other vegetable oils.

Oxidative stability of cactus seed oil

Oxidation of lipid is a major cause of deterioration in the quality of edible oils. It is the cause of important deteriorative changes in their chemical, sensory and nutritional properties (25). Preservation of cactus seed oil has never been investigate, so far.  To get a complete picture of cactus seed oil oxidative stability, we decided to determine the induction period by Rancimat test, an instrument for automatic determination of the oxidation stability of oils and fats (26). Induction time of cactus seed oil, evaluated by the Rancimat accelerated method, was found to be 7 ± 1 hour at 110°C. At the same temperature, we found the Rancimat induction time of 31, 23, 7.5 and 5 hours for argan, olive, soybean and sunflower oils, respectively. Our results show that the stability of cactus seed oil to oxidation is higher than that of sunflower oil and is similar to that of soybean oil. However, cactus seed oil stability of much lower than that of olive and argan oils. Cactus seed oil could be attributed to the presence of tocopherols and phospholipids, compounds that are not present in refined oils as sunflower or soybean oils.

Table 7: Rancimat induction period (hrs) at 110 °C of cactus seed oil and other vegetable oils.

Conclusions

The present study demonstrated, for the first time that press-extracted cactus seed oil has an interesting fatty acid profile (oleic and linoleic) and is a very good source of tocopherol. Its high amounts of tocopherols and linoleic acid could open important applications for this oil. Cactus seed oil composition of moroccan origin is quite similar to that from Tunisia. These important characteristics can soon make possible its application in pharmacology, cosmetics industry, etc.

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