Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 3 Chromatogram of methyl esters from a partially hydrogenated canola oil sample,
using an SP-2560 100 m � 0.25 mm fused silica capillary column. (From Ref. 57.)
methyl ester isomers from partially hydrogenated soybean oil were separated in a
single run on a 100-m capillary column [57]. Several individual trans isomers appeared
to be baseline-resolved by capillary GC. When GC conditions were optimized
to separate trans diene methyl esters, near-baseline resolution was obtained for several
of these isomers, as shown in Figure 4 [58]. The trans triene isomers present in
partially hydrogenated soybean oil were separated into four peaks on a 100-m capillary
GC column [59]. Fractional crystallization and reversed phase HPLC, followed
by GC analysis of hydrazine reduction products, served to identify and quantify these
triene isomers.
Most laboratories use capillary columns to quantitate individual fatty acid isomers
found in hydrogenated oils. The application of capillary GC analysis in the
separation and identification of positional and geometric isomers of unsaturated fatty
acids has been reviewed [60]. This analysis becomes even more complex for geometrical
and positional isomers found in hydrogenated fish oils, which contain many
more fatty acid isomers than are present in hydrogenated vegetable oils. An extensive
analytical study of hydrogenated menhaden oil using capillary GC was conducted
[61,62]. At an iodine value of 84.5, the most unsaturated isomers were eliminated,
although 13.1% diene, 8.3% triene, and 0.4% tetraene isomers were still present.
The 20-carbon isomers were analyzed further to determine the cis and trans bond
positions. A wide range of monoene, diene, and triene cis and tran positional isomers
were identified at virtually every position of the fatty acid chain.
The content of trans-C16:1 in human milk, as determined by GC, is usually
high due to the overlap with peaks attributed to C17 fatty acids [63]. Isolation by
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