Open Access
Issue
OCL
Volume 27, 2020
Article Number 44
Number of page(s) 10
Section Quality - Food safety
DOI https://doi.org/10.1051/ocl/2020039
Published online 07 September 2020

© M. El Yamani et al., Hosted by EDP Sciences, 2020

Licence Creative Commons
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

Olive oil happens to be considered as “the magic elixir of the Mediterranean diet” known for its worldwide appreciation due to its healthy and nutritional aspects that can lead to prevent and reduce the risk rates of many chronic, neurodegenerative and cardiovascular diseases (Huang and Sumpio, 2008; Cicerale et al., 2010; Rizzo et al., 2014).

The nutritional value of olive oil is associated to its composition of fatty acids especially the oleic acid and to its richness of natural antioxidants such as phenolic compounds, tocopherols, flavonoids and carotenoids (Covas, 2007; Stefanoudaki et al., 2010). The high content of unsaturated (polyunsaturated) fatty acids in olive oil makes it very susceptible to oxidation. The oxidative reactions that could occur in oil, especially during storage, impact negatively its nutritional quality and flavor, due to the formation of undesirable volatile compounds potentially toxic for human health (Duh et al., 1992; Kiokias and Gordon, 2004; Pan et al., 2007; Ghanbari Shendi et al., 2018, 2020).

Auto-oxidation, photo-oxidation, enzymatic oxidation and ketone oxidation are the various oxidation reactions that can take place in olive oils leading to their deterioration (Taghvaei and Jafari, 2015). However, auto-oxidation is the most frequent phenomena that proceeds via three cyclical stages, namely induction, propagation and termination (Brand-Williams et al., 1995). During the induction period, alkyl radicals are formed and undergo a reaction with oxygen molecules to form hydroperoxides and peroxy radicals during the propagation phase. These unstable compounds generate secondary oxidation products (i.e. alkanes, alcohols, aldehydes and acids). The termination of chain reactions occurs via combination of free radicals to form stable adducts (Velasco and Dobarganes, 2002). In addition, many studies have shown that deterioration reactions of olive oil are more pronounced at longer storage periods (Rodrigues et al., 2016; Di Serio et al., 2018), and are stimulated by oil exposure to light and air (Pristouri et al., 2010; Li et al., 2014; Houshia et al., 2019).

Addition of some synthetic antioxidants into food formulations, especially olive oil, has been used for long time to overcome the lipid oxidation problems and to reduce their damage (Halliwell et al., 1995; Fki et al., 2005; Bouaziz et al., 2008; Venturi et al., 2017). However, recently the use of natural additives has received a great attention because of safety issues of synthetic forms (Lee et al., 2009, Taghvaei and Jafari, 2015).

Carotenoids, whose consumption is linked to the protection against cardiovascular diseases due to its antioxidant functions (Kiokias and Gordon, 2004), are tested as natural additives to improve oil stability in many investigations. Diverse carotenoid extracts (tomato, marigold, paprika and annatto extracts) were added to sunflower oil and showed a significant delay in the formation of secondary oxidation products (Kiokias et al., 2009). The antioxidant potential of carotenoids extracts were confirmed in other studies (Armando et al., 1998; Alavi and Golmakani, 2017).

Similarly, the use of natural phenols from olives and their by-products for the stabilization of olive oils and functional foods has recently been considered. Fki et al. (2005) reported that the addition of phenolic extracts from olive mill wastewater (OMW) had a protective effect against oil oxidation. Sánchez de Medina et al. (2012) have found that enriched edible oils (i.e. maize, soya, high-oleic sunflower, sunflower, olive and rapeseed oils) with phenolic extracts isolated from olive pomace and leaves showed a similar composition to extra virgin olive oil concerning biophenols content. In view of the above mentioned facts, this work investigated the influence of natural antioxidants on the stability of the virgin olive oil (VOO) stored under room temperatures.

2 Materials and methods

2.1 Materials

Olive oils (cv. Moroccan Picholine) used in this study were collected from an industrial mill unit (super-pressure system) located in Taza province (34°19’48" N, 4°12’0" W, ∼ 534 m asl) (northern Morocco) during 2017 extraction period. Samples of OMW were simultaneously drawn from the same industrial unit to obtain phenol extracts. Carrot (Daucus Carota) samples used for the extraction of carotenoids were purchased from a local supermarket in the same area.

The collected olive oil and OMW samples were put in dark glass and plastic bottles, respectively and stored with carrot samples in a deep freezer at 4 °C until analysis.

2.2 Determination of initial VOO quality indices

Determination of free fatty acids (FFA), peroxide value (PV), and absorbance at specific wavelengths, namely 232 nm and 270 nm (K232 and K270), were carried out following the analytical methods described in the European Union Commission Regulations (EEC, 1991, 2003). FFA (expressed as % of oleic acid [OA]), being the conventional criterion for olive oil quality and their classification, measures hydrolysis breakdown of fatty acid chains. PV (meq O2/kg oil) estimates the active oxygen content; it is a crude indicator of the amount of primary oxidation products (i. e. peroxide compounds). K232 and K270 (calculated from absorption at 232 nm and 270 nm) are related to the presence in olive oil of conjugated dienes and trienes systems, respectively.

Chlorophylls and carotenoids were assessed at 670 nm and 470 nm, respectively, in cyclohexane, using specific extinction values, following the method described by Minguez-Mosquera et al. (1991). The extinction coefficients used were E0 = 613 for pheophytin and E0 = 2000 for lutein, as the major components in the chlorophyll and carotenoid fractions, respectively. The concentrations of both pigments were expressed as mg per kg of oil and calculated using the following equations: Where A is the absorbance and L is the spectrophotometer cell thickness (1 cm).

Total phenols were isolated by triple extraction of a solution of olive oil (10 g) in n-hexane (10 mL) with 10 mL of a methanol-water mixture (60:40, v/v) (Zunin et al., 1995), then their concentration was determined spectrophotometrically (SPECUVIS1; UV-Visible) following the method proposed by Folin and Ciocalteu (1927). Folin Ciocalteu reagent was added to a suitable dilution of the extract, and the absorbance was measured at 750 nm using as standard the caffeic acid (Sigma-Aldrich, St. Louis, MO, USA). Values for total phenols content are given as mg caffeic acid/kg oil.

2.3 Extraction of carotenoids and total phenols

The extraction of carotenoids was carried out using the Soxhlet method (Harwood et al., 1989). The setup consists of a distillation flask, extraction chamber, siphon and condenser. First, a weighed amount of dried carrot was placed in a porous cellulose thimble and put in the extraction chamber. Next, the solvent (acetone) was heated to reflux and vapors passed through the thimble containing the material to be extracted and are liquefied in the condenser. When the liquid reaches the overflow level in the thimble, a siphon aspirates the solution, and the liquid carrying the extracted solutes falls back into the flask. The process is repeated for a number of cycles until complete extraction is achieved and the liquid extract is accumulated in the distillation flask. Finally, carotenoid extract was separated in a rotary evaporator under vacuum at 50 °C; the boiling point of acetone.

Total phenols (TP) were extracted using the analytical methodology described by De Marco et al. (2007). OMW samples were first acidified and washed with hexane in order to remove the lipid fraction, and then a TP extraction was carried out with ethyl acetate which was evaporated to recover the dry phenolic residue.

2.4 Enrichment of VOO and experimental design

Weighed quantities of carotenoid (CE) and phenol (PE) extracts (from carrot and OMW, respectively) were dissolved in an appropriate volume of ethanol/water (50/50; v/v) and added at two concentrations (200 ppm and 400 ppm) to VOO. Carotenoid and phenol extracts were mixed with VOO by stirring for 30 min and ethanol traces were evaporated at 37 °C (Farag et al., 2003). The same procedure was applied to L-Ascorbic Acid (AA) as standard. The used AA (purity of 99%) was obtained from Sigma Chemical Co. (Sigma–Aldrich Company Ltd, Great Britain). VOO enriched with CE, PE and AA and the non-enriched VOO (control sample) were stored in clear and dark glass bottles at room temperatures for 120 days during which the oxidation state was evaluated initially at bottling and then each 30 days (0, 30, 60, 90 and 120 days). In fact, a total of 42 glass bottles, divided into 21 for VOO stored in clear glass bottles and 21 for VOO stored in dark ones, were considered. Each category consisted of: (1 control sample + 2 VOO enriched with CE at 200 and 400 ppm + 2 VOO enriched with PE at 200 and 400 ppm + 2 VOO enriched with AA at 200 and 400 ppm) × 3 replicates.

The experimental design considered to perform statistical analyses was is a Completely Randomized Design with 5 storages times, two storage conditions, seven treatments (1 control + 3 additives at 2 concentrations [6]), and three replicates.

2.5 Oxidation stability measurements

Oxidation stability of VOO enriched with carotenoids and phenols extracts was evaluated during 120 days of storage (at 30 days intervals) by measuring the free fatty acids, the peroxide value, and the extinction coefficients at 232 nm and 270 nm (K232 and K270), according to the European Union Commission Regulation (EEC, 1991, 2003).

2.6 Statistical analyses

All determinations were performed in three replicates. Analyses of variance (ANOVA) were carried out over natural additives and storage time and conditions. Least significant difference (LSD) values were calculated at the 5% probability level. Principal component analyses (PCA) and the relationships between parameters were performed on the basis of a correlation matrix calculated on the mean data of all the replicates. The Statgraphics Centurion XVII package (Stat point Technologies, Inc., Virginia, USA) was used for all the calculations.

3 Results and discussion

3.1 Fresh VOO characteristics and amounts of natural extracts

Olive oils from cv “Moroccan Picholine” were analysed for their initial characteristics before being enriched with natural additives. The results for the legal parameters (FFA, PV, K232 and K270) are listed in Table 1, and indicated that the examined oils belong to the VOO category (EEC, 2003). Indeed, the mean value recorded for FFA was of 1% OA, while the PV, K232 and K270 that measured the hydroperoxides, the conjugated dienes and trienes, were found around 5.643 meq O2/kg, 1.620 and 0.156, respectively. The average contents of carotenoids and chlorophylls were of 1.395 mg/kg and 3.091 mg/kg, respectively, which corroborates those previously reported (Lazzez et al., 2008; Mansouri et al., 2016; El Yamani et al., 2019a). The amount measured for total phenols (410.984 mg/kg caffeic) coincide with the findings of other investigations (Méndez and Falqué, 2007; Ben Youssef et al., 2012). Olive oil composition is generally influenced by cultivar, fruit quality, harvesting time, and storage conditions, as well as processing, production site, harvesting practices, water regime and climatic conditions (Salvador et al., 2001; Torres and Maestri, 2006; Mailer et al., 2010; Bedbabis et al., 2015; Mele et al., 2018; Piscopo et al., 2018; El Yamani et al., 2020).

Carotenoid and phenol extracts derived from carrot and OMW, respectively, were quantified (Tab. 1). The carotenoids content obtained from 100 g of fresh material was around 16 mg, within the range of the results reported by Rebecca et al. (2014). OMW was found rich in phenols (1.26 g/L caffeic) in agreement with the observations of several authors (Belaqziz et al., 2016; El Yamani et al., 2017, 2019b; Arabi et al., 2018).

Table 1

Mean values of analytical characteristics for fresh samples of “Moroccan Picholine” olive oil collected from an industrial mill unit (super-pressure system) located in Taza province (northern Morocco) during 2017 extraction period, and extraction yields of carotenoids and phenols from carrots and olive mill wastewater, respectively.

3.2 VOO enrichment with natural additives

3.2.1 Data variability

Results from the analysis of the variance for the VOO enriched with carotenoid and phenol extracts and stored in clean and dark glass bottles during 120 days are presented in Table 2. They revealed the predominance of storage time influence on all considered parameters, and explained more than 80% of total variance. Storage conditions effect accounted for about 17% of total variance for FFA and less that 8% for PV, K232 and K270. The effect of natural additives was of minor extent for oxidation indices but statistically significant and negligible for FFA.

Table 2

Mean squares from the analyses of variance of free fatty acids (FFA), peroxide value (PV) and extinction coefficients at 232 and 270 nm (K232 and K270) in “Moroccan Picholine” virgin olive oil enriched with natural additives (carotenoid and phenol extracts) and stored during 120 days at room temperatures in clear and dark glass bottles.

3.2.2 Influence of storage time

The evolution of mean values recorded for the analysed parameters of VOO enriched with carotenoid and phenol extracts during storage is shown in Table 3. Regardless the effect of natural additives and storage conditions, all measured parameters increased to statistically the highest values (p < 0.05) in enriched VOO after 120 days indicating that they were subjected to substantial reactions of hydrolysis and oxidation.

The FFA accumulation was duplicated after 60 days of storage, and increased by approximately 2.5 times after 120 days, which is consistent with many previous reports (Méndez and Falqué, 2007; Bubola et al., 2014; Gargouri et al., 2015; Di Serio et al., 2018). The lipolytic enzymes present in the oil mainly the lipase, often derived by microorganisms, decomposed triglycerides into glycerol and free fatty acids (Pereira et al., 2002; Abbadi et al., 2014). The degradation of hydroperoxides and oxidation of aldehydes could also increase the formation of free fatty acids (Abramovič et al., 2007).

The highest increment was observed for PV that quasi-triplicated at the end of the experiment. The lowest rise (38%) was shown for both K232 and K270. The great increment rate for PV was observed during the first 60 days of storage, then it slowed down and those of K232 and K270 became important. Our results are in good harmony with several published studies (Vekiari et al., 2007; Rababah et al., 2011; Rodrigues et al., 2016; Di Serio et al., 2018). The oxidation progress could explain the observed behavior. In fact, there was firstly a formation of hydroperoxides which are then decomposed into secondary oxidation products (Baiano et al., 2005; Del Caro et al., 2006; Escudero et al., 2016).

Table 3

Mean values of free fatty acids (FFA), peroxide value (PV) and extinction coefficients at 232 and 270 nm (K232 and K270) in “Moroccan Picholine” virgin olive oil enriched with natural additives with carotenoid (CE) and phenol (PE) extracts, and ascorbic acid (AA) at different concentrations (0, 200 and 400 ppm) and stored during 120 days at room temperatures in clear and dark glass bottles.

3.2.3 Influence of storage conditions

In relation to storage conditions, significant differences were highlighted between the storage of enriched VOO with carotenoid and phenol extracts at room temperatures in clear and dark glass bottles (Tab. 3). In fact, we detected a delay in the hydrolysis of triglycerides and the oxidation process in enriched VOO stored in dark bottles, which was revealed by the lowest levels of FFA, PV, K232 and K270. In contrast, clear bottles resulted in more advanced deterioration of enriched VOO during storage. Similar results for FFA were displayed by several authors who announced that decomposition of triglycerides, which is responsible of acidity increase in oils, was enhanced by light exposure (Vacca et al., 2006; Pristouri et al., 2010). The formation of hydroperoxides, conjugated dienes and trienes (measured by PV, K232 and K270) was also found to be promoted by light (Caponio et al., 2005; Fadda et al., 2012; Afaneh et al., 2013; Rizzo et al., 2014). The storage in darkness could slow down oxidation rate in olive oil (Del Caro et al., 2006; Pristouri et al., 2010). Moreover, Interesse et al. (1971) indicated that the natural pigments in olive oil act synergistically with the phenols as antioxidants in the dark. However, under light, chlorophylls (especially pheophytin A) have a photosensitizing effect and promoted rapid photooxidation (Sanelli, 1981; Rahmani and Csallany, 1998).

3.2.4 Influence of natural additives

The changes in enriched-VOO with natural additives (carotenoid and phenol extracts) and acid ascorbic (standard) along with the control sample are given in Table 3. Results showed no clear differences among additives and their concentrations on FFA. Similarly, Limón et al. (2015) reported that addition of carotenoids rich extracts from Scenedesmus almeriensis didn’t trigger hydrolytic reactions on olive oils. Non-significant differences were observed on FFA when phenolic compounds and carotenoids were added to refined hazelnut oil (Yalcin, 2011). However, the antioxidant effect of natural additives has been proven in our work and found to be very significant even with lower extent compared to the impact of storage time and conditions. In fact, the VOO “control” displayed a higher accumulation of primary and secondary oxidation products than the enriched VOO. Natural additives had an effect in the retardation of oxidation process, which was greater for phenol extract than that of carotenoids. Moreover, PV, K232 and K270 decreased with increasing the additive concentration from 200 to 400 ppm. However, the effectiveness of ascorbic acid was better despite the effect of both natural additives was very promising. Our findings confirmed those of many researchers that focused their investigation on improving the quality of edible oils and their shelf-life with natural additives from different plant sources and their effectiveness as antioxidants by using different methods (Gordon et al., 2001; Briante et al., 2003; Ranalli et al., 2003; Salta et al., 2007; Abd-El Ghany et al., 2010; Yalcin, 2011; Rafiee et al., 2011).

OMW phenol extracts decreased greatly the oxidation rate (determined by peroxide value) in VOO and sunflower oil (Lafka et al., 2011). Similarly, Fki et al. (2005) reported that OMW extract had an effect in stabilizing husk and refined oils. Farag et al. (2003) indicated that phenols extracted from olive fruits and leaves exerted an antioxidant activity on sunflower oil. Bouaziz et al. (2008) evaluated the antioxidant effect of phenolic extracts recovered from olive leaves on refined and husk olive oils by measuring peroxide values, K232, K270 and Rancimat induction time. They disclosed that the oxidative resistance of oils was significantly improved by the used extracts, which were able to increase the induction time and reduce both lipid hydroperoxides and conjugated dienes and trienes formation. Yahyaoui et al. (2017) noticed that phenolic extracts from aromatic plants and olive leaves could be used to protect extra VOO from oxidation, as well as good substitutes for synthetic antioxidants in the industry. Natural antioxidants extracted from OMW were found highly efficient for the oxidative stabilization of lard (De Leonardis et al., 2007).

Carotenoids, which are considered interesting components in several mechanisms that occur in the human organism (Granado et al., 2003), were recovered from different plant sources and tested to improve the stability of foods and oils. Benakmoum et al. (2008) announced that carotenoids from tomato and its by-products enhanced the thermal stability of the refined olive oil, extra virgin oil and sunflower oil. The industrial tomato by-product (skin and seeds) were also used in another investigation to recover carotenoids in order to produce an olive oil naturally enriched with antioxidants (Bendini et al., 2015). Kiokias et al. (2009) inspected the antioxidant activity of natural carotenoid extracts from tomato, marigold and paprika in the emulsions of sunflower oil in water. They found that the formation of volatile aldehydes was significantly retarded by the addition of the examined carotenoids. The peroxidation was declined and the oxidative stability of olive oils was significantly improved by the addition of carotenoids rich extracts from S. almeriensis, while fatty acids composition and tocopherols content were not influenced (Limón et al., 2015).

In addition, our results indicated that the antioxidant effect of natural carotenoids and phenols was concentration-dependent, and generally augmented with increasing extracts concentration, which was in line with previous researches (Farag et al., 2003; Fki et al., 2005; Kiokias et al., 2009; Lafka et al., 2011; Limón et al., 2015).

The mechanism by which each natural additive acted as an antioxidant could explain the difference in efficiency against oil oxidation between carotenoid and phenol extracts. The antioxidant activity of carotenoids is associated to their highly reactive chemical structure, to the oxygen concentration and to the presence of other antioxidants (Krinsky, 1993; Mordi, 1993). Kiokias and Gordon (2004) recapitulated the action of carotenoids in radical scavenging under low oxygen conditions or in combination with other natural antioxidants, and in the deactivation of singlet oxygen formed by ultraviolet light. β-carotene, the most relevant carotenoids, assured protection against the oxidation of free radicals in lipids by reacting with peroxyl radicals, hence hindering propagation step and favoring termination of the oxidation chain reaction (Burton and Ingold, 1984; Britton, 1995). Niki et al. (1995) reported that antioxidant efficiency of β-carotene was greater in the absence of oxygen. The β-carotene has proved to be an effective quencher of singlet oxygen in the inhibition of photooxidation (Fakourelis et al., 1987). Concerning the antioxidant effect of phenol extract, Lafka et al. (2011) referred its strong protection in the oily system to the amphiphilic properties of the phenolic constituents and suggested that the number of hydroxyl groups improves the hydrogen donor capacity, and therefore enhances the inhibition oxidation. The phenolic compounds act as hydrogen donors and suppress the hydrogen atom abstraction from a fatty acid during the first phase of lipid oxidation, which delay oxidation thereby decreasing hydroperoxides formation (Visioli et al., 2002; Farag et al., 2003). The antioxidant potency of OMW extracts was mostly related to their content of hydroxytyrosol and 3,4-dihydroxyphenyl acetic acid, according to several authors (Bouaziz et al., 2005; Fki et al., 2005; Artajo et al., 2006).

3.2.5 Relationships among parameters and factors

Correlations coefficients among the considered parameters of all VOO samples were calculated and given in Table 4. Positive and significant associations were found among all parameters, with the strongest one was highlighted between K232 and K270 (r = 0.988), both were highly related to peroxide value (r = 0.839 and r = 0.873, respectively). Important relationship was also detected between PV and FFA (r = 0.933). Such findings were observed previously by several researchers who have examined the quality of oils and their oxidation status during storage (Caponio et al., 2005; Marmesat et al., 2009). The FFA were also found positively associated in our work with peroxide value. Frega et al. (1999) disclosed that the concentration of FFA had a prooxidant effect on vegetable oils. Besides, the formation of FFA was related to hydroperoxides content and oxidation of aldehydes, according to Abramovič et al. (2007).

In addition to the correlation studies, experimental data were processed by principal component analysis (PCA) to select significant parameters reflecting the oxidation stability of enriched VOO during storage and the factors most affecting these parameters (Fig. 1). Results showed that most of the variation was captured by the first principal component (PC1) that accounted 89% of the total variance, while PC2 explained about 9%. The obtained PCA also highlighted the presence of four main groups along PC1 arranged according to storage times: The first group consisted of fresh VOO and enriched VOO stored for 30 days, was positioned in the left side of PC1 and characterized by low values of all parameters. The second and third groups were formed by enriched VOO stored for 60 and 90 days, respectively, and interacted with high levels of FFA and PV. The fourth one was located on the right direction of PC1 reacting with high values of K232 and K270, and corresponded to samples stored for 120 days. In addition, the observation of Figure 1 showed that each group was divided into two clusters consisting of enriched VOO stored in clear glass bottles which appeared to be more sensitive to oxidation than those stored in the dark glass bottles.

We have also plotted the evolution of the considered parameters against the additives concentrations, in order to elucidate and compare their effects on the VOO stability (Fig. 2). Results showed that the addition of all extracts in VOO reduced the FFA and the formation or primary and secondary oxidation products. Indeed, this effect was concentration-dependent and the application of additives at 400 ppm was more efficient than enriched VOO with 200 ppm and the control. Moreover, the comparison between natural additives illustrated that phenol extracts were more active in delaying the formation of hydroperoxides and conjugated dienes than carotenoids extracts. However, the ascorbic acid exhibited the most significant antioxidant effect on VOO stability.

Table 4

Correlations between free fatty acids (FFA), peroxide value (PV) and extinction coefficients at 232 and 270 nm (K232 and K270) in “Moroccan Picholine” virgin olive oil enriched with natural additives (carotenoid and phenol extracts) and stored during 120 days at room temperatures in clear and dark glass bottles.

thumbnail Fig. 1

PCA projections on axes 1 and 2 accounting for 98% of total variance. Eigenvalues of the correlation matrix are symbolized as vectors representing traits that most influence each axis. The 70 points representing means of olive oil samples for each storage time (0, 30, 60, 90 and 120 days) and storage condition (dark and clear glass bottles) are plotted on the plane determined by axes 1 and 2.

thumbnail Fig. 2

Evolution of free fatty acids (A), peroxyde value (B), K232 (C) and K230 (D) of “Moroccan Picholine” virgin olive oil enriched with carotenoid (CE) and phenol (PE) extracts, and ascorbic acid (AA) at different concentrations (0, 200 and 400 ppm) during storage.

4 Conclusions

Olive oil is notable Mediterranean diet for its nutritional and antioxidant effect. However, its high content of unsaturated (polyunsaturated) fatty acids makes it very susceptible to oxidation. To this end, application of synthetic antioxidants has cost implications to processors and producers and therefore, use of natural antioxidants such as carotenoids and phenols extend the stability VOO in storage. Therefore, carotenoids and phenols extracted from carrot and olive mill wastewater, respectively, were added at different amounts to VOO and stored in clear and amber glass bottles at room temperatures during 120 days. The efficiency of the natural antioxidants was assessed by measuring the changes reported in the legal quality indices (free fatty acids and oxidations indices) of the VOO.

According to the obtained results, oxidative stability parameters of enriched VOO increased during storage time. In addition, the used natural additives showed an appropriate oxidation prevention activity on VOO, which was concentration-dependent and could be optimized by storage in darkness. This fact could sustain the use of natural antioxidants in fatty foods as safe substitutes of synthetic ones. This topic is, therefore, of great interest and requires in-depth investigation.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgement

The authors wish to express deep gratitude to the owner and workers of the industrial olive mill where olive oil samples were collected. This work was supported by internal funding from Sidi Mohamed Ben Adellah University (Morocco).

References

  • Abbadi J, Ayyad Z, Al-Rimawi F, Sultan W. 2014. Evaluation of the effect of packaging materials and storage temperatures on quality degradation of extra virgin olive oil from olives grown in Palestine. Am J Food Sci Technol 2(5): 162–174. [CrossRef] [Google Scholar]
  • Abd-El Ghany ME, Ammar MS, Hegazy AE. 2010. Use of olive waste cake extract as a natural antioxidant for improving the stability of heated sunflower oil. World Appl Sci J 11(1): 106–113. [Google Scholar]
  • Abramovič H, Butinar B, Nikolič V. 2007. Changes occurring in phenolic content, tocopherol composition and oxidative stability of Camelina sativa oil during storage. Food Chem 104(3): 903–909. [Google Scholar]
  • Afaneh IA, Abbadi J, Ayyad Z, Sultan W, Kanan K. 2013. Evaluation of selected quality degradation indices for palestinian extra virgin olive oil bottled in different packaging materials upon storage under different lighting conditions. J Food Sci Eng 3(5): 267–283. [Google Scholar]
  • Alavi N, Golmakani MT. 2017. Improving oxidative stability of olive oil: Incorporation of Spirulina and evaluation of its synergism with citric acid. Grasas Aceites 68(1): 178. [CrossRef] [Google Scholar]
  • Arabi M, Elias A, Kamel Z, Mansouri B, Toumert I. 2018. Characterization of olive mill wastewater and gamma irradiation effects on some parameters of its composition. J Radioanal Nucl Chem 317(2): 1095–1106. [Google Scholar]
  • Armando C, Maythe S, Beatriz NP. 1998. Antioxidant activity of grapefruit seed extract on vegetable oils. J Sci Food Agric 77(4): 463–467. [Google Scholar]
  • Artajo LS, Romero MP, Morelló JR, Motilva MJ. 2006. Enrichment of refined olive oil with phenolic compounds: evaluation of their antioxidant activity and their effect on the bitter index. J Agric Food Chem 54(16): 6079–6088. [CrossRef] [PubMed] [Google Scholar]
  • Baiano A, Gomes T, Caponio F. 2005. A comparison between olive oil and extra virgin olive oil used as covering liquids in canned dried tomatoes: hydrolytic and oxidative degradation during storage. Int J Food Sci Technol 40(8): 829–834. [Google Scholar]
  • Bedbabis S, Rouina BEB, Salvatore C, Clodoveo ML. 2015. Effect of the seasonal climatic variations on the composition and quality of Chemlali virgin olive oil. Afr J Agric Res 10(49): 4424–4431. [CrossRef] [Google Scholar]
  • Belaqziz M, El-Abbassi A, Lakhal EK, Agrafioti E, Galanakis CM. 2016. Agronomic application of olive mill wastewater: effects on maize production and soil properties. J Environ Manage 171: 158–165. [Google Scholar]
  • Ben Youssef N, Ouni Y, Dabbech N, Baccouri B, Abdelly C, Zarrouk M. 2012. Effect of olive storage period at two different temperatures on oil quality of two tunisian cultivars of Olea europea, Chemlali and Chétoui. Afr J Biotechnol 11(4): 888–895. [CrossRef] [Google Scholar]
  • Benakmoum A, Abbeddou S, Ammouche A, Kefalas P, Gerasopoulos D. 2008. Valorisation of low quality edible oil with tomato peel waste. Food Chem 110(3): 684–690. [Google Scholar]
  • Bendini A, Di Lecce G, Valli E, Barbieri S, Tesini F, Gallina Toschi T. 2015. Olive oil enriched in lycopene from tomato by-product through a co-milling process. Int J Food Sci Nutr 66(4): 371–377. [CrossRef] [PubMed] [Google Scholar]
  • Bouaziz M, Grayer RJ, Simmonds MS, Damak M, Sayadi S. 2005. Identification and antioxidant potential of flavonoids and low molecular weight phenols in olive cultivar Chemlali growing in Tunisia. J Agric Food Chem 53(2): 236–241. [CrossRef] [PubMed] [Google Scholar]
  • Bouaziz M, Fki I, Jemai H, Ayadi M, Sayadi S. 2008. Effect of storage on refined and husk olive oils composition: stabilization by addition of natural antioxidants from Chemlali olive leaves. Food Chem 108(1): 253–262. [Google Scholar]
  • Brand-Williams W, Cuvelier ME, Berset CLWT. 1995. Use of a free radical method to evaluate antioxidant activity. LWT – Food Sci Technol 28(1): 25–30. [CrossRef] [Google Scholar]
  • Briante R, Febbraio F, Nucci R. 2003. Antioxidant properties of low molecular weight phenols present in the Mediterranean diet. J Agric Food Chem 51(24): 6975–6981. [CrossRef] [PubMed] [Google Scholar]
  • Britton G. 1995. Structure and properties of carotenoids in relation to function. Faseb J 9(15): 1551–1558. [CrossRef] [PubMed] [Google Scholar]
  • Bubola KB, Koprivnjak O, Sladonja B, Belobrajić I. 2014. Influence of storage temperature on quality parameters, phenols and volatile compounds of Croatian virgin olive oils. Grasas Aceites 65(3): e034–1. [CrossRef] [Google Scholar]
  • Burton GW, Ingold KU. 1984. Beta-carotene: an unusual type of lipid antioxidant. Science 224: 569–573. [Google Scholar]
  • Caponio F, Bilancia MT, Pasqualone A, Sikorska E, Gomes T. 2005. Influence of the exposure to light on extra virgin olive oil quality during storage. Eur Food Res Technol 221(1-2): 92–98. [Google Scholar]
  • Cicerale S, Lucas L, Keast R. 2010. Biological activities of phenolic compounds present in virgin olive oil. Int J Mol Sci 11(2): 458–479. [Google Scholar]
  • Covas MI. 2007. Olive oil and the cardiovascular system. Pharmacol Res 55(3): 175–186. [CrossRef] [PubMed] [Google Scholar]
  • De Leonardis A, Macciola V, Lembo G, Aretini A, Nag A. 2007. Studies on oxidative stabilisation of lard by natural antioxidants recovered from olive-oil mill wastewater. Food Chem 100(3): 998–1004. [Google Scholar]
  • De Marco E, Savarese M, Paduano A, Sacchi R. 2007. Characterization and fractionation of phenolic compounds extracted from olive oil mill wastewaters. Food Chem 104(2): 858–867. [Google Scholar]
  • Del Caro A, Vacca V, Poiana M, Fenu P, Piga A. 2006. Influence of technology, storage and exposure on components of extra virgin olive oil (Bosana cv) from whole and de-stoned fruits. Food Chem 98(2): 311–316. [Google Scholar]
  • Di Serio MG, Giansante L, Di Loreto G, Di Giacinto L. 2018. Shelf life of extra virgin olive oils: first efforts toward a prediction model. J Food Process Pres 42(7): e13663. [CrossRef] [Google Scholar]
  • Duh PD, Yeh DB, Yen GC. 1992. Extraction and identification of an antioxidative component from peanut hulls. J Am Oil Chem Soc 69(8): 814–818. [Google Scholar]
  • EEC. 1991. Characteristics of olive and olive pomace oils and their analytical methods. Regulation EEC/2568/1991. Off J Eur Communities 248: 1–82. [Google Scholar]
  • EEC. 2003. Characteristics of olive and olive pomace oils and their analytical methods. EEC Regulation 1989/2003. Off J Eur Communities L 295: 57–77. [Google Scholar]
  • El Yamani M, Ghabbour N, Sakar EH, Rharrabti Y. 2017. Olive oil wastewaters from northern Morocco: physicochemical characterization and antibacterial activity of polyphenols. J Mater Environ Sci 8: 2667–2678. [Google Scholar]
  • El Yamani M, Sakar EH, Mansouri F, Serghini-Caid H, Elamrani A, Rharrabti Y. 2019a. Effect of pigments and total phenols on oxidative stability of monovarietal virgin olive oil produced in Morocco. Riv Ital Sostanze Gr 96: 17–24. [Google Scholar]
  • El Yamani M, Sakar EH, Boussakouran A, Ghabbour N, Rharrabti Y. 2019b. Physicochemical and microbiological characterization of olive mill wastewater (OMW) from different regions of northern Morocco. Environ Technol (in Press). https://doi.org/10.1080/09593330.2019.1597926. [Google Scholar]
  • El Yamani M, Sakar EH, Boussakouran A, Rharrabti Y. 2020. Influence of ripening index and water regime on the yield and quality of “Moroccan Picholine” virgin olive oil. OCL 27: 19. [CrossRef] [EDP Sciences] [Google Scholar]
  • Escudero A, Ramos N, Rubia L, Dolores M, Pacheco R. 2016. Influence of extreme storage conditions on extra virgin olive oil parameters: traceability study. J Anal Methods Chem 2016: 1–10. [Google Scholar]
  • Fadda C, Del Caro A, Sanguinetti AM, et al. 2012. Changes during storage of quality parameters and in vitro antioxidant activity of extra virgin monovarietal oils obtained with two extraction technologies. Food Chem 134(3): 1542–1548. [Google Scholar]
  • Fakourelis N, Lee EC, Min DB. 1987. Effects of chlorophyll and β-carotene on the oxidation stability of olive oil. J Food Sci 52(1): 234–235. [Google Scholar]
  • Farag RS, El-Baroty GS, Basuny AM. 2003. The influence of phenolic extracts obtained from the olive plant (cvs. Picual and Kronakii), on the stability of sunflower oil. Int J Food Sci Technol 38(1): 81–87. [Google Scholar]
  • Fki I, Allouche N, Sayadi S. 2005. The use of polyphenolic extract, purified hydroxytyrosol and 3, 4-dihydroxyphenyl acetic acid from olive mill wastewater for the stabilization of refined oils: a potential alternative to synthetic antioxidants. Food Chem 93(2): 197–204. [Google Scholar]
  • Folin O, Ciocalteu V. 1927. On tyrosine and tryptophane determinations in proteins. J Biol Chem 73(2): 627–650. [Google Scholar]
  • Frega N, Mozzon M, Lercker G. 1999. Effects of free fatty acids on oxidative stability of vegetable oil. J Am Oil Chem Soc 76(3): 325–329. [Google Scholar]
  • Gargouri B, Zribi A, Bouaziz M. 2015. Effect of containers on the quality of Chemlali olive oil during storage. J Food Sci Technol 52(4): 1948–1959. [CrossRef] [PubMed] [Google Scholar]
  • Ghanbari Shendi E, Sivri Ozay D, Ozkaya MT, Ustunel NF. 2018. Changes occurring in chemical composition and oxidative stability of virgin olive oil during storage. OCL 25(6): A602. [CrossRef] [EDP Sciences] [Google Scholar]
  • Ghanbari Shendi E, Sivri Ozay D, Ozkaya MT, Ustunel NF. 2020. Determination of chemical parameters and storage stability of extra virgin olive oil extracted by Mobile Olive Oil Processing Unit. OCL 27: 6. [CrossRef] [EDP Sciences] [Google Scholar]
  • Gordon MH, Paiva-Martins F, Almeida M. 2001. Antioxidant activity of hydroxytyrosol acetate compared with that of other olive oil polyphenols. J Agric Food Chem 49(5): 2480–2485. [CrossRef] [PubMed] [Google Scholar]
  • Granado F, Olmedilla B, Blanco I. 2003. Nutritional and clinical relevance of lutein in human health. Brit J Nutr 90(3): 487–502. [CrossRef] [Google Scholar]
  • Halliwell B, Murcia MA, Chirico S, Aruoma OI. 1995. Free radicals and antioxidants in food and in vivo: what they do and how they work. Crit Rev Food Sci Nutr 35(1-2): 7–20. [CrossRef] [PubMed] [Google Scholar]
  • Harwood LM, Moody CJ, Percy JM. 1989. Experimental Organic Chemistry, 2nd ed. Oxford, UK: Blackwell Science Ltd, 778 p. [Google Scholar]
  • Houshia OJ, Abu Eid M, Zaid O, et al. 2019. Alteration of Nabali Baladi Extra Virgin Olive Oil (EVOO) chemical parameters as a function of air and sunlight exposure. OCL 26: 38. [CrossRef] [EDP Sciences] [Google Scholar]
  • Huang CL, Sumpio BE. 2008. Olive oil, the mediterranean diet, and cardiovascular health. J Am Coll Surgeons 207(3): 407–416. [CrossRef] [Google Scholar]
  • Interesse FS, Ruggiero P, Vitagliano M. 1971. Autoxidation of olive oil: influence of chlorophyll pigments. Ind Agr 9: 318–323. [Google Scholar]
  • Kiokias S, Gordon MH. 2004. Antioxidant properties of carotenoids in vitro and in vivo. Food Rev Int 20(2): 99–121. [CrossRef] [Google Scholar]
  • Kiokias S, Dimakou C, Oreopoulou V. 2009. Activity of natural carotenoid preparations against the autoxidative deterioration of sunflower oil-in-water emulsions. Food Chem 114(4): 1278–1284. [Google Scholar]
  • Krinsky NI. 1993. Actions of carotenoids in biological systems. Ann Rev Nutr 13(1): 561–587. [CrossRef] [PubMed] [Google Scholar]
  • Lafka TI, Lazou AE, Sinanoglou VJ, Lazos ES. 2011. Phenolic and antioxidant potential of olive oil mill wastes. Food Chem 125(1): 92–98. [Google Scholar]
  • Lazzez A, Perri E, Caravita MA, Khlif M, Cossentini M. 2008. Influence of olive maturity stage and geographical origin on some minor components in virgin olive oil of the Chemlali variety. J Agric Food Chem 56(3): 982–988. [CrossRef] [PubMed] [Google Scholar]
  • Lee OH, Lee BY, Lee J, et al. 2009. Assessment of phenolics-enriched extract and fractions of olive leaves and their antioxidant activities. Biores Technol 100(23): 6107–6113. [CrossRef] [Google Scholar]
  • Li X, Zhu H, Shoemaker CF, Wang SC. 2014. The effect of different cold storage conditions on the compositions of extra virgin olive oil. J Am Oil Chem Soc 91(9): 1559–1570. [Google Scholar]
  • Limón P, Malheiro R, Casa S, et al. 2015. Improvement of stability and carotenoids fraction of virgin olive oils by addition of microalgae Scenedesmus almeriensis extracts. Food Chem 175: 203–211. [Google Scholar]
  • Mailer RJ, Ayton J, Graham K. 2010 The influence of growing region, cultivar and harvest time on diversity of Australian olive oil. J Am Oil Chem Soc 87: 877–884. [Google Scholar]
  • Mansouri F, Benmoumen A, Richard G, et al. 2016. Characterization of monovarietal virgin olive oils from introduced cultivars in eastern Morocco. Riv Ital Sostanze Gr 93(1): 9321–30. [Google Scholar]
  • Marmesat S, Morales A, Velasco J, Ruiz-Méndez MV, Dobarganes MC. 2009. Relationship between changes in peroxide value and conjugated dienes during oxidation of sunflower oils with different degree of unsaturation. Grasas Aceites 60(2): 155–160. [CrossRef] [Google Scholar]
  • Mele MA, Islam MZ, Kang HM, Giuffrè AM. 2018. Pre- and post-harvest factors and their impact on oil composition and quality of olive fruit. Emir J Food Agric 30(7): 592–603. [Google Scholar]
  • Méndez AI, Falqué E. 2007. Effect of storage time and container type on the quality of extra virgin olive oil. Food Control 18(5): 521–529. [Google Scholar]
  • Minguez-Mosquera MI, Rejano-Navarro L, Gandul-Rojas B, SanchezGomez AH, Garrido-Fernandez J. 1991. Color-pigment correlation in virgin olive oil. J Am Oil Chem Soc 68(5): 332–336. [Google Scholar]
  • Mordi RC. 1993. Carotenoids: functions and degradation. Chem Ind 110: 79–83. [Google Scholar]
  • Niki E, Noguchi N, Tsuchihashi H, Goton N. 1995. Interactions among vitamin E, vitamin C, and beta-carotene. Am J Clin Nutr 62: 1322s–1326s. [CrossRef] [PubMed] [Google Scholar]
  • Pan Y, Zhu J, Wang H, et al. 2007. Antioxidant activity of ethanolic extract of Cortex fraxini and use in peanut oil. Food Chem 103(3): 913–918. [Google Scholar]
  • Pereira JA, Casal S, Bento A, Oliveira MBPP. 2002. Influence of olive storage period on oil quality of three portuguese cultivars of Olea europea, cobrançosa, madural, and verdeal transmontana. J Agric Food Chem 50(22): 6335–6340. [CrossRef] [PubMed] [Google Scholar]
  • Piscopo A, Zappia A, De Bruno A, Poiana M. 2018. Effect of the harvesting time on the quality of olive oils produced in Calabria. Eur J Lipid Sci Technol 120(7): 1700304. [Google Scholar]
  • Pristouri G, Badeka A, Kontominas MG. 2010. Effect of packaging material headspace, oxygen and light transmission, temperature and storage time on quality characteristics of extra virgin olive oil. Food Control 21(4): 412–418. [Google Scholar]
  • Rababah T, Hao F, Yang W, Eriefej K, Al-Omoush M. 2011. Effects of type of packaging material on physicochemical and sensory properties of olive oil. Int J Agric Biol Eng 4(4): 66–72. [Google Scholar]
  • Rafiee Z, Jafari SM, Alami M, Khomeiri M. 2011. Antioxidant properties of olive leaf extract and its application in sunflower oil. J Food Ind Res 21(1): 12–22. [Google Scholar]
  • Rahmani M, Csallany AS. 1998. Role of minor constituents in the photooxidation of virgin olive oil. J Am Oil Chem Soc 75(7): 837–843. [Google Scholar]
  • Ranalli A, Lucera L, Contento S. 2003. Antioxidizing potency of phenol compounds in olive oil mill wastewater. J Agric Food Chem 51(26): 7636–7641. [CrossRef] [PubMed] [Google Scholar]
  • Rebecca LJ, Sharmila S, Das MP, Seshiah C. 2014. Extraction and purification of carotenoids from vegetables. J Chem Pharm Res 6(4): 594–598. [Google Scholar]
  • Rizzo V, Torri L, Licciardello F, Piergiovanni L, Muratore G. 2014. Quality changes of extra virgin olive oil packaged in coloured polyethylene terephthalate bottles stored under different lighting conditions. Packag Technol Sci 27(6): 437–448. [CrossRef] [Google Scholar]
  • Rodrigues N, Dias LG, Veloso AC, Pereira JA, Peres AM. 2016. Monitoring olive oils quality and oxidative resistance during storage using an electronic tongue. LWT – Food Sci Technol 73: 683–692. [CrossRef] [Google Scholar]
  • Salta FN, Mylona A, Chiou A, Boskou G, Andrikopoulos NK. 2007. Oxidative stability of edible vegetable oils enriched in polyphenols with olive leaf extract. Food Sci Technol Int 13(6): 413–421. [Google Scholar]
  • Salvador MD, Aranda F, Fregapane G. 2001. Influence of fruit ripening on “Cornicabra” virgin olive oil quality a study of four successive crop seasons. Food Chem 73(1): 45–53. [Google Scholar]
  • Sánchez de Medina V, Priego-Capote F, Luque de Castro MD. 2012. Characterization of refined edible oils enriched with phenolic extracts from olive leaves and pomace. J Agric Food Chem 60(23): 5866–5873. [CrossRef] [PubMed] [Google Scholar]
  • Sanelli B. 1981. Conservabilita degli oli vegetali in rapporto al grado di indaturazione e del contenuto in clorofilla. II. Irrancidimento. Riv Ital Sostanze Gr 58: 125–131. [Google Scholar]
  • Stefanoudaki E, Williams M, Harwood J. 2010. Changes in virgin olive oil characteristics during different storage conditions. Eur J Lipid Sci Technol 112(8): 906–914. [Google Scholar]
  • Taghvaei M, Jafari SM. 2015. Application and stability of natural antioxidants in edible oils in order to substitute synthetic additives. J Food Sci Technol 52(3): 1272–1282. [CrossRef] [PubMed] [Google Scholar]
  • Torres MM, Maestri DM. 2006. Chemical composition of Arbequina virgin olive oil in relation to extraction and storage conditions. J Sci Food Agric 86(14): 2311–2317. [Google Scholar]
  • Vacca V, Caro AD, Poiana M, Piga A. 2006. Effect of storage period and exposure conditions on the quality of Bosana extra virgin olive oil. J Food Qual 29(2): 139–150. [Google Scholar]
  • Vekiari SA, Papadopoulou P, Kiritsakis A. 2007. Effects of processing methods and commercial storage conditions on the extra virgin olive oil quality indexes. Grasas Aceites 58(3): 237–242. [CrossRef] [EDP Sciences] [Google Scholar]
  • Velasco J, Dobarganes C. 2002. Oxidative stability of virgin olive oil. Eur J Lipid Sci Technol 104(9-10): 661–676. [Google Scholar]
  • Venturi F, Sanmartin C, Taglieri I, et al. 2017. Development of phenol-enriched olive oil with phenolic compounds extracted from wastewater produced by physical refining. Nutrients 9(8): 916. [Google Scholar]
  • Visioli F, Poli A, Gall C. 2002. Antioxidant and other biological activities of phenols from olives and olive oil. Med Res Rev 22(1): 65–75. [Google Scholar]
  • Yahyaoui A, Rigane G, Salem RB. 2017. Oxidative stability of extra virgin olive oil with natural plant extracts during microwave heating. Asian J Chem 29(12): 2655–2659. [CrossRef] [Google Scholar]
  • Yalcin H. 2011. Antioxidative effects of some phenolic compounds and carotenoids on refined hazelnut oil. J Consum Prot Food Saf 6(3): 353–358. [CrossRef] [Google Scholar]
  • Zunin P, Evangelisti F, Pagano MA, Tiscornia E, Petacchi R. 1995. Phenolic compounds in oil obtained from Olea europaea and anti-Dacus treatments. Riv Ital Sostanze Gr 72: 55–59. [Google Scholar]

Cite this article as: El Yamani M, Sakar EH, Boussakouran A, Rharrabti Y. 2020. Activity of two natural additives in improving the stability of virgin olive oil quality during storage. OCL 27: 44.

All Tables

Table 1

Mean values of analytical characteristics for fresh samples of “Moroccan Picholine” olive oil collected from an industrial mill unit (super-pressure system) located in Taza province (northern Morocco) during 2017 extraction period, and extraction yields of carotenoids and phenols from carrots and olive mill wastewater, respectively.

Table 2

Mean squares from the analyses of variance of free fatty acids (FFA), peroxide value (PV) and extinction coefficients at 232 and 270 nm (K232 and K270) in “Moroccan Picholine” virgin olive oil enriched with natural additives (carotenoid and phenol extracts) and stored during 120 days at room temperatures in clear and dark glass bottles.

Table 3

Mean values of free fatty acids (FFA), peroxide value (PV) and extinction coefficients at 232 and 270 nm (K232 and K270) in “Moroccan Picholine” virgin olive oil enriched with natural additives with carotenoid (CE) and phenol (PE) extracts, and ascorbic acid (AA) at different concentrations (0, 200 and 400 ppm) and stored during 120 days at room temperatures in clear and dark glass bottles.

Table 4

Correlations between free fatty acids (FFA), peroxide value (PV) and extinction coefficients at 232 and 270 nm (K232 and K270) in “Moroccan Picholine” virgin olive oil enriched with natural additives (carotenoid and phenol extracts) and stored during 120 days at room temperatures in clear and dark glass bottles.

All Figures

thumbnail Fig. 1

PCA projections on axes 1 and 2 accounting for 98% of total variance. Eigenvalues of the correlation matrix are symbolized as vectors representing traits that most influence each axis. The 70 points representing means of olive oil samples for each storage time (0, 30, 60, 90 and 120 days) and storage condition (dark and clear glass bottles) are plotted on the plane determined by axes 1 and 2.

In the text
thumbnail Fig. 2

Evolution of free fatty acids (A), peroxyde value (B), K232 (C) and K230 (D) of “Moroccan Picholine” virgin olive oil enriched with carotenoid (CE) and phenol (PE) extracts, and ascorbic acid (AA) at different concentrations (0, 200 and 400 ppm) during storage.

In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.