Physicochemical properties of olive oil samples
As shown in Fig. 1a, the percentages of free acidity of oils from different studied regions are between 0.304 and 1.863 for Algerian samples extracted industrially, since they are ranging from 0.248 to 0.688 in the olive oils from foreign countries (Italy, Tunisia, Syria). The free acidity of oil samples of El-Oued traditional, Guelma segoise, and Tlemcen segoise is significantly higher (p < 0.01) as compared to that of the control samples. Besides, the other oil samples showed no significant difference from the control samples. The low values of oil-free acidity indicate weak hydrolysis during oil extraction and storage. Overall, all results comply with the standard values recommended by the International Olive Oil Council (IOOC – International Olive Oil Council, 2008), except those of the traditional oil of El-Oued region (4.79 ± 0.02). Hence, the oil samples can be classified under the category of extra virgin olive oil (69%). Additionally, the amount of acid numbers of the Segoise oil variety is higher than that of the Chemlal variety in both El-Oued and Mila regions, excluding that of Guelma oil (Virgin current 0.7%), Segoise variety from Tlemcen and El-Oued (Virgin 15%). As reported by Dag et al. (2011), high acidity is referred to bad olive oil handling, harvesting, and manufacturing practices. Thus, the very advanced state of the fruit maturity due to the inadequacy of the precautions is occurred during the harvesting or storing of olives. Moreover, the noticed high significant temperature in the region of El Oued (Saharan Climate) promotes the chemical and enzymatic deterioration of the fruits and, consequently the increase in the content of free fatty acids in presence of lipases.
The saponification index
In Fig. 1b, most olive oil samples display saponification indices that meet the National standard values as recommended by the Algerian official journal (The Algerian Official Journal, 2011), reporting a value of 170–200 mg/g, except that found in the Chemlal oils from Mila (211,778 ± 1,403) showing a significant increase (p < 0.05) as compared to control oil samples. The olive oil of Chemlal variety of Mila is less rich in long fatty acid chains than other oils, and hence this parameter becomes inversely proportional to the chain length. On top of that, the olive oil of Guelma has a minimum value of saponification index (163.391 ± 0.701). A study characterizing three varieties of the olive oils of Béjaia city (Northeast Algeria) has reported saponification index values as 185,44, 190.66, and 191,94 mg of KOH/g of oil, which is very far from those of Tbessa and El-Oued (Belarbi-Benmahdi et al., 2009).
The peroxide index
As depicted in Fig. 1c, the peroxide number expressed in milliequivalent of active oxygen per kilogram of oil show values between 2 and 8.25, and comparable to those reported by the IOC standards (IOC, 2021). Also, this parameter is significantly higher in oil samples of El-Oued and Mila (Chemlal) as compared to control samples (p < 0.01). Despite this, all the analyzed oil samples concord with the standard value, by which the oil samples can be classified in the “extra virgin” category (IP ≤ 20). Further, the obtained values of the oil peroxide index are lower compared to those reported by Zegane et al. (2015). (9.22–2.142 meq g of oxygen/ kg of oil). The low values of the peroxide value indicate the quick oil extraction process after harvesting the olives, in addition to the short-term oil storage (samples of the year 2017/2018) in good conditions. Therefore, it is noteworthy to suggest that the oil does not oxidize prematurely and can be kept over time.
The relative density
Figure 1d showing the changes in the relative density of oils in the function of sampling regions reveals values between 0.910 and 0.913. Interestingly, this parameter whether for industrial or traditional oils is comparable to the standard established by the IUPAC (International Union of Pure and Applied Chemistry (IUPAC), In: Paquot C, Hautfenne A, 1987) which varies from 0.910 to 0.916. However, the Guelma oil sample shows a very slightly lower value (ns) than that of the IUPAC.
As shown in Fig. 1e, most of the analyzed samples exhibited comparable values of refractive index; in the CA range (1,4677–1,4705), and consequently, all the oils conform to the standard. Unlike these samples, the refractive index values of the olive oil samples from Tebessa, Souk Ahras, and Guelma are significantly superior (p < 0.05) to that of the standards (as seen in the standards: 14 and 15), and to that of control samples.
Water and volatile materials content
The obtained values of this parameter in the oil samples from El Oued segoise, Tebessa, and Mila (chemlal) showed significantly higher values (p < 0.01) than those seen in the control samples. But generally, all obtained data are consistent with the standards established by CA (Codex Alimentarius) and IOC ((IOC) International Olive Council, 2021) (Fig. 1f).
Determination of olive oil pH
In Fig. 1g, only the Guelma variety shows a significantly higher pH value as compared with the control samples (p < 0.05). Further, the majority of pH values of olive oil samples change slightly compared with standards. However, olive oils of Mascara (3.73) and Mila (Chemlal) (3.72) revealed lower pH values than those of Standards, and so they are characterized by slightly acidic pH compared to the standards.
Specific absorbance in UV (232 and 270nm)
The results of the specific extinction coefficient K232 of the majority of the olive oil samples (nearly 69.23%) whose absorbance values are accordingly comparable to those recommended by IOC ((IOC) International Olive Council, 2021) make it possible to classify the main studied samples in the category of extra virgin oils, except 23.27% of oil samples, including the sample of variety Milla Chemlal which is considered as lampante virgin. On the other hand, the oil samples of El Oued (EO), Souk Ahras (S), and Mascara (Msc) showed a significant increased (EO & S p < 0.001; Msc 0.05) values of the specific extinction coefficient K at 232nm as compared to those of the control samples. The increase in absorbance at 232 nm and 270 nm could be explained by the late harvest and excessive exposure of olives, and the extraction process of oil to oxygen in contact with air and light, or even to a warming of the olives, as well as the dough during crushing (Boulfane et al., 2015). Further, the high values of the Segoise variety (1.035) of Mascara which does not meet IOC standards could be influenced by the used extraction method and other manufacturing practices. Moreover, the specific extinction coefficient K270 showed higher significant values of the oil samples of El Oued and Souk Ahras (p < 0.01) and Mascara (p < 0.05) compared with the controls. It’s noteworthy that the higher value of the extinction at 270nm leads to the richness of oil in secondary oxidation products, and so it has a lower shelf-life. Overall, the El Oued traditional variety of El-oued ranges between a minimum of 0.17 and a maximum of 0.541. Also, 30.76% of the olive oil samples are classified as extra virgin oils, since 23.03% of these samples are considered as lampante virgin, and surprisingly 46.15% of the study samples don't meet the limit authorized by IOC ((IOC) International Olive Council, 2021) (Figs. 1h and 1i).
The extinction coefficient ∆k of olive oil
As indicated in Fig. 1j, the majority of the studied olive oil samples (84.61%) have absorbance values within the limit allowed by IOC (**p < 0.01), however, 23.03% of the oil samples (Guelma 0.0460; El-Oued traditionally extracted oil 0.0725), don't meet the standards fixed by IOC.
Chemical analysis of olive oil pigments
Determination of chlorophylls and carotenoids content
Figure 2a depicts that the level of chlorophyll is significantly higher in olive oils from Souk Ahras (p < 0.001) and El Oued (p < 0.05) when compared with control samples. Also, the minimum value of this parameter was observed in the oil sample of Segoise Mila with an average of 0.48 mg/Kg, since most other studied samples revealed chlorophyll content strictly less than 2 mg/kg. This result is believed to avoid the pro-oxidant action of chlorophyll pigments and, thus to ensure good conservation of the oils (Boulfane et al., 2015). The chlorophyll content contributes to the "fruity" of the olive oil taste, and it tends to decrease with the progression of fruit ripening. Actually, through the extraction process and the method of olives picking, other compounds can be formed in the occurrence of anthocyanins. However, the intensity of this decrease remains strongly dependent on the characteristic metabolism of each variety (Giuliani et al., 2011). As shown in Fig. 2b, the level of carotenoids increased significantly (**p < 0.01) in olive oils from the regions Guelma, El Oued segoise, and the other samples (*p < 0.05) when compared with control samples. The increased level of carotenoids is likely due to the high temperatures and prolonged extraction time, and as a result, the oil exhibits high antioxidant activity. Interestingly, the amount of carotenoids content can vary in function of the variety of fruit, the growing region, the level of ripeness fruit, the extraction method, and the conditions of the oil storage (Serani and Piacenti, 1992; Benamirouche-Harbi et al., 2020).
Total phenolic compounds
The indicated results in Fig. 2c reveal that the level of the phenolic compounds increased significantly in olive oils of El Oued, Souk Ahras, and Tlemcen segoise (** p < 0.01) when compared with two control samples (Italy and Syria), and conversely, the oils of Tunisia sample showed a significant higher (c p < 0.001) value of phenolic contents compared with Italy and Syria. Conclusively, the obtained data prove that the studied olive oils contain an appreciable amount of phenolic compounds. As previously reported (Pirisi et al., 2000), the abundance of phenolic compounds in vegetable waters provides effective antioxidant activity.
The antioxidant activity of olive oils
The anti-radical activity
As indicated in Fig. 3, the olive oil from EL-Oued and Tunisia exhibits significant (p < 0.05) higher percentages of DPPH inhibition activity than that seen in the control samples of Italy and Syria. Hence, the sample of Tunisia shows very high antioxidant inhibition activity 84.96%, since that from the Chemlal variety of the Mila and Guelma cities show percentages of DPPH inhibition activity as 73.45% and 67.70% respectively. A study investigating the antioxidant activity of methanolic extracts using the colorimetric test has shown that olive oil qualitatively contains several secondary metabolites, such as total polyphenols (Falleh et al., 2008). Conclusively, the analysis of the oil antioxidant activity showed that the oil samples from the South (Saharan) Algerian region have a higher activity than the samples from the Northern regions, and similarly the Chemlal variety has a higher antioxidant activity than the Segoise variety.
All data were statistically analyzed by using Principal Component Analysis (PCA) test enabling us to visualize the scores and loadings.
Figure 4 illustrates the projections of the different oil samples on the first two principal components analysis (CPA1 and CPA2). This Figure displays 4 groups of variables; Group 1 (pH, Carotenoids content, free acidity, and Polyphenol content); Group 2 (The saponification index); Group 3 (Refractive index, k232, k270, and relative density), and Group 4 (ΔK, Chlorophyll content and Water content). The first principal component CP1 showing 25.38% of the variability is dominated by the variables: pH, carotenoid, k270, k232, density (positive value), saponification index, and Δk (negative value). Whilst, the second principal component CP2 indicating 22.06% of the total variability essentially reflects the opposition between the percentage of inhibition, refractive index (positive value), free acidity (negative value), and the contents of polyphenols, chlorophyll, and water (negative value). The factorial map of oils from different sampling regions (Fig. 4) reveals an obvious relationship between the different parameters:
1-Carotenoid content, Polyphenol content, and inhibition percentage of DPPH, which is considered a good indicator of the oil antioxidant activity.
2-pH and free acidity correspond to the acidity of studied samples.
3-K232 and K270 provide valuable information about the auto-oxidation of oil due to the shift in the position of the double bonds.
4-Chlorophyll content, water content, and ΔK where their increased values affect the oil quality.
Noteworthy, the relationship between the first and the last parameter is antagonistic; the first improves the quality of the oil and the latter negatively influences the quality.
In light of this study, promising results have been obtained for the interest of olive oil from the Saharan region as a functional food product with antioxidant potential, particularly regarding oxidative stress-related pathologies.