Methods to determine the extent of lipid oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 188

oxidation material or on fractions obtained by adsorption chromatography. The
simplest separation is to separate non-polar from polar lipids, which will allow
differentiating alkyl-chain dimers/polymers from oxygenated dimers/polymers
through the action of alkoxyl or peroxyl radicals. In addition to frying oils,
HPSEC has been successfully applied to the study of oxidation of crude and
refined oils, dispersed lipids and dried microencapsulated oils (MÂrquez-Ruiz
and Dobarganes, 2005).
8.4.3 Gas chromatography (GC) and gas chromatography-mass
spectroscopy (GC-MS)
GC and GC-MS can be used for the analysis of lipid hydroperoxides and other
oxidation products. Since hydroperoxides are labile at high temperatures, they
may need to be reduced to hydroxy derivatives by sodium borohydride or
catalytic hydrogenation (Turnipseed et al., 1993; Grechkin et al., 2005). For GC
separation, the hydroxyl groups need to be acetylated or trisilylated to increase
their volatility. Double bonds in the hydroperoxide molecules are usually
hydrogenated and fatty acid moities are converted to methyl esters. GC-MS is
useful in revealing structural information related to other substituents on the
hydroperoxides such as epoxy or hydroxyl groups, identifying the specific
hydroperoxide structures, and increasing the sensitivity of the analysis. The
drawback of GC is related to lengthy time of analysis because of the need for
reduction and derivatization of the hydroperoxides before analysis.
GC-MS is the preferred method for the analysis of volatile secondary oxida-
tion products, whose analysis would reveal information not only about the
degree of oxidation in different samples but also about the identity of
unsaturated fatty acids in a sample of unknown history. For example, volatile
oxidation products hexanal, heptanal, 2-butanone, 2-propanone, 1-pentanol, 1-
hexanol, pentane and toluene are known to derive mainly from the oxidation of
linoleic acid while octanal, 1-octanol and heptane derive mainly from the
oxidation of oleic acid (Frankel, 1982). Analysis of volatile oxidation products
in oxidized samples is performed by static or dynamic headspace analyses or by
solid-phase microextraction.
In static headspace analysis, the oxidized is placed in a closed container and
the volatile compounds in the sample are allowed time to equilibrate in the
headspace above the sample before an aliquot is injected into a gas (e.g., Snyder
et al., 1985; Girard and Nakai, 1991). While this method is useful to compare
samples of the same matrix, it can not be used to compare different samples,
since the volatile compounds have different solublities/attachment to the matrix.
Dynamic headspace, also called purge and trap or direct thermal desorption,
volatile components are extracted by purge and trap, adsorbed onto a suitable
material, and then rapidly desorbed, e.g. by means of microwave heating and
injected into GC-MS (Vercellotti et al., 1987).
A newer technique is the solid-phase microextraction (SPME), where a piece
of fused silica fiber, ca. 1 cm coated with an adsorbent such as poly(dimethyl-