Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 3 Oxidation reactions in frying oils.
position fluctuate quickly and are not easily predicted. During frying, oils with polyunsaturated
fatty acids, such as linoleic acid, have a distinct induction period of
hydroperoxides followed by a rapid increase in peroxide values, then a rapid destruction
of peroxides [8,13]. Measuring levels of polyunsaturated fatty acids, such as
linoleic acid, can help determine extent of thermal oxidation. Wessels [10] reported
that oxidative degradation produced oxidized triglycerides containing hydroperoxide,
epoxy, hydroxy, and keto groups and dimeric fatty acids or dimeric triglycerides.
Volatile degradation products are usually saturated and monounsaturated hydroxy,
aldehydic, keto, and dicarboxylic acids; hydrocarbons; alcohols; aldehydes; ketones;
and aromatic compounds [13].
c. Polymerization. Polymerization occurs during frying, producing a wide variety
of chemical reactions that result in the formation of compounds with high molecular
weight and polarity (Fig. 4). Polymers can form from free radicals or triglycerides
by the Diels–Alder reaction. Cyclic fatty acids can form within one fatty
acid; dimeric fatty acids can form between two fatty acids, either within or between
triglycerides; and polymers with high molecular weight are obtained as these molecules
continue to cross-link. As polymerized products increase in the frying oil,
viscosity of the oil also increases.
B. Factors Affecting Oil Decomposition
Understanding the mechanism of thermal degradation of a frying oil is difficult because
it is affected by many variables, such as unsaturation of fatty acids, oil temperature,
oxygen absorption, metals in substrates and in the oil, and nature of the
food [14]. A list of factors that affect the processes of hydrolysis, oxidation, and
polymerization and eventually frying oil deterioration are presented in Table 2. Frying
oil degradation can be managed and even inhibited by controlling these factors.
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