Chemistry of Argan Oil

Triglycerides

Like all oils, triacylglycerols (TAGs) are the major constituents of argan oil. Over 99% of argan oil consists of mixtures of TAGs, i.e. glycerol molecules, each esterified with three fatty acids. During oil extraction from the kernal, the hydrophobic TAGs attract other fat- or oil-soluble cellular components. These are the minor components of argan oil such as, triterpenes, sterols, pigments, tocopherols and trace metals. Other components in argan oil are the metabolites from the biosynthesis of TAGs and products of lipolytic activity. These include the monoacylglycerols, diacylglycerols and free fatty acids. 13C NMR methodologies, which are used to characterize oils (Mannina et al., 1999) have been conducted to locate the triglyceridic regiospecificity of the profile of argan oil and the results of this study (Khallouki, 2003) (Table 1), indicated that the method is more convenient and less time consuming. It shows that saturated fatty acids (palmitic or stearic) generally substitute the glycerol extremities (Sn-1 and Sn-3) while oleic acid generally esterifies the glycerol secondary alcohol (Sn-3). Linoleic acid is distributed equally at any of the three positions (manuscript in preparation).

Fatty acids of argan oil

The major fatty acids in argan oil are oleic, linoleic, stearic, and palmitic acids (Charrouf et al., 1999; Khallouki, 2003; Khallouki et al., 2003). The oil has a high content (45%) of oleic acid (C-18:1) with respect to other seed oils, and it is also rich (35%) in polyunsaturated linoleic acid (C-18:2) (Charrouf et al., 1999; Khallouki, 2003; Khallouki et al., 2003). Argan oil has a fatty acid composition similar to that of sesame and peanut oil, marketed in Western Europe.

Minor constituents of argan oil

The minor constituents can be divided into two groups. The first group consists of fatty acid derivatives, such as partial glycerides (mono- and diacylglycerols), triterpenes alcohols and phytosterols. The second group includes classes of compounds not related chemically to fatty acids. These are the hydrocarbons, aliphatic alcohols, tocopherols, pigments, phenolics and trace metals. Most of the minor components found in the unsaponifiable fraction of argan oil are phytosterols, triterpene alcohols, tocopherols and xanthophylls.

Triterpene alcohols

The unsaponifiable matter in argan oil contains a proportion of about 20% of triterpene alcohols (Farines in Charrouf et al., 1999). These are a complex group of plant constituents which consist mainly of five condensed cyclohexane rings with 30 carbon atoms. They can be separated from the sterols by chromatography and the few identified in crude argan oil include lupane, ursane and oleanane derivatives which include b-amyrin,butyrospermol and tirucalol as major triterpenic alcohols and represent 27.3, 18.1 and 27.9 % of the triterpenic fraction, respectively.

Methyl sterols

24-methylene cycloartanol in plants represents the biosynthetic origin of 4-methyl sterols. These sterols are present in small quantities in the triterpenic fractions of the oil. Charrouf et al. (1999) and Khallouki (2003) reported the presence of cycloeucatenol and citrostadienol in argan oil. These methyl sterols do not appear to play any specific biological role and are probably biosynthetic intermediates in the evolution of triterpenic alcohols and sterols.

Sterols

Sterols are tetracyclic compounds with generally 27, 28 or 29 carbon atoms. They constitute a sizeable proportion of the unsaponifiable matter in oil. The total content of sterols in the unsaponifiable fraction of argan oil is about 20%. Farines et al. (1981), Charrouf et al. (1999), Khallouki (2003), Khallouki et al. (2003) report that argan oil contains spinasterol (40%) and its dihydrospinasterol (schottenol, 48%) as major sterols respectively along with d-7-avenasterol and stigmasta-8,22-diene-3-b-ol in lower concentrations. Spinasterol and schottenol are rarely found in vegetable oils. Spinasterol has been described as the charasteristic phytosterol of the sapotaceae family (Gunasekera et al., 1977). Contrary to the composition of fatty acids, the phytosterol composition is very different from that of sesame and peanut oils in which b-sitoterol dominates.

Vitamin E

Vitamin E is a fat-soluble vitamin, which comprises two major homologous series of compounds (tocochromanols), known as tocopherols and tocotrienols. The tocopherols are structurally characterised by a saturated side chain on the chromatin ring, whereas the tocotrienols possess an unsaturated phytyl side chain. Four homologs of each type are known to exist in nature and they have different degrees of antioxidant and vitamin E activities. Vegetable oils, especially the seed oils, are rich sources of tocopherols. The vitamin E content in crude argan oil ranges between 629 to 660 mg/kg and the major tocopherol (500 mg/kg) is the gamma-analogue (75%) (Khallouki, 2003; Khallouki et al., 2003) correcting mistakes in the literature citing atocopherol as the major vitamer (Charrouf et al., 1999).

Carotenoids

Carotenoids are highly unsaturated tetraterpenes, biosynthesised from eight isoprene units. Their morefavoured state is the all-trans. Carotenoids are divided into two main classes: carotenes which are strictly polyene hydrocarbons, and xanthophylls, which contain oxygen. The oxygen in xanthophylls may be in the form of hydroxy (e.g. zeaxanthin and lutein), keto, epoxy or carboxyl groups. Xanthophylls occur in crude argan oil at a level of 42% of the unsaponifiable fraction (Charrouf et al., 1999).

Squalene

Another point to note is that similar to olive oil Owen et al. (2000), argan oil (compared to other seasoning oils) contains, high contents of squalene (up to 3.2 g/kg) (Khallouki, 2003; Khallouki et al., 2003). Hydrocarbons mainly squalene in vegetable oils are present in quantities generally lower than 0.15%, the exceptions are olive and argan oils, which exceed 0.3%.

Phenolic compounds

Definitive quantification for the first time of some other antioxidants such as vanillic, ferulic and syringic acids along with tyrosol in argan oil has also been achieved (Khallouki, 2003; Khallouki et al., 2003). p-Hydroxybenzoic acid and vanillin are also identified in trace amounts and a number of unidentified compounds with UV spectra similar to phenolics were also detected, and warrant further investigations.

Read full article: Secondary metabolites of the argan tree (Morocco) may have disease prevention properties