Establishing the Phenolic Composition of Olea europaea L. Leaves from Cultivars Grown in Morocco as a Crucial Step Towards Their Subsequent Exploitation

Abstract

In Morocco, the recovery of olive agro-industrial by-products as potential sources of high-added value substances has been underestimated so far. A comprehensive quantitative characterization of olive leaves' bioactive compounds is crucial for any attempt to change this situation and to implement the valorization concept in emerging countries. Thus, the phenolic fraction of olive leaves of 11 varieties ('Arbequina', 'Hojiblanca', 'Frantoio', 'Koroneiki', 'Lechín', 'Lucque', 'Manzanilla', 'Picholine de Languedoc', 'Picholine Marocaine', 'Picual' and 'Verdal'), cultivated in the Moroccan Meknès region, was investigated. Thirty eight phenolic or related compounds (including 16 secoiridoids, nine flavonoids in their aglycone form, seven flavonoids in glycosylated form, four simple phenols, one phenolic acid and one lignan) were determined in a total of 55 samples by using ultrasonic-assisted extraction and liquid chromatography coupled to electrospray ionization-ion trap mass spectrometry (LC-ESI-IT MS). Very remarkable quantitative differences were observed among the profiles of the studied cultivars. 'Picholine Marocaine' variety exhibited the highest total phenolic content (around 44 g/kg dry weight (DW)), and logically showed the highest concentration in terms of various individual compounds. In addition, chemometrics (principal components analysis (PCA) and stepwise-linear discriminant analysis (s-LDA)) were applied to the quantitative phenolic compound data, allowing good discrimination of the selected samples according to their varietal origin.

Keywords: Moroccan region; chemometrics; liquid chromatography-mass spectrometry; metabolic profiling; olive leaves; phenolic compounds.

Figure 1

Extracted ion chromatograms (EICs) of the main phenolic compounds identified in a ‘Picholine Marocaine’ olive leaves sample. Numbers correspond with those included in Table 1.

Figure 2

Total phenolic content and content in terms of the different chemical classes (content of secoiridoids, flavonoids in aglycon form, flavonoids in glycosylated form, simple phenols, one phenolic acid and one lignan) of the studied olive leaves samples, expressed in mg/kg DW. Different letters above the bars indicate significant differences at p < 0.05, Turkey’s test (comparison among the 11 cultivars investigated in this study). Abbreviations meaning (in alphabetical order): Arb: ‘Arbequina’; Fran: ‘Frantoio’; Hoj: ‘Hojiblanca’; Kor: ‘Koroneiki’; Lech: ‘Lechín’; Luc: ‘Lucque’; Manz: ‘Manzanilla’; P Lang: ‘Picholine de Languedoc’; PM: ‘Picholine Marocaine’; Pic: ‘Picual’; and Verd: ‘Verdal’.

Figure 3

Scatter plot of the PCA scores projected on PC1, PC2 (a) and PC2, PC3 (b). Abbreviations meaning as in Figure 2. (Even though the statistical treatment was carried out considering the independent extracts and injections of each sample, just the mean value was represented here to facilitate the visual inspection of the figure).