Presentation of a new analytical service: analysis of volatile compounds of extra virgin olive oils by HS-SPME-GC-MS
The knowledge of the profiles of volatile fraction is a fundamental tool to establish rapidly if a food product is compliant or not with a defined standard (quality control) and to understand the reasons of sensory success or unsuccess (preference aspects).
What are the volatile compounds of an oil?
Volatile compounds are natural chemical substances composing the aroma of virgin olive oils. They derive from biochemical reactions produced in olives during milling in olive oil mill and are perceptible by mouth and nose reaching olfactory receptors of retronasal cavity. Volatile compounds are influenced by the cultivar, olive ripening level, extraction temperatures and other different factors. They are characterized by low molecular weight and belong to different chemical classes as aldehydes, alcohols, esters, ethers, hydrocarbons, ketones, furans, terpenes, phenols and others. They are responsible for most of the organoleptic sensations perceived during a virgin olive oil tasting. Their composition determines the organoleptic identity found in each single oil that makes it unique and different from others.
Olive oil chemical analysis, what does the standard say?
The product identity and the market value of a virgin and extra virgin olive oil are determined by the compliance with chemical and physical parameters fixed by legislator, but they are also significantly influenced by their organoleptic profile that is assessed by Panel Test (All. XII Reg. EEC 2568/91 and subsequent amendments). The presence/absence of organoleptic defects makes an oil belong to the category of extra virgin, virgin or lampant (All. I Reg. EEC 2568/91and subsequent amendments). Therefore, it is very important to verify that the oil, besides showing the proper fruity, bitter and spicy attributes, doesn’t show organoleptic defects. These last can be due to many factors such as the health conditions of olives, their ripening level, their variety, the different extraction methods, length and temperatures of process, etc. The intensity of possible defects found can be the cause of downgrading of oil to lampant olive oil if it exceeds the median of 3,5. So, the organoleptic evaluation plays a leading role in product classification of oils.
Chemiservice: an innovative Olive oil chemical analysis service
Even if the Panel Test is based on an official and standardized method, it is characterized by criticalities linked to its own nature of sensorial and not instrumental test as well as to the needed length for its correct application; sometime the before mentioned length is not compatible with the rate of production process that follows production and market needs. To provide an efficient support to organoleptic evaluation, we developed an analysis that, by the use of a simple, reproducible, reliable and quick method, is able to detect in volatile fraction of virgin olive oil the main volatile compounds, called markers, linked to rancid, winey, fusty, muddy sediment and musty defects.
The method allows to assess analytically the volatile compounds of aromatic component of virgin oils using the technique of micro-extraction in solid phase from head space (HS-SPME); it is followed the analysis in gas chromatography with mass revelation. In this way we obtain the quantification of 40 substances (markers), characterizing the aromatic component of virgin oils.
Among the molecules before mentioned, some of them have been associated to positive attributes typical of virgin oils, others have been associated to the presence of one or more organoleptic defects. By the comparison between the indexes of analyzed sample and reference indexes, we can obtain information on fruity intensity and the possible presence of organoleptic defects.
After the assessment of threshold values for single markers present in defective oils (it means the lowest concentration of a compound able to produce an olfactive sensation), a “reference index of the defect” for each negative attribute has been calculated mathematically; when the index of the defect of analyzed sample is higher than the reference index, it is predictable that the analyzed oil will show the presence of the corresponding organoleptic defect also during tasting.
For the assessment of fruity positive attribute too, we have evaluated a reference index to be compared with the indexes calculated for the sample of analyzed olive oil.
Details of method validation
- The method is characterized by high sensitivity;
- For each molecule, very low quantification limits have been obtained;
- The quantification is carried out using the technique of external standard;
- For each molecule detected by the Method a 20% expanded measure uncertainty has been obtained;
- The method has been validated internally.
Representation of the analytical results of the new analytical service of the volatile compounds of extra virgin olive oils
In test report there are:
- The list of analyzed molecules, analytical result, LOQ and measure uncertainty;
- Information on fruity. Fruity index is expressed as “light” “medium” or “intense”, assessed instrumentally with the indexes of attributes calculated on analyzed sample;
- Information on defects. The possible presence of negative attribute/s is reported using an “attention” scale expressed qualitatively as “absent defect” “warning” or “present defect”;
In the summary table attached to the Test Report the results obtained are expressed both in numerical and graphic form to represent the comparison between the calculated assessment indexes and reference indexes.