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monounsaturated fatty acids     peroxide content     sensorial evaluation     composition and sterol content    erytrodiol and uvaol     saturated fatty acids     ECN42     methyl esters     wax ester     halogenated solvents    conclusion

The quality and the purity

Olive Oil is for its most part formed by glicerids (95-99,7%) and by minor compouds (0,3-5%) that have a fundamental role, both from a nutritional and organoleptic and from an analytic point of view, as it is possible to detect the biological provenence and to classify product.

For the consumer what is really important when choosing an olive oil to consume, is the quality and purity of the product. As previously seen on this web-site there are certain parameters that determine whether an oil can be classified as an Extra Virgin or other. However, it is not as simple as taking its acidity content. So this page is for whom wants to go into detail on how the quality and purity of an oil can be detected. Most of the information is from the ONAOO (the National Organization of Olive Oil Tasters). But first of all, let's see what is meant by quality and purity.

The intended meaning of Quality is “the totality of requisites and functions of the aliment which can satisfy the consumer’s needs”. Hereunder we can place sensory characteristics, stability to oxidation, absence of xenobiotics, nutritional values (e.g. essential fatty acids, relationship between saturated fatty acids, mono- and polyunsaturated fatty acids etc.), natural antioxidants etc.
The Purity of an aliment refers to the fact that “it has not been subjected to technologies different from those traditionally used, nor has any substance extraneous to its nature been added”. It is evident that a large part of the aims of the EC Regulations is dedicated to ascertaining the purity of olive oils which, due to their high cost, are the object of illegal practices. One single analysis is not yet available which can establish unequivocably the purity of an oil.
 

Tests on quality

1. The oleic acid percentage is one of the factors that determines the quality of an olive oil and it can easily be measured by the means of titration. The lower the quantity of oleic acidity contained in the oil, the better. To obtain a product which is organoleptically better and which has lower acidity, it is necessary to use an olive which is at the right maturation and to process it quickly after harvest. Micro organisms that grow on the fruit produce enzymes that increase the acidity. Hereunder a figure that shows the molecule compound of the fatty acids found in olive oil:
 

2. The content of peroxides in the oil under examination is expressed by the peroxide value. The higher the number, the greater is the degradation due to oxidation of the oil. In their turn the peroxides are subject to further oxidation which gives rise to the formation of other compounds which are determinable in different ways (aldehydes, ketones etc.) These compounds, called compounds of secondary oxidation, are responsible for making the oil rancid. Because of oxidation and due to the enzymes present in the tissue of the fruit (lipoxygenases), a certain concentration of peroxides is already present in the fruit before pressing. Particular natural circumstances (e.g. temperatures below freezing, dacic infestations, drought etc.), or olives incorrectly harvested and preserved may encourage a further formation of peroxides. Even during milling peroxides can increase greatly through bad processing or due to incorrect hygiene in the olive-press and/or of the vessels. Finally, prolonged exposure of the oil to light or heat sources is another cause of the increase of peroxides. The content of peroxides are determined through titration.
In this context it makes sense that we are told that olive oil should be consumed within two years of its production, it should be kept in a cool and dark place to preserve its organoleptic qualities.

3. In addition to these chemical analysis, a sensorial evaluation (panel test) should be carried out.

Tests on purity

The tests that prove purity are many and strictly chemical. Here's a list of the ones that have been found necessary to determine the pureness of an olive oil, or even better to exclude the probability that an olive oil has been mixed with other substances either chemical or organic (e.g. other cheaper oils).

1. Determination of the composition and content of sterols.
Sterols are compounds which are normally found in oils and natural fats, in concentrations and compositions which vary depending on the origin of the fatty matter. Olive oil has quantities of around 1200 – 1800 mg/Kg (= sterol content). Vegetable oils contain roughly the same type of sterols but in different relationships and the characteristics for each are different (=sterol composition). This specificity should allow the mixture of olive oils with foreign oils to be recognized. In fact, the addition of appreciable quantities of foreign oil to an olive oil will alter its natural sterolic composition. This makes it possible to recognize fraud. However, the careful choice of foreign oils and the amount used may make it difficult to discover fraud. This analysis is carried out by means of gaschromatography.
 

2. Determination of erythrodiol and uvaol.
High values of these two substances may be an indication of the addition of residue oil to the olive oil. The solvent used for the extraction of residue oils (hexane) also dissolves these compounds, which are more abundant in the skins and kernels than in the pulp. However, the so-called “green” oils are high in erythrodiol and uvaol content, even though they have not been obtained through the use of solvents. This fact is due to the repressing of the pastes from the first pressing (“remilling”) and to the great pressure to which they are subjected or to new centrifugation. They have a deep green colour from which they get their name. This analysis is carried out through gaschromatography, usually contextually to sterols .

 3. Determination of saturated fatty acids in position 2 in the triglyceride.
This analysis allows us to have an indication of the presence of any esterified oils in pressed oils. The saturated fatty acids present in the molecule of the triglyceride of an olive oil are linked, for the greater part, to the glicerol in positions 1 and 3, while position 2 is preferably occupied by unsaturated fatty acids. This “uneven” distributioin in the three positions depends on the particular pathway of the biosynthesis of the triglycerides during the oiling of the fruit. On the other hand, the industrial process of chemical synthesis of the triglyceride (esterification) does not discriminate between saturated and unsaturated fatty acids (i.e., it “has no preferences”): this means that the quantity of saturates in position 2 will be greater than that present in a natural oil. This analysis determines this content by means of the use of enzymes (lipase) commonly in commerce.

4. Determination of the difference: ECN42 (HPLC) and ECN42 (theoretic calculation).
The aim of this determination is to ascertain the presence of seed oils added to olive oil. Since it is known the rule according to the fatty acids are distributed in the three positions of the triglyceride when this is formed due to biochemical synthesis in the olive fruit, it is possible to calculate the composition in triglycerides of the oil under examination by starting from the composition of its fatty acids. This calculation is easily made on a computer and with a suitable calculation programme. The composition in triglycerides must also be determined through analysis (by means of HPLC). A particular group of successive peaks, labelled ECN42, is compared for quantity with the corresponding value determined through calculation. In principle, the two values should be identical. In practice, however, there may be differences which must not exceed the legal limits. In fact, the presence of foreign oils, in particular of seed oil, increases this value greatly, thus revealing fraud.

5. Gaschromatographic analysis of methyl esters of fatty acids (and trans isomers).
The aim of this determination is to establish the percentage composition of fatty acids in olive oil, more commonly known as acidic composition. Since, as we know, fatty matters foreign to olive oil have acidic compositions which may be totally different, any mixture may be revealed by this means. In reality this analysis has nowadays lost a great deal of importance even though it was the first gaschromatographic determination carried out on olive oils. As we have seen, there are more efficient ways of reaching the same objectives. At present, EC Regulations give limits of concentration only for a few fatty acids which are considered “tracing”, that is, typical of oils other than olive. They are miristic (C14:0; coconut oil), linoleic (C18:3; linseed oil), arachidic (C20:0; peanut oil), eicosenoic (C20:1; rapeseed oil), beenic (C22:0; peanut oil), lignoceric (C24:0; peanut oil). EEC Regulation n. 1429/92 dated 26/5/92, L150, introduces limits for trans oleic isomer content and for trans linoleic and trans linolenic (commonly known as trans isomers). Illicit industrial procedures which tend to mask a seed oil in order to enable its use in mixtures with olive oil (e.g. de-sterolization, i.e. removal of sterols), cause some modifications in the structure of the fatty acids: in particular, they generate trans isomers. In olive oil, they are normally present in very low concentrations. Higher levels are an indication of unjustified industrial practices. The determination of trans isomer content is carried out contextually to the acidic composition, in particular analytic conditions.

6. Determination of the wax ester content through gaschromatography with capillary column.
The EEC Regulation n. 183/93 dated 29-1-93, L 22, introduces the determination of wax esteres and gives limits for their concentration. Wax esteres are compounds naturally present in olives (these are non gliceridal esters, that is they do not contain glicerol). In particular, they are more abundant on the epicarp of the drupe and, during pressing, some of them are transferred to the oil. The solvent used in the extraction of residue oil also dissolves a certain quantity of wax esteres which, after the removal of the solvent, are abundant in the oil. The aim of this determination is therefore to seek out mixtures of pressed olive oils and residue oils. As wax esters are compounds which contain, combined in the molecule, aliphatic alcohols (alkanols), the latter are present in much greater amounts in residue oils than in pressed oils. Before the introduction of this analysis, the law provided for the determination of the content of aliphatic alcohols. However, when the method was approved, it was already obsolete, since the means of reducing alkanol content was already known (and perhaps in use) (cold treatment in suitable solvents), thus nullifying the efficacy of the analysis.
Later, the possibility of performing the determination directly on the wax esters with 40,42,44 and 46 carbon atoms was studied. In fact, their content remains more or less constant even after the fraudulent treatments mentioned above.

7. Determination of the quantity of volatile halogenated solvents in olive oil
The presence of halogenated solvents (e.g. Freon, trichloroethylene, perchloroethylene, chloroform etc.), may derive either from the use of oils extracted with solvents or from environmental contamination. The presence of these compounds even in oils which are certainly pure has been the object of a great deal of research. It was discovered that in olive mills, the use of drinkable water for extractions by pressing, may cause the formation of halogenated compounds (especially chlorides and bromides) which are extremely soluble in oils, and which thus become more concentrated. Moreover, the contamination of the water strata caused by used industrial waters, in addition to the pollution of the atmosphere caused by halogenated solvents (one may think of the quantity of perchloroethylene used in dry-cleaning) which then penetrate the water strata in rainfall, are factors which influence greatly the levels of concentration of these compounds in oils. This analysis is carried out by gaschromatography.

In Conclusion...
Once again it must be emphasized that until now it has not been possible to reveal the presence of foreign oils by means of one single analysis. As to the different chemical analysis we must entrust ourselves to the authorities as most of us do not have access to such advanced instruments.
But a sensorial analysis by a practised olive oil taster will already take us a long way - and do trust your own taste as well. If there is any foreign taste to the oil, or if tastes mouldy, acidic, metallic or any other taste foreign to the olive, it is a good sign that the oil is either no only olive oil, or it is old, badly made, etc. A good oil tastes of olive, green or mature, with sometimes notes of nuttiness, grass, artichokes, apples, - and sometimes with either a bitter or a spicy aftertaste. If the perception of the defects overpowers the taste of the olive considerably, you should probably not consume the oil.