Antioxidants are used to protect oils, fats, and shortening against oxidative rancidity and to prevent the formation of toxic degradation products and polymers.
Many foods may undergo oxidation, but particularly those containing fats are susceptible to changes in color, odor, taste, and nutritional value. Unsaturated fatty acids are readily peroxidized in the presence of molecular oxygen. The peroxidation products may induce toxic effects. Also, in biological systems peroxidation of lipids may have severe adverse consequences. Peroxidation of polyunsaturated fatty acids is believed to be involved in disturbing the integrity of cellular membranes, the pathogenesis of hemolytic anemia, and pulmonary and hepatic injury. Secondary peroxidation products, e.g., hydroxynonenal, can form adducts with DNA.
The peroxidation of lipids consists of the following steps (LH = lipid):
LH + O2 L- + O2 LOO" + LH LOO" + LOO-LOO" + L-L- + L-
In the initiation step, the unsaturated lipid LH undergoes hydrogen abstraction under the formation of a lipid radical L\ This process can be catalyzed by light, heat, traces of transition metals, and enzymes. The carbon-centered radical tends to be stabilized by intramolecular rearrangement to form a conjugated diene, which readily reacts with molecular oxygen (O2) to yield a lipid peroxide radical, LOO. This, in turn, is capable of inducing the initiation of lipid peroxidation by abstracting a hydrogen atom from another lipid molecule, also leading to the propagation of the oxygenation reaction. The termination step is characterized by the combination of two radicals. Lipid radicals can combine to form dimers, polymers, alcohols, and peroxides. Under normal oxygen tension, the rearrangement of two lipid peroxide radicals (LOO) is most likely to yield LOOL and O2.
Lipid hydroperoxides undergo degradation, leading to the formation of secondary peroxidation products, such as alkanes (e.g., ethane and pentane), aldehydes (e.g., malondialdehyde and hydroxynonenal), ketones, alcohols, and esters.
The purpose of using food antioxidants is to protect food from organoleptic deterioration, decrease in nutritional value, and formation of toxic products by removing radicals. Two types of antioxidants can be distinguished: radical scavengers and synergists.
Radical scavengers, like the phenolic substances butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), interfere with the propagation step, thereby terminating the lipid peroxidation:
A" + LH A" + LOH A" + LOOH LH = lipid
AH = phenolic antioxidant
The antioxidants themselves are converted to resonance-stabilized intermediate radicals A, which is illustrated for BHA in Figure 9.2. The resulting phenoxy radical A^ may either be regenerated to the parent antioxidant AH by reducing agents or further oxidized to a stable quinone, or combine with other phenoxy or lipid peroxy radicals.
Synergists may either regenerate the parent radical scavenging antioxidants from phenoxy radicals (A) formed in the interference with the propagation step, or act as a sequestering agent for transition metals, active catalysts in the initiation, and propagation steps of lipid peroxidation.
Figure 9.2 Radical scavenging by BHA; R' = lipid free radical.
Well-known radical scavengers are a-tocopherol (vitamin E), BHA, BHT, ascorbic acid (vitamin C) and gallate esters (propyl-, octyl- and dodecylgallate). Synergistically-acting antioxidants include ascorbic acid, citric acid, and ethylenediaminetetraacetic acid (EDTA).
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