Quality analysis of virgin olive oils – Part 2

Importance of the Topic

Polymerized triacylglycerides (PTAGs) formation is a key indicator of thermal treatment and refining in vegetable oils. Monitoring PTAG levels in olive oils ensures authenticity, supports quality control, and guards against adulteration and excessive heat exposure during processing or cooking.

Objectives and Overview of the Study

This study aimed to develop and demonstrate a reliable size exclusion chromatography/gel permeation chromatography (SEC/GPC) method with refractive index detection (RID) for quantifying PTAGs and free fatty acids (FFAs) in virgin and partly refined olive oils. Six commercial virgin olive oils and one blend of refined and virgin oils were analyzed to assess PTAG content and detect thermal or refining treatments.

Methodology and Instrumentation

Sample Preparation

• Solid phase extraction (SPE) on silica cartridges using diethyl ether/petroleum ether to isolate the polar fraction containing PTAGs.
• Virgin olive oils diluted in tetrahydrofuran (THF) for direct free fatty acid analysis without SPE.

Chromatographic Conditions

• Mobile phase: THF at 0.8 mL/min isocratic for 35 minutes
• Injection volume: 40 µL
• Column: Two Agilent PLgel 3 µm MIXED-E, 7.5×300 mm in series with a 7.5×50 mm guard
• Column and detector temperature: 35 °C

Instrumentation

• Agilent 1260 Infinity Quaternary Pump (G1311B)
• Agilent 1260 Infinity Autosampler (G1367E)
• Agilent 1290 Infinity Thermostatted Column Compartment (G1316A)
• Agilent 1260 Infinity Refractive Index Detector (G1362A)
• Data analysis: OpenLAB CDS ChemStation Edition Rev. C.01.04

Main Results and Discussion

Standard Mix Performance

• Retention time and peak area precision for TAGs, DAGs, MAGs, and FAMEs showed RSD <0.041% and area RSD <1.9%, ensuring reliable identification and quantification.

PTAG Content in Olive Oils

• All six virgin olive oils contained PTAG levels between 0.002 and 0.013 g per 100 g, well below the 0.05 g/100 g regulatory limit.
• The partly refined blend exhibited 0.224 g/100 g PTAGs, indicating significant thermal or refining treatment.

Thermally Treated Oil

• Oil heated at 180 °C for 20 hours produced extensive polymerization, including dimers, trimers, and higher polymers, visible as broad early-eluting peaks.

Free Fatty Acid Determination

• An extra virgin olive oil sample without SPE yielded 0.554 g FFAs per 100 g, consistent with regulatory limits (≤0.8% for extra virgin grade).

Benefits and Practical Applications

The described SEC/GPC–RID method allows laboratories to:

• Authenticate virgin olive oil and detect illegal refining or overheating.
• Quantify minor PTAG levels for quality assurance in food processing.
• Assess free fatty acid content under the same conditions, streamlining analysis workflows.

Future Trends and Possibilities

Advances may include:

• Integration of multi-detector setups (e.g., UV, MS) for structural elucidation of polymer species.
• High-throughput SPE and automated sample preparation to handle larger sample volumes.
• Development of refined calibration standards for absolute quantification of PTAG sub-classes.

Conclusion

The Agilent 1260 Infinity LC system with SEC/GPC and RID provides a robust and precise approach for monitoring thermal degradation markers and free fatty acids in olive oils. It supports regulatory compliance and quality control in food and industrial settings.

Used Instrumentation

• Agilent 1260 Infinity Quaternary Pump (G1311B)
• Agilent 1260 Infinity Autosampler (G1367E)
• Agilent 1290 Infinity Thermostatted Column Compartment (G1316A)
• Agilent 1260 Infinity Refractive Index Detector (G1362A)
• Agilent PLgel 3 µm MIXED-E columns and guard column
• OpenLAB CDS ChemStation Edition Rev. C.01.04

References

1. Quality Analysis of Virgin Olive Oils – Part 1 Application Note 5991-1894EN 2013
2. Quality Analysis of Virgin Olive Oils – Part 3 Application Note 5991-1896EN 2013
3. Billek G Die Veränderungen von Nahrungsfetten bei höheren Temperaturen Eur J Lipid Sci Technol 94 161–172 1992
4. Gertz C Klostermann J A new analytical procedure to differentiate virgin or non-refined from refined vegetable fats and oils Eur J Lipid Sci Technol 2000 329–336
5. Gertz C Zur Untersuchung und Beurteilung von Frittierfetten Lebensmittelchemie und gerichtl Chemie 31 65–69 1977
6. Guhr G Bestimmung der polaren Anteile in Frittierfetten Fette Seifen Anstrichmittel 83 373–376 1981