Food Lipids: Chemistry, Nutrition, and Biotechnology

containing other esters (�-6, �-9, and saturated) exhibit a triplet at 0.89 ppm. The
area associated with these various signals can be used to obtain semiquantitative
information in terms of �3 fatty acids (�-linolenate), other polyenic, monoenic (oleate),
and saturated acids [94,140].
Acylglycerols show signals associated with the five hydrogen atoms on the
glycerol moiety. There is a one-proton signal at 5.25 ppm (CHOCOR), which overlaps
with the olefinic signals and a four proton signals located between 4.12 and
4.28 ppm (CH 2OCOR). Phospholipids display characteristic signals for phosphatidylcholine
and phosphatidylethanolamine [94].
High-resolution 13 C NMR spectra are more complex than 1 H spectra and provide
more structural information rather than quantitative ones. It is also possible to
locate functional groups such as hydroxy, epoxy, acetylenic, and branched chains in
the molecules. The application of 13 C NMR to TAG allows determination of the
positional distribution of fatty acids on the glycerol backbone [141,142]. The 13 C
resonance of the carbonyl group of fatty acids in the sn-1 and sn-3 positions is well
resolved from those esterified at the sn-2 position. Most unsaturated fatty acids in
the sn-2 position are nondegenerative and could be easily differentiated. 13 C NMR
has been successfully applied for determination of positional distribution of TAG
fatty acids in vegetable oils [143–145] and marine oils [146,147].
The NMR imaging is based on manipulation of magnetic field gradients oriented
at right angles to each other to provide spacial encoding of signals from an
object, which are converted by FT techniques to three-dimensional NMR images
[148]. It produces three-dimensional data by selecting two-dimensional cross-sections
in all directions. Application of NMR imaging or magnetic resonance imaging (MRI)
to foods has been of interest as it is a noninvasive technique that can be applied to
track the dynamic changes in foods during storage, processing, packaging, and
distribution.
Most magnetic resonance images of foods are based on proton resonances from
either water or lipids. Simoneau et al. [149] have applied MRI to the study of fat
crystallization in bulk or dispersed systems. Halloin et al. [150] described two MRI
techniques, spin-echo imaging (SEI) and chemical shift imagining (CSI), for the
study of lipid distribution in pecan embryos. Insect- or fungus-damaged embryos
gave images that were less intense than those of normal embryos, reflecting lower
oil content. When MRI and NMR was employed to determine the oil content of
French-style salad dressings, results were within �2% of expected values and were
in agreement with oil content determined by traditional methods [151]. Pilhofer et
al. [152] have studied the use of MRI to investigate the formation and stability of
oil-in-water emulsions formed with vegetable oil, milk fat, and milk fat fractions.
Distribution of lean and fat in retail meat as a means of quality can be measured
[153] using MRI and also to visualize oil and water concentration gradient during
deep-fat frying food [154].
4. Mass Spectrometry
In conventional MS, compounds in their gaseous state are ionized by bombardment
with electrons (electron impact) in an ionization chamber. The resulting mass spectrum
consists of a characteristic pattern of peaks representing molecular fragments
with different mass to charge (m/z) ratios. Some of these peaks or patterns of peaks
are structurally diagnostic. The parent ion peak that arises from the unfragmented
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.