Comparison of triacylglycerol analysis by MALDI-TOF/MS, fatty acid analysis by GC-MS and non-selective analysis by NIRS in combination with chemometrics for determination of extra virgin olive oil geographical origin. A case study
Graphical abstract
Introduction
Olive oil quality and authenticity topics are gaining interest among manufacturers and customers as well as a direct interest of regulatory agencies. Even though reliable routine methods for quality control exists and are provided by International Olive Council (IOC, 2016), geographical characterization of extra virgin olive oil (EVOO), which is attractive to both producers and consumers from an economical point of view, remains an enormous challenge. EVOOs from certain regions have indeed, an added value and may reach a higher market price, so without development of appropriate characterization methods, the possibilities for frauds and false declarations are clearly evident. Scientists working in this field are therefore, increasingly working on analysis of olive oil components that might be used for discrimination of oils from different geographical regions. In particular, significant success in this field was achieved by analysis of major components in olive oils, namely fatty acids (FA) and triacylglycerols (TAG) (Kammoun & Zarrouk, 2012; Longobardi et al., 2012).
For FA composition analysis, gas chromatography coupled with flame ionization detector (GC-FID) is the most widely used technique. In the last decade, gas chromatography coupled to mass spectrometry (GC–MS) is increasingly used as it combines GC separation power with MS advantages in structural data studies (Alves et al., 2016; Ecker, Scherer, Schmitz, & Liebisch, 2012). FA analysis provides information on main components that form TAG species. All olive oils are characterized by high content of monounsaturated oleic acid, and it was demonstrated that fatty acid composition is influenced by oil geographical origin (Laroussi-Mezghani et al., 2015).
TAG profile may also be interesting in analysis of geographical origin due to TAG preserved structure (Bajoub, Medina-Rodríguez et al., 2016). TAG composition in olive oils is usually assessed by use of high-performance liquid chromatography coupled to refractive index detector (HPLC-RID). Currently, implementation of new methods based on mass spectrometry (MS) is underway with the aim to increase sensitivity, accuracy and rapidity in TAG investigations. For example, analysis of TAG by use of matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) MS showed interesting advantages in TAG analyses, including fast and easy sample preparation as no analyte purification or derivatization are required and adequate resolution in the triacylglycerol mass range (Cozzolino & De Giulio, 2011). We previously demonstrated that TAG profile obtained by MALDI-TOF/MS is sufficient for recognition of different EVOOs (Peršurić, Osuga, Galinac Grbac, Peter-Katalinić, & Kraljević Pavelić, 2017) and that the same method is useful in detection of EVOOs adulteration as well, where combination of MALDI-TOF/MS with statistical analysis proved efficient in detection of 1% of foreign oils addition (Jergović, Peršurić, Saftić, & Kraljević Pavelić, 2017). The final result of MALDI-TOF/MS analysis is a full TAG profile, but a complete structural identification would additionally require a detailed MS/MS analysis as well (Kubo et al., 2013; Peršurić et al., 2017; Pittenauer & Allmaier, 2009).
Oppositely to methods that are oriented on specific compounds, near-infrared spectroscopy (NIRS) is a well-known non-specific analytical technique that might yield important information on certain structures and presence of chemical bonds that are indicative for some functional groups in olive oil as well. Its usage is still limited as the need for precise and exact data on olive oil composition remains a necessity. Even though, NIRS analysis along with established large database may be a powerful and cheap technique for quick and reliable information on olive oils. NIRS is a simple method, and little or no sample preparation steps are required (Cayuela & García, 2018). It provides a low cost analysis and the largest amount of information in comparison with previously mentioned techniques. However, large amounts of data obtained by NIRS require statistical data processing (Bajoub, Bendini, Fernández-Gutiérrez, & Carrasco-Pancorbo, 2016). Recently, NIRS was implemented in studies focused on authentication of food products, especially olive oils, showing a high potential for rapid screening of geographical origin or adulteration of EVOOs (Casale et al., 2012; Dupuy, Galtier, Ollivier, Vanloot, & Artaud, 2010; Laroussi-Mezghani et al., 2015).
With these premises in mind, we decided to evaluate the power of MALDI-TOF/MS, GC-MS and NIRS in combination with chemometrics and develop a general protocol for geographical origin determination of EVOOs. To our knowledge, the power of these three techniques for geographical origin discrimination was not compared so far. Principal component analysis (PCA) was therefore, used for geographical origin sample grouping evaluation, whereas partial least square-discriminant analysis (PLS-DA) was introduced for testing of predictive capabilities of measured variables on accurate classification of EVOO regional category. Additional, the possibility of NIRS as an alternative approach for determination of quality parameters was investigated by PLS analysis.
Section snippets
Chemicals
Phenolphthalein, sodium thiosulphate pentahydrate, potassium hydroxide, potassium iodide, diethyl ether, isooctane and acetic acid were obtained from BDH Prolabo (Lutterworth, United Kingdom). Hexane and methanol were purchased from VWR (West Chester, PA, USA), ethanol from Gram-mol (Zagreb, Croatia), whereas starch and chloroform were obtained from Kemika (Zagreb, Croatia). Tetrahydrofuran (THF), sodium trifluoroactetate, 2, 5-dihydroxybenzoic acid (DHB) and standards bradykinin protein,
Quality parameters of Croatian olive oils
Among 49 olive oils intended for calibration set, 41 samples were classified as EVOO samples according to the values of their standard chemical parameters that fit within defined ranges in EEC/2568/91 and subsequent amendments (European Union Commission, 1991), while 8 samples were classified as virgin olive oil (2 from Kvarner, 2 from Dalmatia) or lampante olive oil (1 from Kvarner, 3 from Dalmatia). Only EVOO samples were included in further TAG and FA analyses and used for construction of
Conclusions
Analytical profiling of TAG and FA by use of MALDI-TOF/MS and GC-MS techniques was tested in combination with chemometrics methods on EVOOs. Statistical PCA analysis showed a classification power for EVOOs geographical origin discrimination, where TAG profile provided better classification ability than FA, later confirmed with PLS-DA as well. Additionally, NIRS as a complementary non-targeted technique for rapid screening of EVOOs, proved valuable in classification of EVOOs. In particular,
Conflicts of interest
The authors have no conflict of interest to declare.
Acknowledgement
We greatly appreciate access to equipment in possession of University of Rijeka within the project “Research Infrastructure for Campus-based Laboratories at University of Rijeka”, financed by European Regional Development Fund (ERDF). We also acknowledge University of Rijeka research support 13.11.1.1.11. Special thanks to Metrohm A.G., Switzerland for making available the NIRS XDS instrument.
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