Application of ³¹P NMR Spectroscopy in Food Analysis. 1. Quantitative Determination of the Mono- and Diglyceride Composition of Olive Oils

Abstract

This paper introduces a facile method to determine the amount of mono- and diglycerides in virgin olive oils. This method is based on the phosphorylation of the free hydroxyls of the mono- and diglycerides with 2-chloro-4,4,5,5-tetramethyldioxaphospholane and the integration of the appropriate peaks in the (31)P NMR spectrum. Quantitative (31)P NMR spectroscopy can be extended to the quantification of other constituents of olive oils bearing functional groups with labile protons.

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

“…5,10,17 Well-separated ranges of 31 P chemical shifts for the three classes of phosphitylated functional groups were observed. The 31 P chemical shifts for all phosphitylated aromatic hydroxyl groups cover a rather wide range (υ 136.4-142.9 (υ D 6.5)), whereas the aliphatic hydroxyl groups cover a narrower range (υ 145.7-149.6 (υ D 3.9)).…”

“…For the detection and quantification of the diacylglycerols, total free sterols and acidity, which give well-separated signals, relatively low magnetic field strengths from 7.05 to 9.4 T (Larmor frequencies 121.5 and 162.0 MHz for the 31 P nucleus) are adequate. 5,6 However, the detection of phenolic compounds in the polar fraction of olive oil may require higher magnetic field strengths up to 11.74 T (202.3 MHz) to improve the sensitivity and resolution of the experiment.…”

“…The present phosphorus reagent does not form isomeric products or side reactions upon derivatization of OH and COOH groups. 5,6,10,17 For instance, phenolcontaining olive oil model compounds bearing hydroxyl and carboxyl groups remained stable for more than 2 weeks after derivatization. 17 Additional advantages offered by phosphorus reagent 1 relative to other tagging reagents have been described in Refs 12,14,18 and 19.…”

“…To obtain reliable quantitative data by using one dimensional (1D) 31 P NMR spectra, several criteria must be considered: (i) Complete derivatization with the tagging reagent 1 must be achieved, as was shown in earlier publications; 5,10,12,14,17 (ii) Thermal equilibrium must be reached by the phosphorus nuclei before the pulse sequence repetition. This was guaranteed by using repetition time at least 5 times greater than the longest spin-lattice relaxation time ( 31 P-T 1 ).…”

“…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).…”