Food Lipids: Chemistry, Nutrition, and Biotechnology

Yen and Duh [69] and Chen and Ho [70] have reported that inhibition of free
radical formation by different antioxidants can be measured using very stable free
radicals such as 1,1-diphenyl-2-picrylhydrazyl (DPPH). The mechanism of the reaction
of antioxidant with DPPH radical is as follows:
• •
(DPPH) � HO— R— OH → (DPPH):H � HO— R— O
•
HO— R— O � (DPPH) → (DPPH):H � O — R — O
DPPH radical, with a deep violet color, receives a hydrogen atom from the antioxidant
and is converted to a colorless molecule. Using this reagent, the free radical
scavenging ability of the antioxidant can be determined by spectrophotometric
methods.
B. Infrared Spectroscopy
Infrared (IR) spectroscopy has also been used for measurement of rancidity, and it
is of particular value in recognition of unusual functional groups and in studies of
fatty acids with trans double bonds. Production of hydroperoxides during oxidation
of lipids gives rise to an absorption band at about 2.93 �m, whereas the disappearance
of a band at 3.20 �m indicates the replacement of a hydrogen atom on a double
bond, or polymerization. It has also been suggested that the appearance of an additional
band at 5.72 �m, due to C—O stretching, indicates the formation of aldehydes,
ketones or acids. Furthermore, changes in the absorption bands in the 10- to 11-�m
region indicates cis,trans isomerization and probably formation of conjugated bonds.
Determination of oxidative deterioration of lipids using IR method is simple, rapid,
and requires small amounts of sample (20 mg).
van de Voort et al. [71] and Sedman et al. [72] have investigated the feasibility
of employing Fourier transform infrared (FTIR) spectroscopy to assess the oxidative
status or forecast the oxidative stability of oils. These authors constructed a spectral
library by recording the FTIR spectra of oils spiked with various compounds representative
of common oil oxidation products. Table 1 shows that each of the various
types of oxidation products gives rise to discernible and characteristic absorptions in
the FTIR spectrum. Similar absorption bands were detected in the spectra of oils
oxidized under accelerated conditions and monitored in real time by FTIR spectroscopy.
On the basis of the results of this study, the authors proposed a quantitative
approach whereby the oxidative status of an oil could be determined through calibrations
developed with oils spiked with appropriate compounds representative of
the functional groups associated with typical oxidative end products. These concepts
were subsequently put into practice with the development of a calibration for the
determination of peroxide value. A similar approach may be used to develop a parallel
method for evaluating p-anisidine values.
C. Chemiluminescence Spectroscopy
Burkow et al. [73] reported that hypochlorite-activated chemiluminescence could
provide a useful means for evaluation of antioxidants in edible oils. Due to high
sensitivity and ability to detect small changes in the degree of oxidation of lipids,
this method may be employed to evaluate the effects of antioxidants on oils during
low temperature storage (about 35�C) within a 24-hour period. Chemiluminescence
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