**4. Discussion**

In this work, the potential of four different characterization techniques to detect traces of adulteration in EVOO were analyzed. Photoluminescence, Raman, and Fouriertransform infrared spectroscopies demonstrated the successful detection of small traces of adulteration on the order of 5%, while the thermal conductivity analysis showed small but constant fluctuations as a function of adulterant oil concentration. Notably, we demonstrated four different characterization methods that are able to rapidly assess the purity of EVOO. Photoluminescence showed a linear decrease in the peak intensity and position as the adulterant oil concentration was increased due to a decrease in the amount of chlorophyll and pheophytin, which are naturally present in EVOO but absent in the adulterant oils. Raman spectroscopy also presented a clear difference between the spectra of EVOO and adulterant oils (even in olive-pomace oil, which is also derived from olives) was also found. Notably, two peaks at ~1155 cm<sup>−</sup><sup>1</sup> and 1523 cm<sup>−</sup><sup>1</sup> were detectable only in EVOO. These modes are associated with the polyene chain of the carotenoids that are naturally presented in EVOO but absent in the adulterants. A clear decrease in the intensity ratio between the peaks at 1523 cm<sup>−</sup><sup>1</sup> (only presented in EVOO) and 1656 cm<sup>−</sup><sup>1</sup> (a common mode presented

in all the studied oils) was observed as a function of the adulterant-oil concentration. While a rough comparison between the IR spectra did not show appreciable differences, a statistical analysis showed grouping of the spectra and distinguished a remarkable difference in the PCA scores between pure EVOO and adulterated oils, demonstrating detection of as low as 5% adulterant concentration via FTIR spectroscopy. It is important to note that, while even PCA did not show significant differences between EVOO and EVOO–pomace mixtures, a deeper analysis using a two-dimensional correlation treatment was sensitive to small fluctuations around 2900 cm<sup>−</sup>1. This result is a nascent effort that demonstrates the potential of 2DCOS analysis for the detection of EVOO adulterated with oils of very similar origin. Finally, an appreciable fluctuation in the thermal conductivity of EVOO was observed for different amounts of adulterant oils. Thermal conductivity has previously been overlooked as a simple but useful figure of merit for assessing food authenticity, but is also a useful manner in which purity can be ascertained. These results highlight the potential of these techniques to detect adulteration, and indicate that the results of the current study can be used as a starting point for the development of spectroscopic methods that allow for the effective and efficient detection of adulteration in olive oils by aiding in identification and classification. While each technique independently may fail to reliably capture small amounts of adulteration in EVOO given the complexity and chemical variability in the oils, a combination of all of them together provides a more holistic base for authentication. For example, as was observed in the case of the FTIR spectra, it is difficult to differentiate EVOO from olive-pomace oils, due to their common origin, though other techniques such as Raman can clearly distinguish the two. Future subsequent development of multiple sensors incorporating and combining these techniques will allow for the acquisition of complete spectral data sets that are critical for precise EVOO authentication. Beyond the authentication of EVOO, the combination of spectroscopic and thermal techniques has the potential to facilitate simplified authentication throughout the food industry.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/foods11091304/s1, Figure S1: Sample preparation; Figure S2: IR spectra second derivative and pc loading of EVOO, pomace, corn, and soy-nu<sup>t</sup> oils; Figure S3: Photoluminescence of EVOO adulterated with different concentrations of: (a) corn, (b) soy-nu<sup>t</sup> blend, (c) high oleic sunflower, (d) sunflower oils, and (e) olive-pomace oils; Figure S4: Normalized Raman spectra of EVOO adulterated with different concentrations of: (a) ol-ive-pomace, (b) soy-nu<sup>t</sup> blend, and (c) corn oils; Figure S5: Normalized Raman spectra of EVOO measured directly from its package; Figure S6: 2DCOS map of pure EVOO; Table S1: Schematic representation of FTIR dataset for PCA; Table S2: Acid content of the studied edible oils [48].

**Author Contributions:** Conceptualization, E.C.-A., C.M.S.T. and A.C.-A.; Data curation, E.C.-A., B.P., M.K. and R.C.N.; Formal analysis, E.C.-A., B.P. and M.K.; Funding acquisition, C.M.S.T. and A.C.-A.; Investigation, E.C.-A., B.P. and R.C.N.; Methodology, E.C.-A. and A.C.-A.; Software, E.C.-A. and A.C.-A.; Supervision and validation R.S.F. and C.M.S.T.; Writing—original draft, E.C.-A., R.S.F. and R.C.N.; Writing—review and editing, E.C.-A., R.C.N. and R.S.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** ICN2 is supported by the Severo Ochoa program from the Spanish Research Agency (AEI, gran<sup>t</sup> no. SEV-2017-0706) and by the CERCA Programme/Generalitat de Catalunya. ICN2 authors acknowledge the support from the Spanish MICINN project SIP (PGC2018-101743-B-I00). A.C.-A. acknowledges the support from Fondecyt Iniciación 11200620. R.C.N. acknowledges funding from the EU-H2020 research and innovation program under the Marie Sklodowska Curie Fellowship (Grant No. 897148).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Raw data can be provided by the corresponding author (ECA) on reasonable request.

**Acknowledgments:** The infrared-spectroscopy measurements were performed at MIRAS beamline at ALBA Synchrotron with the collaboration of ALBA staff.

**Conflicts of Interest:** The authors declare no conflict of interest.
