1. Introduction
Olea europaea L., commonly known as the olive tree, is a small tree species typically distributed by the Mediterranean countries [
1]. Olive oil, its main derived product, has experienced an increase in its popularity due to its organoleptic characteristics and its associated beneficial health effects [
2]. The olive tree is one of the species that was first cultivated. In fact, there are records for the first cultivated trees which date from 6000 years in Asia Minor regions according to the International Olive Council (IOC) [
3]. Its origin is linked to Mediterranean civilizations and regions, characterized by soft and rainy winters and hot dry summers [
4]. Although its industry uses large areas for the cultivation of trees, nowadays, its cultivation has been spread to regions of all continents (excluding Antarctica). Still, Mediterranean countries remain the main olive producers, led by Spain, Italy, and Greece. In consequence, the European Union (EU) is responsible for the 70% of the global olive production, generating a production value of 7000 million euros each year, becoming a key factor for the agro-industrial sector development and thus, a social and economic engine for the EU southern regions [
5].
The French writer Georges Duhamel once wrote “There, where the olive tree gives up, is where the Mediterranean ends. The tree of light is the nature and culture of the Mediterranean” [
6]. This statement corroborates the joining and simultaneous development of a culture-bound to olive products and particularly olive oil. So much, that olive oil is a crucial part of the commonly known as Mediterranean diet (MED). This diet consists of a balanced combination of low animal protein consumption with a high intake of fruits, vegetables and cereals and olive oil as the principal source of fat in many foods [
3]. MED has been usually associated with a low incidence of cardiovascular diseases (CVD) risk in EU southern regions, which likewise, show higher life expectancy when compared to EU northern countries and the United States of America (USA) [
7,
8]. Several studies have been conducted to evaluate and prove MED as the main protective agent in the primary prevention of chronic diseases. Research has pointed out that the beneficial health effects of MED are attributed to a large extent to olive oil.
Some of those beneficial properties are gathered up in
Figure 1. Its unique composition and biological properties are largely responsible for this association. Olive oil composition is mainly formed by triglycerides and a variety of several compounds in small quantities. Among the glyceride fraction, olive oil shows a high content of fatty acids and particularly, an elevated proportion of monounsaturated fatty acids (MUFA). Unsaturated acids are up to 85% of its composition, due to its high content in oleic acid (C18:1), which might range between 70–85% and other fatty acids as linoleic or palmitoleic acid. In the case of saturated fats, they entail around de 14% of oil composition, basically due to palmitic and stearic acids. Regarding minor compounds, they signify less than 2% of olive oil composition, and the best representatives of this group are phenolic compounds, although this minor group also includes some lipophilic compounds like α-tocopherol (vitamin E). Likewise, there are several hydrophilic phenolic compounds, among which the simple phenol hydroxytyrosol and the secoiridoid oleuropein must be highlighted [
1,
8]. Besides, olive oil is also a source of pigments like carotenoids [
3]. Phenolic compounds are known for their biological properties. In particular, these olive oil compounds have shown potential as antioxidant, anti-inflammatory and antimicrobial agents [
9]. However, their concentration is related to different factors: olive varieties, environmental factors, time of harvest and extraction, and storage conditions [
1]. To sum up, olive oil is considered as a superfood due to its health properties derived from its unique composition; this is, its lipid profile and its bioactive compounds content [
10]. Moreover, its singular composition has attracted so much attention that olive oil has been studied as a source of these bioactive compounds but also their derived residues have shown high concentrations of these molecules and thus a great potential for industrial recovery and related applications, such as formulation of new high added value products [
10].
Figure 2 shows a detailed procedure of the extraction of olive oil from
Olea europaea L. of different varieties, whose extracting process follows a common line formed by different steps: collection, grading, washing, crushing, malaxing, separation and centrifugation, storage and packaging. This process is not aimed at improving oil quality; however, attention must be paid to its development to not cause a loss of quality [
13,
14]. Depending on the modifications of this process and the resultant organoleptic characteristics of the product, among others, the EU defines on the Article 118 of European Commission (EC) Regulation (EC) No 1234/2007 six main types of oils, which are delineated also by the IOC [
15]. The specific attributes of each type of oil are shown in
Table 1. Regarding all these groups, virgin olive oil (VOO) must be highlighted due to their organoleptic characteristics and beneficial properties. Among VOOs, the EU establishes three types of oils: extra virgin olive oil (EVOO), VOO, and lampante olive oil. Furthermore, IOC adds an extra subgroup called ordinary virgin oil. All VOOs are characterized for being obtained by mechanical processes (only washing, decantation, centrifugation and filtration) under specific thermal conditions which do not cause any alteration. Afterward, they are divided according to their acidity, which gives an idea of the content in free fatty acids based on the percentage of oleic acid. Lower acidity values guarantee a high-quality oil, showing it has been obtained from healthy olives and under ideal conditions. Moreover, they are submitted to sensory analysis to asses some requirements [
16].
Particularly, EVOO is obtained directly from olives, that is, pure olive juice. It is considered the highest quality oil, and in general, it is characterized for having a low acidity, up to 0.8% and a sensory grade higher than 6.5 points, thus, having perfect aroma and flavor [
3,
10]. Regarding sensory aspects, EVOO must show a fruity note higher than 0 and, more importantly, a median of zero defects [
18]. In addition, and to offer the most possible information to the consumer, these oils are usually labeled as intense, medium or light depending on their positive attributes [
19]. However, sensory analysis is always more difficult to unify. On the other hand, EVOO is susceptible to different chemical alterations during its production. Hydrolytic and oxidative degradation are the main causes of undesirable product formation so that water plays an important role since lipases (naturally present at olive pulp or seeds) are active in an aqueous phase. Lipolytic reactions lead to the formation of free fatty acids, thus increasing the acidity of the oil. To avoid these reactions, the filtration step is of high importance, as well as paying attention to other factors such as avoiding microorganisms or mechanical crushing, among others. Moreover, water may affect the transformations of different phenolic compounds [
18,
20]. Regarding oxygen, EVOO must have a peroxide index (mEqO
2/kg) ≤20. Oxygen is crucial for the lipoxygenase cascade reactions and its consequential positive sensory notes, but its excess can cause defects [
20]. Moreover, EVOO must also show specific spectrophotometric values: absorption λ = 233 nm, ≤ 2.50 and λ = 270 nm, ≤ 0.22, which are quality and authenticity indicators, respectively [
18]. Besides, contaminants (i.e., mycotoxins or pesticides, among others) are another crucial factor in avoiding associated risks [
21]. Considering all the characteristics that EVOO is required to fulfill, quality control is a must. Both IOC and EC have approved different standards in matters of purity (oil composition) and quality (organoleptic profile, acidity, peroxide value, among others) as well as sensory analysis to ensure that alterations have not occurred during the process [
10]. Additionally, another remarkable aspect is traceability. Traceability must be assured during all the obtaining process of EVOO, including the four basic steps (harvesting, milling, storage, and packaging batches) of the procedure to guarantee the identity of the product and its production chain. This approach also leads to the next step, this is certification, carried out by normalization and certification quality organisms in order to prove the rigorous process system employed at EVOO production [
22,
23].
Regarding its composition, EVOO is mainly composed of triglycerides (97–99%) and minor compounds (1–3%), which are the principal responsible for its biological properties and sensory attributes. It has a high content of MUFA (65–83%), especially oleic acid, and some polyunsaturated fatty acids (PUFA) such as linoleic acid, which is considered a potent fatty acid on the reduction of low-density lipoprotein (LDL) cholesterol. This lipid profile and also high ω6/ω3 ratio have been linked to protective effects on coronary, autoimmune and inflammatory disorders but also as anti-thrombotic and regulators of blood pressure [
24,
25,
26]. Concerning bioactive compounds, their main representatives are the same of oil in general, namely phenolic compounds such as hydroxytyrosol and derivatives (oleuropein and tyrosol), tocopherols but also other compounds as hydrocarbons (i.e., squalene) or pigments like provitamin A compounds [
3,
26]. However, it must be mentioned that some of these compounds such as squalene might be lost during refinery, so they can only be found on this type of oil [
25].
As mentioned, all these bioactive compounds are known for their biological properties and positive effects on human health. EVOO inclusion in the diet and its bioactive molecules have been studied to identify its effects. EVOO is known for having a high content of antioxidant compounds with protective properties against free radicals. Therefore, it has been pointed out that its high consumption is related to a generally low risk of suffering colon, breast or skin cancer as well as beneficial effects on aging and coronary diseases [
1]. It has also been proposed as a preventing tool of Alzheimer’s and other neurodegenerative diseases [
3], as anti-inflammatory [
7] and also as immune-stimulating [
24]. Another study proved that rats fed with EVOO in substitution of lipids and complemented with physical exercise, could avert cartilage diseases as osteoarthritis [
9]. Additionally, EVOO consumption has shown positive effects on gut microbiota [
27]. Some studies have researched the bioavailability of phenolic compounds of EVOO and found that 55–60% of them can be absorbed, most of them at small intestine [
11]. Moreover, their different compounds have shown other beneficial properties like antimicrobial, antitumor or protective agents against cellular damage [
26,
28]. More specific studies have also related EVOO treatment with positive gene regulation and with micro ribonucleic acid (miRNA) modulation of target genes associated with synaptic plasticity as well as to motor and cognitive behavior [
29]. There are multiple bibliographic references directed towards proving all these promoting effects. Nevertheless, a specific study must be highlighted, the PREDIMED trial (prevention through MED, in Spanish). This is one of the largest nutritional studies ever conducted in Spain, which evaluated the effects on primary prevention of CVD when following a MED supplemented by EVOO or nuts mix [
30]. This project groups together different studies, which have brought to light several positive consequences: reduction of CVD risk, reduction of C-reactive protein, reduction of atrial fibrillation, prevention of diabetes and metabolic syndrome, reduction of diastolic blood pressure, higher protection against breast cancer or lower prevalence of non-alcoholic fatty acid liver disease [
25,
31,
32,
33]. However, more epidemiologic studies and controlled trials are necessary to better validate and understand the beneficial effects of EVOO consumption. At last, it is worth mentioning that new disciplines (encompassed as nutrigenomics) are also working on new approaches for evaluating the health-promoting effects, characterizing new markers, and understanding their action mechanisms [
29,
34].
Regarding all the presented information, this article aims to review the current knowledge about the unique EVOO composition and its related bioactivities. Therefore, a revision focused on its beneficial and health-promoting properties, its chemical composition as well as some quality parameters is presented. To achieve this purpose, a systematic search in several databases was carried out based on the topic described, all in all, ~5000 documents have been published. Most of the publications correspond to research articles and book chapters, 56.3% and 15.6%, respectively, followed by reviews (12.5%). Among them, bibliographic references were chosen according to their concordance with the covered topics. The extensive information compiled was further classified into the different sections presented in this review. After analyzing the considerable amount of data available on this field, it can be said that EVOO is a current topic; although it has been known and used for centuries, it is still under study, which means EVOO has potential properties and/or applications yet to be discovered.
2. Main Components of EVOO
Virgin olive oils are oils obtained from the fruit of the olive tree (
Olea europaea L.) solely by mechanical or other physical means under conditions, particularly thermal conditions, that do not lead to alterations in the oil, and which have not undergone any treatment other than washing, decantation, centrifugation and filtration [
35]. The use of said physical techniques allows the preservation of many compounds that make EVOO one of a kind among plant oils. Its uniqueness is due to the abundance of fatty acids, PUFA and MUFA but also the occurrence of many bioactive molecules, like hydrophilic phenols, phytosterols, tocopherols and carotenes that provide several functional properties as well as a long storage time due to their high oxidative stability [
36,
37,
38]. Other vegetable oils, like palm oil, are rich in saturated fats, which are more stable during the cooking or frying processes than the unsaturated ones, avoiding degradation to toxic compounds, but they do not have beneficial properties for the human health as the unsaturated one. On the other hand, sunflower oil is rich in unsaturated fats, especially in linoleic and oleic acids that enhance its healthy properties but decrease its thermal stability [
39]. EVOO has a good PUFA:MUFA balance, which confers it stability properties against oxidative thermal degradation, particularly regarding the formation of volatile aldehydes, so EVOO is a proper and recommendable oil to use in food frying [
40]. This relation between PUFA and MUFA and the low content of saturated fats also makes EVOO one of the healthiest vegetable oils to be consumed raw because it helps reduce LDL cholesterol levels in the human body [
41].
The composition of EVOO is a result of several factors like genotypic potential, environmental factors, fruit ripening, harvest time, agricultural factors (irrigation, sunlight, orchard management) and also technological factors like the method applied for oil extraction or the storage conditions [
42]. The concentration of the minor and major fruit components changes and depends on all those variables. Apart from that, the olives ripening process lasts a few months in which the atmospheric, environmental and agricultural conditions play a very important role despite the strict genetic control that can be applied [
43,
44]. During the maturation and ripening process, the photosynthetic activity decreases as the oil content in the fruits increases [
45]. In the first stage of ripening, denominated green stage, the ripe fruits have already acquired their final size, so the maturation proceeds, and the chlorophylls in the skin are slowly swapped by anthocyanins, turning the olives from green to dark violet or purple until the end of the ripping process. These changes in color define the spotted, purple and black stages [
43,
46]. Olives have the highest phenolic compound content at the phase between green and darker skin, and therefore, the degree of maturation is an important factor to define the right harvest time that will originate the best quality olive oil [
47].
Figure 3 shows a summary of representative chemical structures of some relevant compounds present in EVOO.
2.1. Primary Metabolites
2.1.1. Lipids
Lipids are one of the principal sources of energy for all living beings and additionally, they are also involved in many physiological functions, as their role as a structural component of cell membranes, nervous system, the production of hormones, brain development and also on the promotion of liposoluble vitamins absorption.
EVOO is widely used in the human diet, especially in the MED and has been long renowned for its many health-promoting properties. Its consumption is associated with reduced risk of several chronic illnesses, like diabetes, hypertension, obesity and CVD [
48,
49]. These health properties are related to the presence of bioactive compounds like phenolic compounds but also with the high content in MUFA. Olive oil has a high content in oleic acid transforming it into a healthy fat, especially when compared with other vegetable oils [
40]. This lipid can decrease the risk of CVD due to its effects on the lipids present in the blood vessels [
50]. According to available data, there is 65.2–80.8% of MUFA in the lipidic fraction of olive oil [
51]. Other fatty acids found in the total fatty acids composition of olive oils are palmitic acid, palmitoleic acid, stearic acid, linoleic acid, α-linolenic acid, and other minor ones that are listed in
Table 2.
Triacylglycerols constitute a big part of the edible oil and a high percentage of the saponifiable fraction is constituted by MUFA [
27]. The principal triacylglycerol detected in olive oil is oleic–oleic–oleic (OOO), representing about half of the total triacylglycerol portion found in EVOO. Other triacyclglycerols also present are palmitic–oleic–oleic (POO), oleic–oleic–linoleic (OOL), palmitic–oleic–linoleic (POL) and stearic–oleic–oleic (SOO) [
43,
52]. Diacylglycerols and monoacylglycerols have been identified in VOO at concentrations of 1–2.8% and 0.25, respectively [
53].
Four classes of sterols also take place in olive oil and are commonly used to check its genuineness because their presence is linked to the quality of the oil. These four classes are common sterols (4-Desmethylsterols), 4α-Methylsterols, triterpene alcohols (4, 4-Dimethylsterols) and triterpene dialcohols [
43]. Common sterols in EVOO are present in both free and esterified forms [
59]. The leading components of this sterol fraction are campesterol, β-Sitosterol and Δ5-Avenasterol [
60,
61], and in smaller amounts, it is also possible to find stigmasterol, cholesterol, cholesterol, brassicasterol, sitostanol, ergosterol, campestanol, Δ7-Cholestenol, Δ7-Avenasterol, Δ7-Stigmasterol, Δ7-Campesterol, Δ5,24-Stigmastadienol, Δ5,23-Stigmastadienol, Δ7,24-Ergostadienol, Δ7,22-Ergostadienol, 22,23-Dihydrobrassicasterol and 24-Methylene-cholesterol [
62,
63]. The total sterol content of EVOO varies between 1000 and 2000 mg/kg, being the first value the inferior limit set by the EU Commission [
43]. β-Sitosterol is the main compound in the sterol fraction with values between 75% and 90% of the total sterol fraction, while Δ5-Avenasterol has values between 5% and 20% [
62]. Crop year, cultivar, ripeness of the fruit, storage time of the olives before oil extraction and geographic influences all contribute to sterol composition of the final EVOO obtained [
64,
65,
66]. At the same time, storage time and conditions of the final product are also factors that can originate several important changes particularly in the concentrations of each individual sterol [
43]. 4-Monomethylsterols are present in smaller amounts and signify part of sterol biosynthesis as intermediates. They can be found in their free and esterified forms [
67]. The most common are gramisterol, obtusifoliol, cycloeucalenol and citrostadienol [
60,
62], and their concentrations vary between 50 and 360 mg/kg of oil [
60,
68]. Triterpene alcohols, also identified as 4,4-Dimethylsterol, are a very complex fraction that can be in free and esterified form, and whose main compounds are butyrospermol, β-Amyrin, cycloartenol and 24-Methylenecycloartanol. In smaller amounts or trace quantities, cyclosadol, cyclobranol, dammaradienol, germanicol, 24-Tirucalladienol, 24-Methylene-24-Dihydroparkeol, α-Amyrin, taraxerol, 7, parkeol and tirucallol can also be found [
62]. Total triterpene alcohol levels range from values of 350 to 1500 mg/kg [
68,
69]. Lastly, among the triterpene dialcohols class, erythrodiol (5α-olean-12-ene-3β, 28-diol, homo-olestranol) in free and esterified form and uvaol (Δ12-Ursen-3β,28-diol) are the major triterpene dialcohols found in EVOO [
70], and their presence is mainly affected by cultivation characteristics [
68]. EVOO contains levels of total erythrodiol from 19 to 69 mg/kg of oil, and the free form is inferior to 50 mg/kg [
59,
68].
2.1.2. Tocopherols
Three isoforms of tocopherols are present in EVOO: α-, β- and γ-tocopherol. α-Tocopherol can be found in its free form and represents more than 90% of the identified section with ranges from 206.5 to 270.9 mg/kg of oil to 191.5 to 292.7 mg/kg of oil, values that fluctuate with variables as the year of harvest and spacing between olive trees [
71]. Both, the distance between plants and the crop year influenced statistically tocopherols amount [
43,
71]. Besides, the high levels of this type of tocopherol may be linked to the high levels of chlorophyll pigments and the simultaneous necessity for singlet oxygen deactivation [
72].
Lower quantities of β-Tocopherol (~10 mg/kg), γ-Tocopherol (~20 mg/kg) and δ-Tocopherol (~10 mg/kg) can also be found on EVOO. The total tocopherol concentration seems to decrease in the ripping of the fruits, and the refining or the hydrogenation process causes their degradation, so they are only found in the EVOO and VOO [
73].
2.1.3. Carbohydrates
There are two hydrocarbons mainly present in olive oil, 2,6,10,15,19,23-Hexamethyl-2,6,10,14,18,22-Tetracosahexaene also known as squalene and β-Carotene, which will be addressed in the pigments section of this review. Squalene is the last metabolite synthesized before the sterol ring formation. Some of the beneficial health effects of olive oil are partially linked with the presence of squalene, and it has also demonstrated antitumoral effects against certain types of cancer [
74,
75]. This compound constitutes more than 90% of the hydrocarbon fraction and is the most abundant compound in the unsaponifiable matter, with concentrations ranging from 200 to 7500 mg/kg oil [
57]. In a different study, squalene was reported in even higher concentrations, up to 12,000 mg/kg oil. Squalene content depends on several variables like the type of olive cultivation and the oil extraction technique applied, and it decreases largely during the refining process so it is present in larger quantities in EVOO and VOO [
76].
The remaining fraction of carbohydrates in EVOO is composed of triterpene and diterpene, isoprenoid polyolefins, hydrocarbons and n-paraffins [
43,
76].
2.2. Secondary Metabolites
2.2.1. Phenolic Compounds
The principal group of antioxidants in EVOO are hydrophilic phenols, and these compounds are extremely relevant when it comes to determining the quality of the oil regarding their sensory characteristics, like bitterness, pungency and stability [
38,
77], as well as determining the organoleptic characteristics of aroma and flavor of each EVOO [
53]. The oxidative stability of EVOO depends not only on the olive variety and quality but also on the harvesting time; cultivation area; the degree of unsaturation and the levels of antioxidants present from tocopherols, hydrophilic phenols and carotenes. Besides, factors like oil extraction system and storage conditions also influence its conservation [
78].
The correlation of the phenolic content of olive oil and oxidative stability was studied showing that these two are interconnected [
79]. Furthermore, EVOO phenolic compounds provide benefits for human health in the prevention of several chronic diseases [
80,
81]. Various studies indicate that EVOO phenolic compounds have antioxidant, anti-inflammatory, antimicrobial and antitumoral activities, and they can also modulate gene expression to protect proteins that take part in the cellular mechanisms involved in the inflammation process, the oxidative stress resistance and in lipid metabolism [
82,
83]. Therefore, the major antioxidant substances found in EVOO are polar phenolic compounds that can be present in free, bound or esterified forms [
43], and usually, its total phenolic content ranges between 50 and 1000 mg/kg [
84], being more common in concentrations between 100 and 300 mg/kg [
43]. Likewise, each EVOO has a different phenolic profile, content and composition due to the differences discussed above [
78].
Phenolic compounds have been largely reported in EVOO composition, with more than 30 different compounds identified [
47,
85], being the major phenolic acids present in EVOO hydroxybenzoic,
p-Coumaric, ferulic, gallic, syringic, vanillic, caffeic,
o-coumaric and sinapic acids [
53]. Other types of polyphenols that can also be found in EVOO are flavonoids, lignans, hydroxy-isocromans, secoiridoids and phenolic alcohols. The major flavonoids found in EVOO are luteolin, apigenin and many of their derivates [
86,
87], whereas the main lignans present are (+)-pinoresinol and (+)-1-Acetoxypinoresinol [
88], being the usual lignan content in EVOO between 1 and 100 mg/kg [
89].
Secoiridoids are rare phenolic compounds present in plant species, nevertheless, they are found in abundance in Oleaceae species, particularly in
O. europaea leaves and fruits. However, they are insoluble in oil and therefore only a small percentage of these compounds ends up in the final EVOO after the mechanic extraction process. Nevertheless, they are one of the most important micronutrients on EVOO for their sensorial and heath properties [
38,
80]. The most common secoiridoids are demethyloleuropein, oleuropein, ligstroside and their aglycones, the last ones accounting for approximately 90% of the phenolic compounds in EVOO [
90]. Secoiridoids are hydrolyzed through crushing and malaxation by enzymatic reactions catalyzed by endogenous b-glucosidases yielding secoiridoid aglycons [
91]. The bitterness of olive oil is due to the secoiridoids present, especially the dialdehydic form of oleuropein aglycone [
92].
Isochromans are only found at low concentration in EVOO, and the two mainly found are 1-Phenyl-6,7-dihydroxy-isochroman and 1-(3′Methoxy-4′-hydroxy)phenyl-6,7-dihydroxy-isochroman [
93]. The concentration of these compounds increases during the extraction process because of the hydrolytic process that originates carbonyl compounds and hydroxytyrosol, which are isocromans derivatives [
88]. Finally, the principal phenolic alcohols found in EVOO are tyrosol (p-Hydroxyphenyl ethanol) and hydroxytyrosol (2-[3,4-Dihydroxyphenyl] ethanol). These are present in small concentrations in fresh olive oil but tend to increase along the storage process because of the hydrolysis of olive oil secoiridoids [
94].
2.2.2. Pigments
The lipophilic carotenoid and chlorophyll pigments occurring in olive oil are responsible for its characteristic color [
95]. The coloration of EVOO is greener in the presence of green olives that have higher chlorophyll content whereas using mature olives with higher carotenoid content we obtain a more yellowish oil, so the final color is a result of the proportions of these pigments [
96]. EVOO has a large variety of carotenoids and chlorophylls, from β-Carotene, violaxanthin, neoxanthin, lutein and other xanthophylls to chlorophyll a and b, pheophytin a and b and other minor derivatives [
97,
98]. These pigments can be found in amounts up to 100 ppm of total carotenoids and major pigments like pheophytin up to 25 ppm, β-carotene up to 15 ppm and lutein up to 10 ppm [
96], although these values depend on various factors. The final concentration of each pigment in the final EVOO relies on the physicochemical characteristics of the fruit, the geographic origin, climate and irrigation conditions and the mechanic extraction process used. Storage conditions and final packaging also play a role in pigment concentration and type [
96,
99,
100].
Quality and adulteration of EVOO are sometimes analyzed through the measuring of pigment compounds because they are correlated with EVOO nutritional value, freshness and antioxidant properties [
99,
101]. In addition, pigments can also be used for the authentication of EVOO, by measuring the chlorophyll and carotenoid pigments of EVOO and comparing them through a quality index, in which the total chlorophylls to total carotenoids ratio must be around 1 and the ratio of minor carotenoids to lutein must be around 0.5, to declare it an authentic olive oil [
102]. These parameters are valid for any olive oil regardless of the studied variety. Furthermore, other pigments like violaxanthin, lutein and total pigment content can be useful as a tool to identify a monovarietal EVOO [
102]. Chlorophylls, carotenoids and other minor pigments like lutein and violaxanthin can be stable for more than one year in storage regardless of the degree of ripeness and variety of the olives used to produce that oil [
103].
The degradation of chlorophylls occurs as a consequence of a pheophytinization reaction that starts from the malaxation step during the extraction of the EVOO and increases throughout storage time [
104]. During that process, the chlorophylls naturally present (a and b) are slowly but irreversibly converted into pheophytins a and b, where the central Mg
+2 ion of the porphyrin ring is exchanged with two hydrogen atoms making the molecules more stable. These eventually turn to pyropheophytins by the removal of the carboxymethyl group, which are the ultimate products of chlorophyll degradation [
105].
6. Conclusions
EVOO, also colloquially known as “liquid gold” is a natural product of unquestionable value and not only in the monetary sense of the term but also for its recognized properties and advantages on health. It is considered food for its nutritional value and is practically a requirement of the MED, contributing to the benefits associated with it. Its chemical composition allows EVOO to be classified as a lipophilic product since lipids are the main compounds, especially MUFA, followed by PUFA. This lipid fraction is responsible for protective properties on coronary, autoimmune and inflammatory disorders, granting anti-thrombotic and regulation effects of blood pressure Although in a smaller quantity, other compounds such as tocopherols or polyphenols are also present, which are associated with the powerful antioxidant and inflammatory activity of EVOO, among other qualities. For all these reasons, the inclusion of this golden ingredient in the diet, in addition to offering characteristic organoleptic properties, provides substances capable of preventing the appearance or development of diseases of various nature, from heart and circulatory diseases to metabolic disorders, including carcinogenic processes. Compared to other types of olive oil, EVOO must meet more stringent requirements that give it the right to possess that nomenclature. However, it is a shame that, during distribution and storage, the previously achieved quality is corrupted, leaving some of its most distinguished and desirable properties on the way. Therefore, the factors that promote the degradation of its components must be known, intending to reduce the negative impact that may originate in EVOO and thus increasing its shelf-life. These factors can be divided into intrinsic factors, such as the variety of olives and their cultivation conditions, about which little can be done once EVOO has been produced; and external factors, which include exposure to light, temperature, time or the type of material used for packaging. Due to the functional properties it presents, other applications for EVOO could also be considered, such as its use in therapies in which other treatments are not very effective, like certain neurodegenerative diseases or as a vehicle for the administration of certain pharmacological compounds in a comfortable way, by simply adding them to the diet. Future studies aimed at optimizing and maximizing the capabilities and applications of this product will, without a doubt, be welcomed and well received by the industry, not only food and agriculture but also by the pharmaceutical and cosmetics industries.