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Mechanism of Oxidation in Foods of Animal Origin                 7
  VetBooks.ir  two free radicals combine to terminate the process. The lipid radical reacts


            very quickly with atmospheric oxygen making a peroxyl radical which again
            may abstract hydrogen from another acyl chain, resulting in the formation
            of a lipid hydroperoxide and a new radical. By themselves, lipid hydroper-
            oxides are not considered harmful to food quality; however, they are further
            degraded into compounds, especially aldehydes that are responsible for off-
            flavors (Erickson, 2003).
               To break  the  repeating  sequence  of propagating  steps, two types of
            termination reactions are encountered: radical–radical coupling and radical–
            radical  disproportionation,  a process in which two stable products are
            formed from free radicals by an atom or group transfer process. In both
            cases, non-radical products are formed. However, the termination reactions
            are not always efficient. When coupling gives rise to tertiary tetroxides, they
            decompose to peroxyl radicals at temperatures above −80 °C and to alkoxyl
            radicals at temperatures above −30 °C. On the other hand, secondary and
            primary peroxyl radicals, terminate efficiently by a mechanism in which the
            tetroxide decomposes to give molecular oxygen, an alcohol, and a carbonyl
            compound (Erickson, 2003). On the other word, termination of free radical
            oxidative reaction occurs when two radical species (peroxyl, alcoxyl, or
            alkyl) react with each other to form a non-radical adduct. Free radical reac-
            tions can also be terminated when one of the lipid radicals reacts with an
            AH proper, because hydrogen abstraction by a peroxide radical from the AH
            molecule produces an inert AH radical (Kolakowska, 2002).



            1.3.2  PHOTOOXIDATION

            Photooxidation involves the formation of hydroperoxides in a direct reac-
            tion of singlet oxygen ( O ) addition to unsaturated lipids, without radical
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                                   2
            formation. The  O  emerges during a reaction of sensitizers (e.g., chloro-
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                            2
            phyll,  hemoglobin,  myoglobin,  and  riboflavin)  with  atmospheric  oxygen
            (triplet oxygen, O ) (Kolakowska, 2002). Photosensitizations also can occur
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                            2
            in vivo (Halliwell et al., 1995). The  O is 1450 times more reactive than
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                                               2
            molecular oxygen. It is inserted at the end carbon of a double bond, which is
            shifted to an allylic position in the trans configuration. The resulting hydro-
            peroxides have an allylic trans double bond, which renders them different
            from hydroperoxides formed during autoxidation. Hydroperoxides formed
            during photooxidation are more easily cyclicized than hydroperoxy epid-
            ioxides (Frankel, 1998). In addition, light, particularly ultraviolet light may
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