Phytochemicals

Olive oil polyphenols: classification, properties, and health benefits

Olive oil polyphenols are phytochemicals synthesised by the olive tree (Olea europaea), of which the drupe and extra virgin oil represent one of the prime sources.[1]

Olive oil, the primary source of lipids in the Mediterranean diet, contains macronutrients, represented by triglycerides, and micronutrients, including vitamin E and polyphenols.[2][3]

The concentration and qualitative profile of the polyphenols in olive oil are determined by a complex interaction of factors ranging from the cultivar to the technological process of extraction.[4]

Olive oil polyphenols are considered the main contributors to its organoleptic properties, its oxidative stability, and the positive health effects associated with its regular consumption.[5]

Note: the olive is a fleshy, indehiscent fruit botanically classified as a drupe. The drupe consists of three main layers: the exocarp or outer skin, the mesocarp, which is the pulp rich in water and lipids from which most of the oil is extracted, and the endocarp, the woody stone that encloses the seed.[6]

Summary: Key Points

  • Chemical properties: olive oil polyphenols include simple structures, such as phenolic acids and alcohols, and complex molecules, such as secoiridoids and lignans.
  • Agronomic and extraction factors: their concentration and qualitative profile depend on cultivar, climate, geographic region, drupe ripening stage, and extraction processes.
  • Biological actions: they exert systemic antioxidant, free radical scavenging, and anti-inflammatory functions by modulating the NF-κB and Nrf2 molecular pathways.
  • Health benefits: they protect LDL lipoproteins from oxidation, improve insulin sensitivity, and preserve cardiovascular function.
  • EFSA validation: a daily intake of 20 grams of olive oil containing at least 5 mg of hydroxytyrosol and its derivatives helps protect blood lipids from oxidative stress.

Contents

Olive oil polyphenols

Olive oil polyphenols comprise both simple molecules, such as phenolic acids and phenolic alcohols, and more complex structures, such as flavonoids, lignans, and secoiridoids—the latter representing the largest fraction.[7]
Polyphenols, which possess potent antioxidant properties, are present in the olive pulp and pass into the oil during pressing.[5]

Flavonoids

Flavonoids represent a smaller yet biologically active fraction of olive oil polyphenols. They include glycosidic derivatives of flavonols, such as rutin or quercetin-3-rutinoside, flavones, such as luteolin-7-glucoside, and anthocyanins, such as delphinidin glycosides.[7]

Phenolic acids and phenolic alcohols

Among the phenolic acids, the first simple structured polyphenols to be observed in olive oil include:

  • hydroxybenzoic acids, such as gallic acid, protocatechuic acid, vanillic acid, syringic acid, and 4-hydroxybenzoic acid, all with a C6–C1 structure;
  • hydroxycinnamic acids, such as caffeic, p-coumaric, and o-coumaric acids, all with a C6–C3 structure.[8]

The most abundant phenolic alcohols are hydroxytyrosol or 3,4-dihydroxyphenylethanol, and tyrosol or 2-(4-hydroxyphenyl)ethanol.[9]

Hydroxytyrosol

Hydroxytyrosol is present both free and esterified with elenolic acid to form oleuropein and its aglycones, as well as being a component of the verbascoside molecule. Furthermore, it can be found in various glycosidic forms, depending on the hydroxyl group to which the sugar moiety is attached.[10]

Structural formula of hydroxytyrosol, a phenolic alcohol and one of the polyphenols in olive oil.
Structural Formula of Hydroxytyrosol

It is one of the main phenolic compounds found in olives, extra virgin olive oil, and olive mill wastewater.[8]

In nature, its concentration, like that of tyrosol, increases during the ripening of the fruit, in parallel with the hydrolysis of compounds with a higher molecular weight, while the total content of phenolic compounds and α-tocopherol, a form of vitamin E, decreases. It can therefore be considered an indicator of the ripeness of olives.[11]

In fresh extra virgin olive oil, hydroxytyrosol is mostly present in esterified form, whereas over time, due to hydrolysis reactions, the non-esterified form becomes the predominant one. Finally, its concentration correlates with the stability of the oil.[12]

Secoiridoids

They are the most complex olive oil polyphenols and the product of the secondary metabolism of terpenes.[13]

Secoiridoids are compounds that bind a sugar and are characterised by the presence of elenolic acid in their structure, both in its glycosidic and aglycone forms, the common molecule found in secoiridoid glycosides of the Oleaceae family.[14]

Unlike tocopherols, flavonoids, and phenolic acids and alcohols found in many fruits and vegetables belonging to different botanical families, secoiridoids are present only in plants of the Oleaceae family.[15]

The main ones are oleuropein, demethyloleuropein, ligstroside, and nuzenide. Oleuropein, demethyloleuropein, and verbascoside are abundant in the pulp, but are also found in other parts of the fruit. Nuzenide is present only in the seeds.[16]

Oleuropein

Oleuropein, the most important secoiridoid and the main olive oil polyphenol, is the ester between hydroxytyrosol and elenolic acid.

Structural formula of oleuropein, a secoiridoid and one of the polyphenols in olive oil.
Structural Formula of Oleuropein

It is present in very high quantities in olive leaves and in all parts of the fruit, including the pit.[17]

It accumulates in the olive during the growth phase, reaching up to 14% of its net weight; as the fruit ripens, its quantity decreases. When the drupe turns dark brown, a color due to the presence of anthocyanins, the reduction of its concentration becomes more evident. It has also been shown that in green cultivars, its content is higher compared to black cultivars.[18]

During the reduction of its levels and those of other secoiridoids, an increase in compounds such as flavonoids, verbascosides, and simple phenols is observed. The reduction of its content is also accompanied by an increase in its glycosylated byproducts, which reach their maximum values in black olives.[19]

Lignans

Lignans represent another key class of olive oil polyphenols, particularly (+)-pinoresinol and (+)-1-acetoxypinoresinol.

(+)-Pinoresinol is a common compound in the lignan fraction of several plants, such as sesame (Sesamum indicum) and seeds from the Forsythia genus, which belongs to the Oleaceae family. It is also found in the pit of olives.

(+)-1-Acetoxypinoresinol and (+)-1-hydroxypinoresinol, along with their glycosides, are also present in olive bark.[20]

Structural formulas of selected lignans, a class of polyphenols present in olive oil.
Structural Formulas of Selected Lignans

Lignans are absent from the pericarp of drupes, as well as from the twigs and leaves that may accidentally be co-pressed with the olives. Consequently, the exact mechanism by which they enter the oil, becoming one of its most prominent phenolic fractions, remains unclear.[21]

Neither (+)-1-acetoxypinoresinol nor (+)-pinoresinol is found in seed oils, and both are virtually absent from refined olive oils. In contrast, their concentration in extra virgin olive oil can reach up to 100 mg/kg.[22]

Classification, localization, and chemical characteristics of olive oil polyphenols
Family Chemical class Localization in the drupe Main compounds and notes
Simple fraction Phenolic alcohols Pulp and olive mill wastewater Hydroxytyrosol, tyrosol. Indicators of fruit ripening stage; free form concentration increases over time due to hydrolysis.
Phenolic acids Pulp and skin Hydroxybenzoic acids (C6–C1: gallic, protocatechuic, vanillic) and hydroxycinnamic acids (C6–C3: caffeic, p-coumaric).
Complex fraction Secoiridoids Pulp, leaves, and pit (nuzenide in seeds) Oleuropein, demethyloleuropein, ligstroside. Exclusive to the Oleaceae family; responsible for bitter and pungent notes.
Lignans Pit and bark (absent in pericarp) (+)-pinoresinol, (+)-1-acetoxypinoresinol. Absent in seed and refined olive oils; reach up to 100 mg/kg in extra virgin olive oil.
Flavonoids Pulp and skin Flavonols (rutin), flavones (luteolin-7-glucoside), and anthocyanins (delphinidin). Minor but biologically active fraction.

Influences on olive oil polyphenol content

The concentration of olive oil polyphenols is the result of a complex interaction between various factors, both environmental and related to the oil extraction process, such as:

  • the cultivation location and climate;
  • the cultivar (variety);
  • the degree of ripeness of the olives at the time of harvest. Their level usually decreases with excessive ripening of the drupes, although there are exceptions to these rules: for example, olives grown in warmer climates, despite their faster ripening, produce oils that are richer in polyphenols;
  • the extraction process. Notably, refined olive oils retain negligible polyphenol concentrations.[4]

Variations in polyphenol composition directly affect the sensory profile, nutritional value, and oxidative stability of the oil. For example, compounds such as hydroxytyrosol and oleuropein contribute directly to the characteristic bitter and pungent notes of extra virgin olive oil.[23][24][25]

Health effects

Regular consumption of extra virgin olive oil, a cornerstone of the Mediterranean diet, is associated with significant health benefits resulting from the synergistic action of the lipid fraction, rich in unsaturated fatty acids, particularly oleic acid, and its polyphenolic content.[3]

While oleic acid plays a key role in maintaining normal blood cholesterol levels, the antioxidant, free radical-scavenging, and systemic anti-inflammatory properties are attributed to the olive oil polyphenols.[2][5]

Among the most documented benefits is cardiovascular protection, mediated by the ability of phenols to prevent the oxidation of low-density lipoproteins (LDL) and to protect the vascular endothelium.[26]

At the molecular level, the anti-inflammatory action of hydroxytyrosol and secoiridoids occurs through the downregulation of the transcription factor NF-κB, inhibition of pro-inflammatory enzymes COX-2 and iNOS, and activation of the cytoprotective pathway Nrf2.[27]

At the metabolic level, the phenolic fraction improves insulin sensitivity and counters the endothelial dysfunction typical of metabolic syndrome.[28] The European Food Safety Authority (EFSA) has validated the official health claim stating that olive oil polyphenols contribute to the protection of blood lipids from oxidative stress. This beneficial effect is achieved with a daily intake of 20 grams of olive oil (approximately two tablespoons) containing at least 5 mg of hydroxytyrosol and its derivatives.[29]

References

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Domande Frequenti

What are the main polyphenols found in extra virgin olive oil?

Extra virgin olive oil contains simple phenols such as hydroxytyrosol and tyrosol, phenolic acids, and flavonoids. however, the predominant fraction consists of secoiridoids, mainly oleuropein and ligstroside aglycones, which are responsible for its bitter and pungent sensory notes.

How do olive oil polyphenols protect blood lipids from oxidation?

Olive oil polyphenols act as powerful antioxidants that shield low-density lipoproteins (LDL) from oxidative damage. This protective effect on vascular structures and blood lipids is supported by strong clinical evidence and has been officially validated by the european food safety authority.

What daily intake of olive oil is recommended for health claims?

According to the official EFSA health claim, the protective benefit against oxidative stress is achieved with a daily intake of 20 grams of olive oil (about two tablespoons) containing at least 5 mg of hydroxytyrosol and its chemical derivatives, such as oleuropein complex.

What factors influence the final polyphenol content in olive oil?

Polyphenol levels depend on olive cultivar, regional climate, ripening stage at harvest, and extraction technology. Improper thermal processing or industrial refining drastically depletes these bioactive compounds, leaving refined olive oils with negligible polyphenol amounts.