Differently to high-resolution NMR, broad band NMR does not distinguish among hydrogen nuclei in different atomic environments. Two techniques can be employed, namely wide-line NMR or pulsed NMR, and both distin- guish between hydrogen atoms in liquid and solid environ- ments. Nowadays, the most widely used technique is pulsed NMR. By using this technique, a measurement related to the total number of hydrogen nuclei is followed by a sec- ond measurement after 70 µs to determine only those hydrogen nuclei in a liquid environment. This determina- tion depends on the fact that the signal for hydrogen nuclei in solid triacylglycerols decays much more quickly (less than 1% remains after 70 µs) than that of hydrogen nuclei in a liquid environment (which requires about 10000 s).
These measurements require only about 6 seconds and are used routinely for the study of margarine and other con- fectionery fats and of cocoa butter and similar substances. However, though the measurement is so quick, it may have
Fats: Physical Properties 9-19
TABLE 9.34
Chemical Shift Assignments of Main 1H NMR Signals
of Fats and Oilsa
Chemical
Shift (ppm) Assignment
Glycerol and Unsaturated Protons
5.40–5.26 O9/O10/L9/L10/L12/L13/Ln9/Ln10/Ln12/Ln13/Ln15/Ln16 5.26–5.20 β-Glycerol 4.32–4.10 α-Glycerol Saturated Protons 2.75 Ln11/Ln14 2.72 L11 2.34 L2/Ln2 2.23 O2 2.22 S2 2.03 Ln17 2.00 L8/L14 1.99 Ln8 1.95 O8/O11 1.57 L3/Ln3 1.56 O3 1.55 S3 1.28 L chain/Ln chain 1.23 O chain 1.19 S chain 0.93 Ln18 0.84 L18 0.82 O18/P16/St18
aAssignments are abbreviated by fatty acid and carbon number. 1(3)-
and 2-Positions of glycerol are designated by the Greek symbols αand
β, respectively. Labeling of acyl chains: S, saturated; P, palmityl; St, stearyl; O, oleyl; L, linoleyl; Ln, linolenyl chain. Depending on the oil composition and experimental conditions, a lower number of signals is usually observed.
to be preceded by a lengthy tempering routine. Without controlling tempering the results would not be reproduced from day to day or between laboratories. The tempering regime varies with the kind of fat but a typical procedure for cocoa butter involves melting at 100°C then holding at 60°C (1 h), 0°C (1.5 h), 26°C (40 h), 0°C (1.5 h), and finally at the measuring temperature for 1 h. For many fats the long tempering at 26°C can be omitted (3).
For oilseed breeders, the NMR method is often used to estimate the oil content of oilseeds in a nondestructive manner. This information is of commercial value and can assist seed breeders and agronomists in their studies to develop improved varieties. Pulsed NMR is also employed in lipid crystallization studies (163).
2. High-Resolution 1H NMR
The use of 1H NMR in the study of oils, fats and food
lipids has increased particularly because of the great amount of information that high field instruments can pro- vide in a very short period of time (164). The NMR spec- trum consists of a series of sharp signals whose frequencies and multiplicities can be related to the chem- ical nature of the different hydrogen atoms (methyl, meth- ylene, olefin, etc.) and whose intensities are directly related to the number of hydrogens producing the signal (165,166). In this spectrum, all hydrogen atoms having the same chemical surroundings produce signals at the same frequency. The position of a resonance signal in the
TABLE 9.35
Chemical Shift Assignments of Main 13C NMR Signals of Fats and Oilsa
Chemical Shift (ppm) Assignment Chemical Shift (ppm) Assignment Chemical Shift (ppm) Assignment
Carbonyl Carbons Aliphatic Carbons 29.22 L5β
176–174 Fatty acids 34.25 P2β/St2β 29.21 O5α/Ln5β 173.29 P1α 34.23 O2β 29.19 L5α/Ln5α 173.26 O1α 34.20 L2β/Ln2β 29.17 O6β/St4α 173.25 St1α 34.09 P2α/St2α 29.16 P4α 173.22 L1α/Ln1α 34.06 O2α 29.15 O6α/L6β 172.88 P1β 34.04 L2α/Ln2α 29.14 L6α/Ln6β 172.85 O1β/St1β 31.98 St16αβ 29.13 O4α/St4β/Ln6α 172.82 Ln1β 31.96 P14αβ 29.12 P4β 172.81 L1β 31.94 O16αβ 29.11 L4α Olefinic Carbons 31.55 L16αβ 29.10 Ln4α 131.96 Ln16αβ 29.80 O12αβ 29.09 O4β 130.23 Ln9α 29.76 O7β/St11αβ/St12αβ/St13αβ/St14αβ 29.07 L4β 130.22 L13β 29.74 O7α/P11αβ/P12αβ/St10αβ 29.06 Ln4β 130.21 L13α/Ln9β 29.73 P10αβ 27.26 O11αβ 130.06 O10β 29.72 St9αβ/St8αβ 27.23 L14αβ/Ln8αβ 130.04 O10α 29.70 P8αβ/P9αβ/St7β 27.21 O8αβ/L8αβ 130.01 L9α 29.68 P7β/St7α 25.66 L11αβ 129.98 L9β 29.66 P7α 25.65 Ln11αβ 129.74 O9α 29.65 L7β 25.56 Ln14αβ 129.71 O9β 29.63 L7α 24.95 P3β/St3β 128.32 Ln12β 29.62 Ln7β 24.92 O3β 128.31 Ln12α 29.61 Ln7α 24.91 St3α 128.26 Ln13α 29.56 O14αβ/St5β 24.90 L3β/P3α/Ln3β 128.25 Ln13β 29.54 P5β 24.88 O3α 128.12 L10β 29.53 St5α 24.86 L3α/Ln3α 128.10 L10α 29.52 P5α 22.73 St17αβ 127.93 L12α 29.42 St15αβ 22.72 P15αβ 127.92 L12β 29.40 P13αβ 22.71 O17αβ 127.80 Ln10β 29.37 L15αβ 22.59 L17αβ 127.79 Ln10α 29.36 O13αβ/O15αβ 22.57 Ln17αβ 127.15 Ln15α 29.35 St6β 14.29 Ln18αβ 127.14 Ln15β 29.33 P6β 14.13 P16αβ/St18αβ
Glycerol Carbons 29.32 St6α 14.12 O18αβ
68.93 β-Glycerol 29.31 P6α 14.08 L18αβ
62.13 α-Glycerol 29.24 O5β
aAssignments are abbreviated by fatty acid and carbon number. 1(3)- and 2-Positions of glycerol are designated by the Greek symbols αand β, respec-
tively. Labeling of acyl chains: S, saturated; P, palmityl; St, stearyl; O, oleyl; L, linoleyl; Ln, linolenyl chain. Depending on the oil composition and experimental conditions, a lower number of signals is usually observed.
spectrum is called the chemical shift (δ). In the 1H NMR
spectra of fats and oils the resonances appear between δ4.10 and 5.40 ppm for glycerol and unsaturated protons, and saturated protons signals appear between δ 0.80 and 2.80 ppm. Table 9.34 collects the assignation of most common signals.
By using the information contained in the spectra,1H
NMR has been employed, among others, to determine the iodine value, number of double bonds, average molecular weight, proportion of acyl groups in the triacylglycerol molecule, n-3 polyunsaturated fatty acid proportion, and docoxahexahenoic acid content (167–176). Some attempts to apply 1H NMR spectroscopy to oil authenticity have
also been carried out (177).
1H NMR spectroscopy may also be employed to
determine minor oil components, but the signals of these components should not overlap with those of the main components, their concentration be high enough to be detected, and high field equipment be employed. Thus, the determination of saturated and unsaturated aldehydes in virgin olive oils as well as diacylglycerols have been described (170,177,178).
3. High-Resolution 13C NMR
High-resolution 13C NMR spectra are more complex than 1H spectra and they do not provide quantitative information
so easily. Nevertheless, they contain much more structural information (chemical shifts and intensities) if this can be teased out of the data provided with each spectrum.13C
NMR resonances of fats and oils can be grouped into four well-defined spectral regions: carbonyl carbons ranging from 173.3 to 172.8 ppm; unsaturated carbons ranging from 132.0 to 127.1 ppm; glycerol carbons ranging from 69.1 to 61.6 ppm; and aliphatic carbons ranging from 34.3 to 14.0 ppm. The assignation of the different signals has been the objective of many studies and it is nowadays clearly resolved (179–181). The main resonances observed in the 13C NMR spectra of fats and oils are col-
lected in Table 9.35.
Information contained in these spectra has been employed for edible oil authenticity determination and quality controls, including the analysis of fatty acid com- position and distribution of fatty acids in the triacylglyc- erol molecule, the free fatty acid, iodine value, and diacylglycerol determination, the analysis of minor com- ponents, the oil stability prediction, the determination of polar components and oil colors, etc. (182–198). All these data suggest that with only one analysis, NMR allows the determination of a large number of components with very little or without any manipulation of the oil samples that nowadays need many different analyses. In addition, the application of multivariate statistics to NMR spectral data increases considerably the potential of the technique. However, and because minor components of the oils are
playing an essential role in defining oil authenticity and quality, concentration of these compounds (either by using a chromatographic procedure or by the use of unsaponifiables) or their observation during routine analysis by using special probes seem to be a necessary requisite to achieve a routine application of NMR to most aspects of oil analysis.
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