Page 169 - Natural Antioxidants, Applications in Foods of Animal Origin

148 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir having oxidative and non-oxidative characteristics (Scott, 2012). Fats

contained in food are chemically composed of triglycerides and oxidation
leading to the rancidity of foods occurs at the unsaturated sites of triglycer-
ides. Oxidation in fish and fish products occurs in both free fatty acids and
fatty acyl groups. The oxidation mechanism consists of three steps:
Initiation (Formation of Fatty Acid Radicals): This is the first step in
the oxidative mechanism of lipids. It involves the abstraction of hydrogen
from fatty acid to form a fatty acid radical known as the alkyl radical. Initia-
tors react by binding a hydrogen atom from an unsaturated lipid leading to
formation of a free radical. Stabilization of the free radical by delocalization
over the double bonds results in double bond shifting or formation of conju-
gated double bonds. This leads to production of cis or trans configuration
(Damodaran et al., 2007). Pro-oxidants such as ionizing radiation or light,
transition metals, and temperature are responsible for initiation of oxidation
reactions(Fennema et al., 2008).

RH → R + H
.
RH → ROO + H .
.
where ROO is a lipid peroxy radical, R is a lipid radical, and RH is an unsat-
.
.
urated lipid. The peroxides may break down to carbonyls, form polymers or
react with vitamins, proteins, pigments, and so forth. The ease of formation
of fatty acid radicals increases with an increase in unsaturation. Hydrogen
abstraction becomes easier and lipid oxidation is faster due to decrease in
carbon–hydrogen bond energy as a result of bond dissociation. For instance,
linolenic acid has been estimated to be 10–40 times more susceptible to
oxidation than oleic acid due to its rate of bond dissociation (Damodaran et

al., 2007).
Propagation (Fatty Acids Radical Reaction): The initial reaction in
this step involves the addition of an oxygen molecule which binds to the
fatty acid radical (alkyl radical) leading to the formation of peroxyl-fatty
acid radical and a weak covalent bond. As a result of the weak covalent
bonds of unsaturated fatty acids, they are susceptible to react with peroxyl
radicals. Furthermore, the high energy of peroxyl radicals allows them to
promote the abstraction of hydrogen from another molecule. This reaction
leads to formation of fatty acid hydroperoxyl and fatty acid radical. Further
hydrogen addition to the peroxyl radical results in formation of a fatty acid
HPO and a new alkyl radical on another fatty acid. Thus, the reaction moves
from one fatty acid to another.