Page 119 - Natural Antioxidants, Applications in Foods of Animal Origin

98 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir free radicals present in the system (Masuda et al., 2002; Amakura et

al., 2000). (b) The second mechanism involves the removal of reac-
tive oxygen species (ROS) and reactive nitrogen species (RNS) initi-
ators by quenching the chain initiator catalyst (Hamid et al., 2010).
Examples of antioxidants which scavenge free radicals are phenolic
compounds (tocopherols, butylated hydroxytoluene (BHT), butyl-
ated hydroxyanisole (BHA), tert-butylhydroquinone (TBHQ), propyl
gallate (PG), lignans, flavonoids, and phenolic acids, carotenoids,
and so forth.
• Antioxidants that react with transition metals to form complexes, and
thus avoid the catalytic effect of the metals in the oxidation process.
Metal chelators decrease oxidation by preventing metal redox cycling,
forming insoluble metal complexes, or providing steric hindrance
between metals and food components or their oxidation intermedi-
ates (Graf & Eaton, 1990). The most common metal chelators used
in foods contain multiple carboxylic acid (e.g., ethylene diamine tetra
acetic acid (EDTA) and citric acid) or phosphate groups (e.g., poly-
phosphates and phytate). Chelators are typically water soluble but
many also exhibit lipid solubility (e.g., citric acid), thus allowing it
to inactivate metals in the lipid phase. Lignans, polyphenols, ascorbic
acid, and amino acids such as carnosine and histidine can also chelate
metals (Decker et al., 2001). Phenolics, which possess hydroxyl and
carboxyl groups are able to bind particularly well with metals like Fe
or Cu (Jung et al., 2003).
• Antioxidants that decompose peroxides and produce stable substances
which are unable to produce radicals, such as selenium (Se) containing
glutathione peroxidase, an antioxidative enzyme, which inactivate
free radicals and other oxidants, particularly hydrogen peroxide.
• Antioxidants which inactivate the singlet form of oxygen: In the
presence of a photosensitizer, such as chlorophylls and pheophy-
tins, singlet oxygen may be formed from ordinary triplet oxygen by
the action of light. This singlet form of oxygen is very reactive; it
is extremely important to deactivate it back to the triplet form very
rapidly to prevent the photo-oxidation process. Tocopherols, carot-
enoids, curcumin, phenolics, urate, and ascorbate can quench singlet
oxygen (Das & Das, 2002; Choe & Min, 2005).
• Antioxidants which prevent the enzymatic activity required for auto-
oxidation. Examples are flavonoids, phenolic acids, and gallates,
which deactivate the lipoxygenase.