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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
1
2
formation. The O emerges during a reaction of sensitizers (e.g., chloro-
1
2
phyll, hemoglobin, myoglobin, and riboflavin) with atmospheric oxygen
(triplet oxygen, O ) (Kolakowska, 2002). Photosensitizations also can occur
3
2
in vivo (Halliwell et al., 1995). The O is 1450 times more reactive than
1
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