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A Natural Antioxidant

OLIVE OIL: A NATURAL ANTIOXIDANT

by Prof. MAURIZIO SERVILI, Department of Economic-Estimative and Food Science - Food Technology and Biotechnology Section - University of Perugia

Extra virgin olive oil is obtained exclusively by pressing olives and is enjoyed directly without any further physical-chemical treatments.

Extra virgin olive oils are made up of 98% triglycerides, while the other 2% is made up of what are known as minor components because of their quantity, but certainly not their importance. The fatty fraction includes all the fatty acids that form the basis of the "traditional" nutritional value of extra virgin olive oil, such as oleic, linoleic and linolenic acids.

In the past, the nutritional quality of olive oil was attributed to its high levels of oleic acid, but since currently high levels of this acid are also found in certain sunflower and rapeseed mixtures, it is precisely these minor components that make extra virgin olive oil a fatty substance with unique chemical properties.

Although they only represent 2% of the oil's weight, minor components are a group of over 230 substances that belong to different chemical classes: volatile compounds, aliphatic and terpenic alcohols, aldehydes, chlorophylls, sterols, carotenoids and phenols.

Over 180 volatile compounds are what give extra virgin olive oils their many different kinds of fragrance, such as grass, flowers, green apple, tomato, almond... Currently, only the relationship between "grassy fruitiness" and the aldehydes and medium-chain alcohols formed during mechanical oil extraction has been documented.

Chlorophylls and pheophytins give the oil a green colour, while carotenes, such as lutein and ß-carotene give it a yellow colour.

Among minor components there are also natural antioxidants, including tocopherols and hydrophilic phenolic substances, which play an important role in giving extra virgin olive oil its nutritional and health value. These components are formed, during the oil extraction process, from substances that are already in the olives: hydroxytyrosol and tyrosol belonging to the phenyl-alcohols class and oleuropein, demethyloleuropein, ligustroside and verbascoside, known as secoiridoides.

Recently, the presence of another secoiridoide in olive pits has been confirmed: nuzhenide; as well as another kind of phenolic compound known as lignans, found both in the pulp and in the pits, and to which pinoresinol and acetoxypinoresinol belong.

Phenolic substances have an important antioxidant action, meaning they protect the oil from the natural oxidizing process that leads to it becoming rancid. However, several studies have also highlighted a number of health properties:

Inhibiting platelet aggregation;
Involvement in the synthesis of thromboxane in human cells;
Inhibiting the oxidation of phospholipids and cholesterol;
Inducing apoptosis and cell differentiation in tumour cells.

This shows that, aside from protecting oil from going rancid and therefore making it last longer, these components are important in preventing cardiovascular diseases and may play an important role in preventing certain types of tumour.

Even the sensory properties of extra virgin olive oil are closely related to phenolic compounds, which are responsible for its bitterness and spiciness. In this context, currently, precise information is also available regarding the relationship between molecular structure and taste.

Thus, natural antioxidants are what give oil its bitterness and spiciness but they also have beneficial health properties, which means that spicy and bitter oils "are good for your health".

Virgin olive oil's sensory and health properties are highly influenced by the farming and technological production conditions, which need to be managed properly so as to obtain high quality oil.

Applied analytical methodology

Assessment of total polyphenols. The total polyphenol concentration in the oil has been expressed in mg/kg of hydroxytyrosol, and has been assessed with the colorimetric method using the Folin-Ciocalteau reagent as described by Montedoro et al. 1992.

HPLC assessment of the α-tocopherol. The α-tocopherol concentration has been expressed in mg/kg of α-tocopherol, as described by Psomiadou and Tsimidou, 1998, applying the following modifications: a column was used in normal phase, Waters μPorasil 300 mm x 3.9 mm (Milford, MA, USA) with a particle diameter of 10 μm. The solvents used for the elution consisted of a mixture of n-hexane with 0.5 % isopropyl alcohol (solvent A) and n-hexane with 10& isopropyl alcohol (solvent B). The solvent flow in the column was 2 mL/min. The solvent gradient during the test was the following: 100 % A / 0 % B for 4 minutes, 60 % A / 40 % B in 14 minutes, 40 % A / 60 % B in 4 minutes., 100 % A / 0 % B in 3 minutes and this last gradient was maintained for 5 minutes; for a total duration of analysis of 30 minutes.

Oils have been divided into three categories in relation to their natural antioxidant content (sum of the total polyphenols and the α-tocopherol). Since as time goes on there can be a loss of natural antioxidants, each category has been assigned a tolerance value.

Results

Class 1: Natural antioxidant content less than 300 mg/kg.

Class 2: Natural antioxidant content between 300 and 500 mg/kg.

Class 3: Natural antioxidant content greater than 500 mg/kg.

Bibliography

Montedoro GF., Servili M.. Baldioli M., Miniati E. Simple and hydrolyzable compounds in virgin olive oil. 1. Their extraction, separation and quantitative and semiquantitative evaluation by HPLC. J Agric. Food Chem. 40 (1992) 1571-1576.

Psomiadou E., and Tsimidou M. Simultaneous HPLC Determination of tocopherols, carotenoids, and chlorophylls for monitoring their effect on virgin olive oil oxidation. J. Agric. Food Chem. 1998, 46, 5132-5138.

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