Health aspects of oxidized dietary fats - Oxidation in Foods and Beverages and Antioxidant Applications - page 147

antioxidant like vitamin E. In soybean oil containing natural tocopherols, -
carotene was demonstrated to be effective in protecting against light oxidation
(Frankel, 1998). During frying, -carotene was not effective in protecting oils
from thermooxidation (Kajimoto et al., 1992).
7.3.2 Loss of polyphenols
Besides tocopherols, polyphenols are further important phenolic compounds
acting as antioxidants in vegetable oils. Polyphenols, like tocopherols, act as
chain breakers by donating a hydrogen radical to alkylperoxyl radicals, which
are formed during the propagation step of lipid oxidation, thereby forming a
resonance stabilized radical. In contrast to tocopherols, polyphenols are only
present in significant amounts in virgin oils because they are largely lost during
oil refining. The most prominent representatives of polyphenols are tyrosol,
hydroxytyrosol, simple phenolic acids and esterified derivatives of tyrosol and
hydroxytyrosol (Tsimidou, 1998).
Similar to tocopherols, polyphenols rapidly decompose at frying tempera-
tures (160±190 ëC) as demonstrated by Beltran-Maza et al. (1998) and Pellegrini
et al. (2001). However, polyphenols are effective stabilizers of -tocopherol
during frying as shown in frying experiments where oil samples with a similar
-tocopherol content but increasing contents of polyphenols were investigated
(Pellegrini et al., 2001).
7.3.3 Loss of phytosterols
Phytosterols are the major constituents in the nonsaponifiable fraction of
vegetable oils and are found mainly in either their free form or as steryl-fatty
acid esters (Moreau et al., 2002; Piironen et al., 2000). The amount of phyto-
sterols in vegetable oils varies greatly, from 70mg/100 g in palm oil to over
1000 mg/100 g in evening primrose oil (Phillips et al. 2002; Verleyen et al.,
2002). Although more than 100 different phytosterol structures have been
identified, the most predominant phytosterols in vegetable oils are campesterol,
-sitosterol, stigmasterol and D5-avenasterol (Moreau et al., 2002; Piironen et
al., 2000; Phillips et al. 2002). Brassicasterol is also a predominant phytosterol
in oils from plants in the Brassicaceae family, such as canola. The saturated
stanols, sitostanol and campestanol are found in corn oil and oils from grains
such as wheat and rice.
It is well known that phytosterols can undergo oxidation reactions when
heated, leading to a variety of polar and nonpolar compounds (Dutta and Savage,
2002). Heating experiments revealed that the phytosterol loss is extreme when
oils are heated without frying (Lampi et al., 2000; Boskou and Morton, 1975). In
contrast, the loss of phytosterols is markedly lower during deep-fat frying
(Ghavami and Morton, 1984; Winkler et al., 2007; Dutta and Appelqvist, 1996;
Dutta and Appelqvist, 1997; Dutta, 1997). For instance, Winkler et al. (2007)
demonstrated that the reduction of phytosterol content ranged between 4 and 6%