Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 1 The mid-infrared spectrum of an edible oil collected on an attenuated total reflectance
crystal. Labels indicate absorption bands or regions associated with triacylglycerols
or other constituents that may be present in an oil or free fatty acid. (From Ref. 137.)
region where compounds containing hydroxyl groups are absorbed, including water
(H-OH) and hydroperoxides (RO-H) and their breakdown products. At some lower
frequencies (3025–2850 cm �1 ) is the CH stretching region where three bands are
visible: a weak cis double bond CH absorption (CH—CH) and strong bonds due to
the CH 2 groups of the aliphatic chains of TAGs as well as the terminal methyl groups.
Just beyond this region is secondary oxidation products of lipids such as aldehydes
and ketones that absorb energy albeit weakly. Toward the center of the spectrum is
a very strong band due to the C—O stretching absorption due to the ester linkage
attaching the fatty acids to the glycerol backbone of TAGs. Next to it is a band due
to COOH group of FFAs if the lipid is hydrolyzed. In the same region there would
be carbonyl absorption bands of aldehydes (R-CHO) and ketones (R-CO-R) if the
oil is oxidized. Continuing to lower frequencies area is the finger printing region
(1500–900 cm �1 ) as the pattern of bands in this region is very characteristic of
molecular composition of the lipid and could be used to identify different components.
At the low-frequency end of the fingerprinting region, a band due to the
CH—CH bonding absorption of isolated trans double bonds could be seen when
trans-containing TAGs are present in the oil. The corresponding absorption of conjugated
dienes containing trans-trans and cis-trans double bonds appears at slightly
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Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.