Health aspects of oxidized dietary fats - Oxidation in Foods and Beverages and Antioxidant Applications - page 149

(Psomiadou and Tsimidou, 2002). The losses of squalene during a half year of
storage of virgin olive oil in the dark at room temperature were in the range
between 26 and 47% (Manzi et al., 1998). Under accelerated storage conditions
(60 ëC), the loss of squalene in extra virgin olive oil was below 20% (Hrncirik
and Fritsche, 2005). During roasting (20min at 150 ëC), the squalene loss in
amaranth was about 12%. In addition, it has been reported that squalene is
remarkably stable during domestic and commercial frying. During pan-frying of
French fries in different edible oils, squalene concentration in the fat showed
only marginal losses (Chiou et al., 2009). Similarly, during domestic pan-frying
and deep-frying of potatoes, the squalene content of the frying oils was only
slightly reduced (Kalogeropoulos and Andrikopoulos, 2004), a finding that was
also confirmed by Chiou et al. (2009). Because a considerable amount of
squalene is absorbed by the fried foods, squalene becomes part of the diet
(Kalogeropoulos and Andrikopoulos, 2004).
7.4 Formation of lipid peroxides in dietary fats during
oxidation
During heat treatment of fats (e.g., deep-frying, baking, roasting) several
chemical reactions occur such as hydrolysis, oxidation, and polymerization of
the oil. Hydrolysis increases the amount of free fatty acids, mono- and
diacylglycerols, and glycerols in oils (Choe and Min, 2007; Chung et al., 2004).
The thermal oxidation, in particular of unsaturated fatty acids, occurs at a greater
rate than hydrolysis during deep-fat frying (Choe and Min, 2007). In the initial
step of fatty acid oxidation, an alkyl radical is formed by removing hydrogen.
These alkyl radicals further react rapidly with oxygen to produce a peroxy-
radical, which abstracts hydrogen from other fatty acids and produces hydro-
peroxide (free radical chain reaction or propagation step). The hydroperoxides
are relatively unstable and are decomposed to alkoxy radicals and hydroxy
radicals by homolysis of the peroxide bond. Typical hydroxyl derivatives
identified in heated fats rich in linoleic acid are 9-hydroxy- and 13-
hydroxyoctadecadienoic acids (9-HODE, 13-HODE), which are formed from
decomposition of the respective hydroperoxides (Toschi et al., 1997).
Besides fatty acid radicals several low molecular volatile compounds such as
aldehydes, ketones, carboxylic acids, and short-chain alkanes and alkenes are
formed during oxidation. The alkyl radicals can also react with other alkyl
radicals, alkoxy radicals, and peroxy radicals to form dimers, trimers, and
polymers which are linked by ±C±C±, ±C±O±C±, and ±C±O±O±C± bonds
(Stevenson et al., 1984; Choe and Min, 2005). Fatty acid radicals also can react
with other radicals within one fatty acid leading to cyclic fatty acid monomers
(CFAM). The formation of polymerization products in general depends on the
type of fat (e.g., degree of unsaturation of fatty acids) and the heating
temperature. As the heating temperature increases the amounts of polymers
increase (Cuesta et al., 1993; Sanchez-Muniz et al., 1993; Takeoka et al., 1997).