Health aspects of oxidized dietary fats - Oxidation in Foods and Beverages and Antioxidant Applications - page 144

oxidizability increases with increasing degree of unsaturation. Cosgrove et al.
(1987) demonstrated that the oxidizabilities of PUFA are linearly dependent on
the number of bis-allylic positions present in the molecule; e.g., the measured
oxidizability of linoleic acid, linolenic acid, arachidonic acid, eicosapentaenoic
acid, and docosahexaenoic acid was found to increase about two-fold for each
bis-allylic methylene group.
The increased susceptibility of unsaturated fatty to oxidation is also shown
during heating of dietary fats. For instance, deep-fat frying experiments of
potato chips in refined, bleached, and deodorized soybean oil and vanaspati
(partially hydrogenated vegetable oil blend) at different frying temperatures
(170, 180, and 190 ëC) revealed a decrease in iodine values (an indicator of the
amount of unsaturated fatty acids within a fat) and a significant loss of PUFA
during frying (Tyagi and Vasishtha, 1996). The iodine value of soybean oil and
vanaspati decreased from their initial values of 129.8 and 74.7 to 96.2 and 59.6,
respectively, after 70 h of frying. Moreover, these experiments clearly showed
that the extent of PUFA loss is directly correlated with frying time and
temperature: The longer the frying time and the higher the frying temperature
the greater the loss of PUFA. The authors of that study (Tyagi and Vasishtha,
1996) observed the highest losses of PUFA in soybean oil with a 79% decrease
in trienoic acids and a 60% decrease in dienoic acids.
Trace amounts of heavy metals, such as iron and copper, decreases the
oxidative stability of fats even at low temperatures and have a marked
accelerating effect on the rates of lipid oxidation, because transition metals
promote hydrogen proton abstraction from fatty acids due to single electron
transfer which occurs during the change of oxidation states (Velasco and
Dobarganes, 2002). Hence, even trace amounts of heavy metals are considered
to be detrimental for fat quality (Rossell, 1998).
7.2.2 Loss of amino acids
Nutrient losses during deep-fat frying also occur due to the reaction of lipid
oxidation products with amines, amino acids, and proteins in fried foods
(Pokorny, 1981; Gardner et al., 1992). Aldehydes, dialdehydes, and epoxides
derived from the decomposition of hydroperoxides can react with amino groups
to produce imino Schiff bases, which themselves polymerize by aldol con-
densation to dimers and complex high-molecular weight brown macromolecules
known as melanoidins (Frankel, 1998). This complex reaction known as
Maillard reaction or as non-enzymatic browning, leads to browning of the fried
food. Although this reaction significantly contributes to the development of the
typical flavor of fried foods, it also causes significant losses of labile amino
acids, such as lysine, histidine, tryptophane, cysteine, cystine, methionine and
tyrosine, and therefore reduces the nutritive value of food proteins. Interestingly,
the intensity of browning is correlated with the loss of these amino acids. Loss of
amino acids can also occur due to oxidation of amino acids by lipid radical
compounds. For instance, methionine is oxidized to sulfoxide, and cystine can