Food Lipids: Chemistry, Nutrition, and Biotechnology

triacylglycerol polymers, and triacylglycerol dimers. Although diacylglycerol levels
were significantly correlated (r = 0.945), free fatty acids were not significantly correlated
(r = 0.27) with the number of fryings. Arroyo and coworkers also found that
hydrolytic changes paralleled thermoxidative changes, as evidenced by high correlations
between levels of triglyceride polymers and triglyceride dimers (thermoxidative
process) and diglycerides (hydrolytic process) with the number of fryings [14].
Dobarganes et al. [37], who measured triglyceride species and polar compound level
and distribution, found no significant differences in the frying oils and lipids extracted
from fried food for either total polar compounds or polar compound distribution.
Thus, the study results indicated no preferential adsorption of altered oil
compounds on the fried potato surface. Billek et al. [33] compared four methods to
assess frying oils and reported good correlations between results with gel permeation
chromatography (GPC), liquid chromatography (LC) on a silica gel column, polar
and nonpolar components column chromatography (CC) on silica gel and petroleum
ether–insoluble oxidized fatty acids. However, they found that measuring petroleum
ether–insoluble oxidized fatty acids was time consuming and inaccurate. The GPC
method was able to determine dimeric and oligomeric triacylglycerols in frying oil
irrespective of the presence of oxidized compounds, whereas the LC method indicated
the total amount of polar and oxidized compounds. Separation of polar and
nonpolar components by CC was simple and quick.
Wessels [10] reported that methods to analyze frying oils, including measurement
of peroxide value, benzidine value, petroleum ether–insoluble oxidized fatty
acids, acid value, smoke point, UV absorbance, refractive index, iodine value, viscosity,
color, and fatty acid composition, were of limited significance. Abdel-Aal and
Karara [38] measured changes occurring in corn oil during heating and during frying
of potato chips and onion rings by refractive index, acid value, peroxide value, total
carbonyls, benzidine value, and oil color (which all increased) and iodine value
(which decreased). These changes were more pronounced in oil that was used intermittently
rather than continuously. Furthermore, onion rings were more detrimental
to the oil than potato chips, possibly because of the breading material that accumulated
in the oil. These investigators observed significant differences in the physiochemical
changes of the oil extracted from the fried foods and the frying oil.
B. Volatile Decomposition Products
Since many of the volatile decomposition products volatilize during frying, it is
difficult to get an accurate representation of oil deterioration by instrumental and
chemical analyses of these compounds. Methods that measure volatile compounds
directly or indirectly include peroxide value, gas chromatographic volatile compound
analysis, and sensory analysis (Table 4).
1. Peroxides
Fritsch [1] noted that peroxide value is not a good measure of heat abuse in frying
oils because peroxides are unstable at frying temperature (Fig. 6). Usuki et al. [39]
confirmed this observation with pan frying (thin-film heating) of soybean oil, which
produced high peroxide values of 230�C. No thermostable peroxides were detected
after the oil was fractionated by silicic acid column chromatography.
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