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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Table 1 Peak Positions of the Functional Group Absorptions of Reference Compounds<br />

Representative of Products Formed in Oxidized Oils<br />

Compounds Vibration<br />

Water �OH<br />

�HOH<br />

Hexanol<br />

�ROH<br />

tert-Butyl hydroperoxide<br />

�ROOH<br />

Hexanal �RHC—O<br />

�RHC—O<br />

2-Hexenal a<br />

�RHC—O<br />

�RHC—O<br />

�RC—CH—HC—O<br />

�RC—CH—HC—O<br />

2,4-Decadienal a<br />

�RHC—O<br />

�RHC—O<br />

�RC—CH—HC—O<br />

�RC—CH—HC—O<br />

4-Hexen-3-one a<br />

�RC(—O)HC—CHR<br />

�RC(—O)HC—CHR<br />

�RC(—O)HC—CHR<br />

Oleic acid �RCOOH<br />

�RC(—O)OH<br />

a All double bonds in the trans form.<br />

Frequency (cm �1 )at<br />

peak maximum<br />

3650 <strong>and</strong> 3550<br />

1625<br />

3569<br />

3447<br />

2810 <strong>and</strong> 2712<br />

1727<br />

2805 <strong>and</strong> 2725<br />

1697<br />

1640<br />

974<br />

2805 <strong>and</strong> 2734<br />

1689<br />

1642<br />

987<br />

1703 <strong>and</strong> 1679<br />

1635<br />

972<br />

3310<br />

1711<br />

generally originates from electronically excited stages, such as singlet molecular<br />

oxygen in lipid peroxidation [74]. The chemiluminescence method has been tested<br />

for estimating the degree of deterioration of edible oils containing antioxidants [75]<br />

as well as for shelflife dating of fish samples [76].<br />

D. NMR Spectroscopy<br />

High-resolution nuclear magnetic resonance (NMR) spectroscopy makes it possible<br />

to determine various types of hydrogen atoms (protons, 1 H) in triacylglycerol (TAG)<br />

molecules. This is due to the fact that hydrogen atoms in a strong magnetic field<br />

absorb energy, in the radiofrequency range, depending on their molecular environment.<br />

During oxidation of food lipids, changes occur in the environment in which<br />

protons in an oxidizing TAG molecule are located. These changes may be monitored<br />

by employing 1 H NMR spectroscopy [77–80]. For this purpose, the oil is dissolved<br />

in CDCl3 <strong>and</strong> its NMR spectrum recorded (Fig. 10). The sharp signal at the extreme<br />

right side of the spectrum (high applied field) is due to tetramethylsilane (TMS)<br />

added to the solution to serve as an internal st<strong>and</strong>ard. The spectrum shows eight<br />

groups of signals labeled a–h. These signals are assigned: a, hydrogens directly<br />

attached to double-bonded carbons (olefinic protons) <strong>and</strong> the methine proton in the<br />

glyceryl moiety (� 5.1–5.4); b, hydrogens in the two methylene groups in the glyceryl<br />

moiety (� 4.0–4.4); c, hydrogens in the CH2 groups attached to two doublebonded<br />

carbon atoms (diallylmethylene protons) (—HC—CH2—CH—; � 2.6–2.9);<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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