**5. Conclusions**

This study definitely assesses the constant and relevant presence in olive oils of a not-oleic mono-unsaturated fatty ester called *cis*-vaccenic ester. It resolves the literature controversies concerning the assignment of some 13C-NMR resonances but, more importantly, it brings back the expected coherency between NMR and chromatography data. The serendipitous comparison of GC-FID and NMR profiles for the "exotic" *Capparis spinosa* oil evidenced the overwhelming amount of *cis*-vaccenic ester in this matrix but also unambiguously confirmed 13C-NMR assignments also validated in olive oil. By reconsidering the NMR and GC-FID of olive oils, it turned out the surprising quantitative contribution (around 3%) of *cis*-vaccenic ester. The official GC method does not always perform the required resolution to resolve and quantify oleic and *cis*-vaccenic esters and this is leading to the undistinguished quantification of both mono-unsaturated fatty esters. It opens up grea<sup>t</sup> potential for any technique able to clearly resolve *cis*-vaccenic moieties (just like 13C-NMR) in the study of extra-virgin olive oils.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2304-8158/9/4/384/s1, Table S1: Analyzed samples coming from awarded BIOOIL competition 2014. The used code is connected to the provenance and to the known cultivar. Table S2: Quantitative data and relative deviation for 20 main variables, as measured through MARA-NMR processing method working on mono dimensional 1H and 13C-NMR experiments for 33 samples. Table S3: General scheme of MARA-NMR referred just to the first sample. There are several blocks: namely 1H-NMR integrations with assignment (100 entries), DPFGSE 1H-NMR integrations (17 entries, not used in this study), 13C-NMR integrations (90 entries) along with some sum of integrals belonging to the same spectral block. Where possible, assignments are performed respecting the chemical position indicated also in other studies about the NMR of olive oil compounds (see Figure 1), for the fatty esters the abbreviation is followed by a number indicating the distance from the carboxyl position (generally from 1 to 18). These first rows will be used in the following equations according to the style of (1) (see main text) conveyed as square sum in the raw called RHO. The RHO value is minimized playing around with the quantitative variables so that the theoretical outcome is best-fitting the real (independent) variables, Figure S1. Stack-plot of eight olive oils coming from Sicily. The reported assignment follows the labeling used in the main manuscript (scheme 1). The expanded regions around 22 and 32 ppm show the clear presence of cis-vaccenic acid signals useful for the quantification within MARA-NMR quantification. The aromatic region is already reported in Figure 2 of the main text.

**Author Contributions:** Conceptualization, A.R. and A.S. (Andrea Salvo); methodology, A.R.; software, A.R.; validation, A.R., G.L.L.T. and A.S. (Andrea Salvo); formal analysis, A.R., G.L.L.T.; investigation, A.R.; data curation, A.R., A.S. (Andrea Salvo) and G.L.L.T.; writing—original draft preparation, A.R.; writing—review and editing, A.R and G.L.L.T.; visualization and supervision, A.R., A.S. (Antonello Santini), A.S. (Andrea Salvo) and G.D.; background, N.C. and A.S. (Antonello Santini). All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Once again we thank the institution "University of Messina," which, in spite of poor means and the difficult situation of the south Italian research, does not give up.

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