Health aspects of oxidized dietary fats - Oxidation in Foods and Beverages and Antioxidant Applications - page 146

decline in vegetable oils under deep-frying conditions, as demonstrated in frying
experiments with different fats (Miyagawa et al., 1991; Marquez-Ruiz et al.,
1999). The rate of tocopherol decomposition during deep-fat frying depends
amongst others on the type of fat and the tocopherol isomer examined: During
oxidation at 100 ëC and during simulated frying at 180 ëC, tocopherols were
shown to be more slowly depleted in PUFA-rich fats such as soybean oil, corn
oil and safflower oil than in SFA-rich fats such as palm oil and coconut oil or
partially hydrogenated fats (Frankel et al., 1959; Yuki and Ishikawa, 1976). This
effect has been explained by the more rapid decomposition of PUFA
hydroperoxides before they react with tocopherols (Frankel et al., 1959). The
rate of decomposition of -tocopherol during deep-fat frying of potatoes in a
mixture of soybean and rapeseed oil at 180 ëC was faster than that of -
tocopherol followed by -tocopherol (Miyagawa et al., 1991).
7.3 Loss of non-nutritive components during oxidation
Fats and oils contain varying amounts (1±2%) of non-glyceride components. To
this fraction several non-nutritive components such as carotenoids, polyphenols,
phytosterols, squalene, and chlorophylls belong. For most edible oils,
carotenoids are largely removed and destroyed during conventional processing
by deodorization at elevated temperatures. In other oils, such as virgin olive oil
and corn oil, the carotenoids are desirable for optical reasons, and maintained by
partial or full omission of the bleaching step or by deodorizing at lower tem-
peratures. Squalene and phytosterols have little effects on lipid oxidation, but
some types of phytosterols are claimed to have protective effects on oxidative
stability of the oils during heating. Chlorophylls a and b and its degradation
products pheophytins a and b are very active promoters of photosensitized
oxidation, and are therefore removed as completely as possible in the bleaching
step.
7.3.1 Loss of carotenoids
Other antioxidants that are contained in virgin vegetable oils in significant
amounts (i.e., olive oil: 3.1 to 9.2 mg/kg; Gandul-Rojas and Minguez-Mosquera,
1996) are carotenoids, such as -carotene, lutein, neoxanthin, violaxanthin,
luteoxanthin, atheraxanthin, mutatoxanthin, and -cryptoxanthin.
Frying experiments with red palm oil, which has a high native carotene
content, revealed a complete loss of -carotene during the frying process
(Manorama and Rukimi, 1992). Carotenoids, in particular -carotene, are very
efficient singlet oxygen quenchers and prominent antioxidants in the light
(Burton and Ingold, 1984). However, due to their highly conjugated double
bonds, carotenoids add radicals, resulting in unstable radical adducts which
initiate new lipid oxidation reactions. Hence, to be effective for inhibition of
photosensitized oxidation of PUFA, -carotene must be protected by an