Food Lipids: Chemistry, Nutrition, and Biotechnology

the isolated trans band is another group of CH absorption bending vibrations including
a very strong cis absorption [137].
The IR absorption signal could be employed to analyze and obtain information
about qualitative structural and functional groups of lipids. In principle, since IR
band intensities are linearly related to the concentration of the absorbing molecular
species, quantitative information about the lipid can also be obtained [137]. IR spectroscopy
has been applied to solid lipids to obtain information about polymorphism,
crystal structure, conformation, and chain length. In oils, IR is commonly used to
determine the presence and the content of trans unsaturation. Single trans double
bonds show a characteristic absorption band at 968 cm �1 , and the frequency does
not change for additional double bonds unless these are conjugated. There is no
similar diagnostic IR absorption band for cis unsaturation; however, Raman spectra
show strong absorption bands at 1665 � 1cm �1 (cis-olefin), 1670 � 1cm �1 (transolefin)
and 2230 � 1 and 2291 � 2cm �1 (acetylene) for the type of unsaturation
shown [94]. Kates [83] has provided the characteristic IR absorption frequencies that
have diagnostic values for identification of major classes of lipids. It has also been
reported that ionic forms of phospholipids influence the absorption bands associated
with phosphate groups that influence the interpretation of the spectra [83]. The FTIR
spectrometer finds its uses in measuring iodine value, saponification value, and FFAs
[138]. Oxidative stability of lipids as reflected in the formation of peroxides and
secondary oxidation products may also be determined by FTIR [137,139].
3. Nuclear Magnetic Resonance Spectroscopy
Low-resolution pulsed 1 H NMR spectroscopy is employed to determine solid-fat
content of lipids as well as the oil content of seeds as discussed earlier in this chapter.
High-resolution 1 H NMR applied to vegetable oils gives several signals with designated
chemical shifts, coupling constant, splitting pattern, and area. This information
can be used to obtain structural and quantitative information about lipids.
Methyl stearate (saturated fatty acid ester) may give five distinct 1 H NMR signals
as summarized in Table 5. Similar signals appear in methyl oleate and linoleate, but
methyl oleate also gives signals for olefinic (5.35 ppm, 2H) and allylic (2.05 ppm,
4H) hydrogen atoms, and for linoleate these are at 5.35 (4H), 2.05 (4H, C8, and
C14) and 2.77 ppm (2H, C11). When a double bond gets close to the methyl group,
as in �-linolenate and other �-3 esters, the CH 3 signal is shifted to 0.98 ppm; oils
Table 5 Chemical Shift (�H) for Methyl Alkanoates Observed
for 1 H NMR
�H (ppm) Splitting H Group a
0.90
1.31
1.58
2.30
3.65
Triplet
(Broad)
Quintet
Triplet
Singlet
a
Assigned hydrogen is designated as H.
Adapted from Ref 94
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
3
2n
2
2
3
CH 3
(CH 2) n
—CH 2CH 2COOCH 3
—CH 2CH 2COOCH 3
—CH 2CH 2COOCH 3