Food Lipids: Chemistry, Nutrition, and Biotechnology

An intersystem energy transfer occurs in the complex, resulting in a dissipation of
energy and the eventual release of triplet oxygen.
C. Control of Lipoxygenases
Lipoxygenases are active lipid oxidation catalysts found in plants and some animal
tissues. Lipoxygenase activity can be controlled by heat inactivation and plant breeding
programs that decrease the concentrations of these enzymes. Phenolics are capable
of indirectly inhibiting lipoxygenase activity by acting as free radical inactivators,
but also by reducing the iron in the active site of the enzyme to the
catalytically inactive ferrous state [16].
IV. INACTIVATION OF OXIDATION INTERMEDIATES
Several compounds that can exist in foods indirectly influence lipid oxidation rates.
While these substances do not always directly interact with lipids, they may interact
with metals or oxygen to form reactive species. Examples of such compounds include
superoxide anion, peroxides, and photosensitizers.
A. Superoxide Anion
Superoxide anion is produced by the addition of an electron to molecular oxygen.
Superoxide participates in oxidative reactions by maintaining transition metals in
their reduced, active states, by promoting the release of metals bound to proteins
such as ferritin, and through the pH-dependent formation of its conjugated acid, the
perhydroxyl radical, which can directly catalyze lipid oxidation [26]. Because superoxide
anion participates in oxidative reactions, biological systems contain superoxide
dismutase (SOD).
Two forms of SOD are found in eukaryotic cells, one in the cytosol and the
other in the mitochondria [75]. Cytosolic SOD contains copper and zinc in the active
site, while mitochondrial SOD contains manganese. Both forms of SOD catalyze the
conversion of superoxide anion to hydrogen peroxide by the following reaction:
�
2O2 � 2H� → O2 � HO 2 2
Other compounds can also possess superoxide dismutase-like activity. The most
notable of these are complexes of amino acids and peptides with transition metals.
Cupric ions complexed to lysine, tyrosine, and histidine are capable of catalyzing
the dismutation of superoxide [76]. Histidine-containing peptides complexed to
nickel [77], copper [78], and zinc [78,79] also contain superoxide dismutase-like
activity. It should be noted that the SOD-like activity of metal–amino acid or peptide
complexes generally is orders of magnitude lower than those of proteinaceous SOD.
B. Peroxides
Peroxides are important intermediates of oxidative reactions because they decompose
via transition metals, irradiation, and elevated temperatures to form free radicals.
Hydrogen peroxide exists in foods as a result of direct addition (e.g., aseptic processing
operations) and formation in biological tissues by mechanisms including the
dismutation of superoxide by SOD and the activity of peroxisomes [84]. Hydrogen
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