Food Lipids: Chemistry, Nutrition, and Biotechnology

which is defined as the number of milligrams of KOH required to neutralize the free
acids in 1gofsample, is a measure of FFA content. FFAs of oils can be determined
colorimetrically by dissolving oil in chloroform (or benzene), then allowing the FFAs
to react with a cupric acetate solution. The organic solvent turns to a blue color due
to the FFA–cupric ion complex, which has a maximum absorbance between 640 and
690 nm [74].
As there is a band attributed to the carboxyl group (COOH) in the center region
of the mid-IR spectrum, Fourier transform IR (FTIR) spectroscopy can be used to
determine the content of free fatty acids [75,76].
3. Oxidative Stability and Oxidation Products
Owing to their degree of unsaturation, lipids are very susceptible to autoxidation.
Autoxidation occurs via a self-sustaining free radical mechanism that produces hydroperoxides
(primary products), which in turn undergo scission to form various
aldehydes, ketones, alcohols, and hydrocarbons (secondary products). The presence
of secondary lipid oxidation products influences the overall quality of a lipid. Methods
of determination of oxidative stability and oxidation products are discussed in
detail in another chapter.
4. Refractive Index
The refractive index (RI) of an oil is defined as the ratio of the speed of light in
vacuum (practically in air) to the speed of light in oil at a specified temperature.
This ratio also provides a measure of purity of oils and may be used as a means of
identifying them. The RI is measured with a refractometer, usually at 20–25�C for
oils and 40�C for solid fats, which generally liquify at 40�C. The RI declines linearly
with decreasing IV; thus, it is also used as an index for reporting the degree of
hydrogenation of the oil [5].
5. Saponification Value
The saponification value provides an indication of the average molecular weight of
lipids. It is defined as the amount of KOH, in milligrams, required to saponify 1 g
of fat, i.e., to neutralize the existing FFAs and those liberated from TAG [5].
6. Solid Fat Index
The solid fat index (SFI), an empirical expression of the ratio of liquids in fat at a
given temperature, is measured as the change in specific volume with respect to
temperature. As a solid fat melts, the volume of the sample increases, and this change
is measured by dilatometry. Detection of analysis of phase transformation of fat may
also be performed, because lipids expand upon melting and contract upon polymorphic
change to a more stable fat [5]. Use of low-resolution pulse NMR and FTIR
[77–81] for determination of solid fat content has been detailed in the literature.
B. Chromatographic Procedures for Lipid Characterization
Lipid extracts are complex mixtures of individual classes of compounds and require
further separation to pure components if needed. Analysis of chemical components
of lipid (e.g., lipid classes, fatty acids, trans fatty acids, sterols, tocopherols, pig-
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