Kinetics of Diglyceride Formation and Isomerization in Virgin Olive Oils by Employing ³¹P NMR Spectroscopy. Formulation of a Quantitative Measure to Assess Olive Oil Storage History

Abstract

Diacylglycerol isomers and free acidity were determined for five extra virgin olive oils of different initial acidities by employing a facile (31)P NMR methodology as a function of storage time and storage conditions. The kinetic treatment of the hydrolysis of triacylglycerols (TGs) and the isomerization of 1,2-diacylglycerols (1,2-DGs) to 1,3-diacylglycerols (1,3-DGs) during storage of 18 months at ambient temperature in the dark and light and at 5 degrees C in the dark showed that the isomerization is strongly dependent on the rate of the TGs hydrolysis, the initial free acidity (H(0)) of the virgin olive oil samples, and storage conditions. Although the time-evolution of the diacylglycerols (DGs) depends on the TGs hydrolysis, the ratio D of the concentration of 1,2-DGs to the total amount of DGs was found to be independent of this factor. From the kinetic expression of the ratio D, a quantitative measure was formulated that allows the estimation of the storage time or age of virgin olive oils. Application of this quantitative measure to several olive oil samples of known and unknown storage history resulted in a very good agreement with respect to the actual storage time for up to 10-12 months of storage. For a longer storage period, where the isomerization of DGs is close to its equilibrium state, the calculated age index is only indicative.

31P NMR spectroscopy in the quality control and authentication of extra‐virgin olive oil: A review of recent progress

“…At lower magnetic field strengths, the diacylglycerol resonances are overlapped by the strong resonances of the triacylglycerols, 1 questioning therefore the ability of this magnetic resonance method for reliable quantitative determination of these minor components at lower magnetic field strengths. 6 On the other hand, the large range of chemical shifts (¾1000 ppm) reported for the 31 P nucleus 7 ensures a good separation of the diacylglycerol signals. This is shown in Fig.…”

“…2(b)). Apart from mono-and diacylglycerol signals, the 31 P NMR spectra in Fig. 2(a) and (b) show one additional signal at υ 144.96 attributed to the phosphitylated total free sterols (STE).…”

“…2). The study of diacylglycerols (but not monoacylglycerols due to their low concentration in olive oil, Ä0.3%) in olive oil by 31 P NMR spectroscopy was feasible at even lower magnetic field strengths.…”

“…This method 5 is based on the derivatization of the labile hydrogens of functional groups, such as hydroxyl and carboxyl groups, of olive oil constituents with the phosphorus reagent 2-chloro-4,4,5,5-tetramethyldioxaphospholane (1) according to the reaction scheme illustrated in Fig. 1, and the use of the 31 P chemical shifts to identify the labile centers (compound 2). The present review is focused on the basic aspects underlying the 31 P NMR methodology, including instrumentation, preparation of samples and the phosphorus reagent, experimental conditions, chemical shifts of the phosphitylated compounds and finally its application to authentication and quality assessment of extra-virgin olive oil (EVOO).…”

“…1, and the use of the 31 P chemical shifts to identify the labile centers (compound 2). The present review is focused on the basic aspects underlying the 31 P NMR methodology, including instrumentation, preparation of samples and the phosphorus reagent, experimental conditions, chemical shifts of the phosphitylated compounds and finally its application to authentication and quality assessment of extra-virgin olive oil (EVOO).…”