Food Lipids: Chemistry, Nutrition, and Biotechnology

shown that the ratio of olefinic to aliphatic protons, measured by NMR, decreases
continuously as long as the oxidation reaction proceeds. They suggested that the
NMR technique could be useful for measuring oxidative deterioration of oils containing
PUFAs, even at stages beyond the point at which peroxide value profile
reaches a maximum. Saito and Udagawa [78] have used this method to evaluate
oxidative deterioration of brown fish mean and suggested that NMR methodology is
suitable for comparing the storage conditions of the fish meal as well as estimating
the effect of antioxidants in both fish meal and fish oil. These authors reported good
correlations between peroxide values and NMR data. However, Wanasundara and
Shahidi [80] found that linear relationships between peroxide values and NMR data
were not as suitable as those of TOTOX values and NMR data. It is obvious that
TOTOX values correlate better with R ao and R ad than peroxide values since both
TOTOX and NMR data estimate overall changes that occur in fatty acid profiles as
reflected in both primary and secondary oxidation products of lipids. Thus, NMR
methodology offers a rapid, nondestructive, and reliable technique for estimating the
oxidative state of edible oils during processing and storage.
E. Chromatographic Techniques
Different chromatographic techniques have been developed and applied to quantitate
oxidation products in variety of substances, including model compounds, oils, and
food lipids, subjected to oxidation under very different conditions, from room to
frying temperatures. For quantification of free MA, reversed phase HPLC using ionpairing
reagent or size exclusion separation followed by monitoring the absorbance
at 267 nm has been described [81–83]. MA and 4-hydroxynonenal (4-HNE) can also
be derivatized with 2,4-dinitrophenylhydrazine at room temperature to form dinitrophenylhydrozone
(DNP) derivatives. The DNP derivatives could be solubilized in
organic solvents and separated on a reversed phase HPLC and detected at 300–330
nm, depending on the type of hydrozone formed [84–88]. GC methods to quantify
MA have also been reported. The advantages of the GC methods are increased sensitivity,
particularly when used with MS detection, and the possibility for simultaneous
analysis of several aldehydes. Reduction of MA to 1,3-propanediol with borane
trimethylamine [89] forms a butyldimethylsilyl ether, which can be analyzed by GC-
MS with an HP-5 capillary column (25 m long), temperature programmed from
115�C to 165�C and [ 2 H8]propanediol as the internal standard. MA can also be converted
to 1-methylpyrozole by reaction with N-methylhydrazine at room temperature
for a 1-hour period; this derivative could be recovered by extraction with dichloromethane
and analyzed on a DB-Wax capillary column (30 m long), temperature
programmed from 30�C to 200�C, and a nitrogen-phosphorus detector [90,91]. Oils
heated at high temperatures (oil used for frying) and (TAGs) oxidize and form polymeric
TAG and hydrolytic products (e.g., diacylglycerols and fatty acids). Solid
phase extraction with silica could be used to separate polar and nonpolar fractions
of oxidized oils. The polar fraction can be analyzed by high-performance size exclusion
column (highly cross-linked styrene–divinylbenzene copolymer) using a refractive
index detector. Polar compounds are separated as an inverse order of
molecular weight; TAG polymers, TAG dimers, oxidized TAG monomers, diacylglycerols,
monostearine, and fatty acids [92]. It has been observed that oxidized
monomers show a progressive increase during early stages of oxidation. According
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