Food Lipids: Chemistry, Nutrition, and Biotechnology

of foods. However, industrially, frying oil quality is usually checked by measurement
of color and/or free fatty acid content in order to tell an operator when a fat is ending
its useful life. The foaming characteristics of used fats would also lead one to the
same conclusion.
A quick colorimetric test kit is now available for measuring oil quality [63].
Blumenthal et al. [64] developed a spot test to measure free fatty acids in which
drops of used fat are placed on a glass covered with silica gel containing a pH
indicator in order to give a three-color test scale of blue, green, and yellow. This
may indicate the amount of free fatty acids in a sample. Northern States Instrument
Corp. (Lino Lakes, MN) has developed an instrument that measures the dielectric
constant of insulating liquids. The instrument is a compact unit, relatively inexpensive,
simple to operate, and requires only a few drops of oil for each measurement.
For evaluation of frying fats, the instrument must be calibrated first with a fresh oil
sample prior to its use in frying operations.
Determination of total polar matter in frying fats appears to be emerging as a
reliable method for assessing the useful life of fats and oils subjected to frying and
is an official method in Europe. Total polar matter is determined by dissolving the
fat in a relatively nonpolar solvent, such as toluene or benzene, and running through
a silica gel column that adsorbs the polar compounds. After evaporation of the solvent,
the nonpolar fat can be weighed and the total polar matter calculated from the
weight difference data or determined directly by their elution from the column with
diethyl ether or a mixture of chloroform and methanol. Sebedio et al. [65] illustrated
that polar and nonpolar fractions of fried oils can be quantitatively estimated using
Iatroscan thin-layer chromatography–flame ionization detection (TLC-FID) system
with Chromarod SII. This method requires a very small sample and is much faster
than silicic acid column separation.
IV. RECENT DEVELOPMENTS FOR QUANTITATION OF
LIPID OXIDATION
A. ESR Spectroscopy
Lipid oxidation in foods and biological systems has conventionally been tested by
monitoring either primary or secondary oxidation products. Over the last 20 years
or so, advances in pulse radiolysis [66] and electron spin resonance (ESR) [67]
techniques have facilitated the detection and study of short-lived free radical intermediates.
ESR spectroscopy allows selective detection of free radicals. The technique
depends on the absorption of microwave energy (which arises from the promotion
of an electron to a higher energy level) when a sample is placed in a variable
magnetic field. A major limitation in the detection of free radicals by ESR is the
requirement that radical concentrations remain higher than 10 �8 M. Radical lifetimes
in solution are very short (