Sensitivity of Proton NMR Relaxation and Proton NMR Diffusion Measurements to Olive Oil Adulterations with Vegetable Oils.

Donatella Ancora,Ana Sepe,Boštjan Zalar,Tomaž Apih,Anton Gradišek,Valentina Domenici,Jerneja Milavec,Mario Cifelli

doi:10.1021/acs.jafc.1c00914

Abstract

Olive oils and, in particular, extra-virgin olive oils (EVOOs) are one of the most frauded food. Among the different adulterations of EVOOs, the mixture of high-quality olive oils with vegetable oils is one of the most common in the market. The need for fast and cheap techniques able to detect extra-virgin olive oil adulterations was the main motivation for the present research work based on 1H NMR relaxation and diffusion measurements. In particular, the 1H NMR relaxation times, T1 and T2, measured at 2 and 100 MHz on about 60 EVOO samples produced in Italy are compared with those measured on four different vegetable oils, produced from macadamia nuts, linseeds, sunflower seeds, and soybeans. Self-diffusion coefficients on this set of olive oils and vegetable oil samples were measured by means of the 1H NMR diffusion ordered spectroscopy (DOSY) technique, showing that, except for the macadamia oil, other vegetable oils are characterized by an average diffusion coefficient sensibly different from extra-virgin olive oils. Preliminary tests based on both NMR relaxation and diffusometry methods indicate that eventual adulterations of EVOO with linseed oil and macadamia oil are the easiest and the most difficult frauds to be detected, respectively.

Highlights

Extra-virgin olive oil (EVOO) represents one of the most important and nutritionally valuable edible oil and is a basic ingredient of the Mediterranean diet
The analysis of 1H nuclear magnetic resonance (NMR) T1 dispersions of EVOOs and the analysis of T1 at low magnetic fields have been used to investigate supramolecular structural features, such as the occurrence of inverse-micelle-like organization of triglycerides in extra-virgin olive oils,[22] and dynamic information, such as correlation times associated with rotational motions and self-diffusion constants.[23−25,27] A low-field (LF) 1H NMR relaxation method based on the reconstruction of 2D and 3D plots to correlate T1 and T2 distributions has been applied to check the thermal oxidation of linseed oils[36] and to detect several types of adulterations of vegetable oils.[37,38]
We report an original study based on 1H NMR relaxation and 1H NMR diffusion measurements applied to several vegetable oils, namely, soybean (SoO), macadamia nut (MO), linseed (LO), sunflower (SuO), and about 60 extravirgin olive (EVOO) oils produced in Italy in two different regions, Apulia and Tuscany

Summary

■ INTRODUCTION

Extra-virgin olive oil (EVOO) represents one of the most important and nutritionally valuable edible oil and is a basic ingredient of the Mediterranean diet. Transverse (T2),[35] at different Larmor frequencies can give indirect information about several chemical and physical properties of edible oils having a variable fatty acids’ composition, and it has been used to study the effect of thermal oxidation and desiccation processes.[22,23,26,32] The analysis of 1H NMR T1 dispersions of EVOOs and the analysis of T1 at low magnetic fields have been used to investigate supramolecular structural features, such as the occurrence of inverse-micelle-like organization of triglycerides in extra-virgin olive oils,[22] and dynamic information, such as correlation times associated with rotational motions and self-diffusion constants.[23−25,27] A low-field (LF) 1H NMR relaxation method based on the reconstruction of 2D and 3D plots to correlate T1 and T2 distributions has been applied to check the thermal oxidation of linseed oils[36] and to detect several types of adulterations of vegetable oils.[37,38] This rapid and relatively cheap LF NMR relaxation method seems useful to study the effect of oxidation in several vegetable oils, such as the macadamia,[39,40] linseed,[41] sunflower[42,43] and other blended oils,[44,45] which are used for consumption and for painting, energy, and biomass applications. A comparison between EVOO and vegetable oil samples in terms of the relaxation times (T1 and T2) measured at low resolution NMR setups (i.e., 2 and 100 MHz) and of the average diffusion coefficient (D) measured by means of the DOSY 1H NMR technique, is reported and discussed in view of the applicability of these NMR methods in discriminating several adulterations of EVOOs with vegetable oils

■ MATERIALS AND METHODS

■ REFERENCES

NMR study of phenolics in the vegetation water of three cultivars of